MINERALS

CONCEPT

A mineral is a naturally occurring, typically inorganic substance with a specific chemical composition and structure. An unknown mineral usually can be identified according to known characteristics of specific minerals in terms of certain parameters that include its appearance, its hardness, and the ways it breaks apart when fractured. Minerals are not to be confused with rocks, which are typically aggregates of minerals. There are some 3,700 varieties of mineral, a handful of which are abundant and wide-ranging in their application. Many more occur less frequently but are extremely important within a more limited field of uses.

HOW IT WORKS

Introduction to Minerals

The particulars of the mineral definition deserve some expansion, especially inasmuch as mineral has an everyday definition somewhat broader than its scientific definition. In everyday usage, minerals would be the natural, nonliving materials that make up rocks and are mined from the earth. According to this definition, minerals would include all metals, gemstones, clays, and ores. The scientific definition, on the other hand, is much narrower, as we shall see.

The fact that a mineral must be inorganic brings up another term that has a broader meaning in everyday life than in the world of science. At one time, the scientific definition of organic was more or less like the meaning assigned to it by nonscientists today, as describing all living or formerly living things, their parts, and substances that come from them. Today, however, chemists use the word organic to refer to any compound that contains carbon bonded to hydrogen, thus excluding carbonates (which are a type of mineral) and oxides such as carbon dioxide or carbon monoxide. Because a mineral must be inorganic, this definition eliminates coal and peat, both of which come from a wide-ranging group of organic substances known as hydrocarbons.

A mineral also occurs naturally, meaning that even though there are artificial substances that might be described as "mineral-like," they are not minerals. In this sense, the definition of a mineral is even more restricted than that of an element, discussed later in this essay, even though there are nearly 4,000 minerals and more than 92 elements. The number 92, of course, is not arbitrary: that is the number of elements that occur in nature. But there are additional elements, numbering 20 at the end of the twentieth century, that have been created artificially.

PHYSICAL AND CHEMICAL PROPERTIES OF MINERALS.

The specific characteristics of minerals can be discussed both in physical and in chemical terms. From the standpoint of physics, which is concerned with matter, energy, and the interactions between the two, minerals would be described as crystalline solids. The definition of a mineral is narrowed further in terms of its chemistry, or its atomic characteristics, since a mineral must be of unvarying composition.

A mineral, then, must be solid under ordinary conditions of pressure and temperature. This excludes petroleum, for instance (which, in any case, would have been disqualified owing to its organic origins), as well as all other liquids and gases. Moreover, a mineral cannot be just any type of solid but must be a crystalline one—that is, a solid in which the constituent parts have a simple and definite geometric arrangement that is repeated in all directions. This rule, for instance, eliminates clay, an example of an amorphous solid.

Chemically, a mineral must be of unvarying composition, a stipulation that effectively limits minerals to elements and compounds. Neither sand nor glass, for instance, is a mineral, because the composition of both can vary. Another way of putting this is to say that all minerals must have a definite chemical formula, which is not true of sand, dirt, glass, or any other mixture. Let us now look a bit more deeply into the nature of elements and compounds, which are collectively known as pure substances, so as to understand the minerals that are a subset of this larger grouping.

Elements

The periodic table of elements is a chart that appears in most classrooms where any of the physical sciences are taught. It lists all elements in order of atomic number, or the number of protons (positively charged subatomic particles) in the atomic nucleus. The highest atomic number of any naturally occurring element is 92, for uranium, though it should be noted that a very few elements with an atomic number lower than 92 have never actually been found on Earth. On the other hand, all elements with an atomic number higher than 92 are artificial, created either in laboratories or as the result of atomic testing.

An element is a substance made of only one type of atom, meaning that it cannot be broken down chemically to create a simpler substance. In the sense that each is a fundamental building block in the chemistry of the universe, all elements are, as it were, "created equal." They are not equal, however, in terms of their abundance. The first two elements on the periodic table, hydrogen and helium, represent 99.9% of the matter in the entire universe. Though Earth contains little of either, our planet is only a tiny dot within the vastness of space; by contrast, stars such as our Sun are composed almost entirely of those elements (see Sun, Moon, and Earth).

ABUNDANCE ON EARTH.

Of all elements, oxygen is by far the most plentiful on Earth, representing nearly half—49.2%—of the total mass of atoms found on this planet. (Here the term mass refers to the known elemental mass of the planet's atmosphere, waters, and crust; below the crust, scientists can only speculate, though it is likely that much of Earth's interior consists of iron.)

Together with silicon (25.7%), oxygen accounts for almost exactly three-fourths of the elemental mass of Earth. If we add in aluminum (7.5%), iron (4.71%), calcium (3.39%), sodium (2.63%), potassium (2.4%), and magnesium (1.93%), these eight elements make up about 97.46% of Earth's material. Hydrogen, so plentiful in the universe at large, ranks ninth on Earth, accounting for only 0.87% of the planet's known elemental mass. Nine other elements account for a total of 2% of Earth's composition: titanium (0.58%), chlorine (0.19%), phosphorus (0.11%), manganese (0.09%), carbon (0.08%), sulfur (0.06%), barium (0.04%), nitrogen (0.03%), and fluorine (0.03%). The remaining 0.49% is made up of various other elements.

Looking only at Earth's crust, the numbers change somewhat, especially at the lower end of the list. Listed below are the 12 most abundant elements in the planet's crust, known to earth scientists simply as "the abundant elements." These 12, which make up 99.23% of the known crustal mass, together form approximately 40 different minerals that account for the vast majority of that 99.23%. Following the name and chemical symbol of each element is the percentage of the crustal mass it composes.

Abundance of Elements in Earth's Crust

• Oxygen (O): 45.2%
• Silicon (Si): 27.2%
• Aluminum (Al): 8.0%
• Iron (Fe): 5.8%
• Calcium (Ca): 5.06%
• Magnesium (Mg): 2.77%
• Sodium (Na): 2.32%
• Potassium (K): 1.68%
• Titanium (Ti): 0.86%
• Hydrogen (H): 0.14%
• Manganese (Mn): 0.1%
• Phosphorus (P): 0.1%

Atoms, Molecules, and Bonding

As noted earlier, an element is identified by the number of protons in its nucleus, such that any atom with six protons must be carbon, since carbon has an atomic number of 6. The number of electrons, or negatively charged subatomic particles, is the same as the number of protons, giving an atom no net electric charge.

An atom may lose or gain electrons, however, in which case it becomes an ion, an atom or group of atoms with a net electric charge. An atom that has gained electrons, and thus has a negative charge, is called an anion. On the other hand, an atom that has lost electrons, thus becoming positive in charge, is a cation.

In addition to protons and electrons, an atom has neutrons, or neutrally charged particles, in its nucleus. Neutrons have a mass close to that of a proton, which is much larger than that of an electron, and thus the number of neutrons in an atom has a significant effect on its mass. Atoms that have the same number of protons (and therefore are of the same element), but differ in their number of neutrons, are called isotopes.

COMPOUNDS AND MIXTURES.

Whereas there are only a very few elements, there are millions of compounds, or substances made of more than one atom. A simple example is water, formed by the bonding of two hydrogen atoms with one oxygen atom; hence the chemical formula for water, which is H₂O. Note that this is quite different from a mere mixture of hydrogen and oxygen, which would be something else entirely. Given the gaseous composition of the two elements, combined with the fact that both are extremely flammable, the result could hardly be more different from liquid water, which, of course, is used for putting out fires.

The difference between water and the hydrogen-oxygen mixture described is that whereas the latter is the result of mere physical mixing, water is created by chemical bonding. Chemical bonding is the joining, through electromagnetic attraction, of two or more atoms to create a compound. Of the three principal subatomic particles, only electrons are involved in chemical bonding—and only a small portion of those, known as valence electrons, which occupy the outer shell of an atom. Each element has a characteristic pattern of valence electrons, which determines the ways in which the atom bonds.

CHEMICAL BONDING.

Noble gases, of which helium is an example, are noted for their lack of chemical reactivity, or their resistance to bonding. While studying these elements, the German chemist Richard Abegg (1869-1910) discovered that they all have eight valence electrons. His observation led to one of the most important principles of chemical bonding: atoms bond in such a way that they achieve the electron configuration of a noble gas. This concept, known as the octet rule, has been shown to be the case in most stable chemical compounds.

Abegg hypothesized that atoms combine with one another because they exchange electrons in such a way that both end up with eight valence electrons. This was an early model of ionic bonding, which results from attractions between ions with opposite electric charges: when they bond, these ions "complete" each other. Metals tend to form cations and bond with nonmetals that have formed anions. The bond between anions and cations is known as an ionic bond, and is extremely strong.

The other principal type of bond is a covalent bond. The result, once again, is eight valence electrons for each atom, but in this case, the nuclei of the two atoms share electrons. Neither atom "owns" them; rather, they share electrons. Today, chemists understand that most bonds are neither purely ionic nor purely covalent; instead, there is a wide range of hybrids between the two extremes, which are a function of the respective elements' electronegativity, or the relative ability of an atom to attract valence electrons. If one element has a much higher electronegativity value than the other one, the bond will be purely ionic, but if two elements have equal electronegativity values, the bond is purely covalent. Most bonds, however, fall somewhere between these two extremes.

INTERMOLECULAR BONDING.

Chemical bonds exist between atoms and within a molecule. But there are also bonds between molecules, which affect the physical composition of a substance. The strength of intermolecular bonds is affected by the characteristics of the interatomic, or chemical, bond.

For example, the difference in electronegativity values between hydrogen and oxygen is great enough that the bond between them is not purely covalent, but instead is described as a polar covalent bond. Oxygen has a much higher electronegativity (3.5) than hydrogen (2.1), and therefore the electrons tend to gravitate toward the oxygen atom. As a result, water molecules have a strong negative charge on the side occupied by the oxygen atom, with a resulting positive charge on the hydrogen side.

By contrast, molecules of petroleum, a combination of carbon and hydrogen, tend to be nonpolar, because carbon (with an electronegativity value of 2.5) and hydrogen have very similar electronegativity values. Therefore the electric charges are more or less evenly distributed in the molecule. As a result, water molecules form strong attractions, known as dipole-dipole attractions, to each other. Molecules of petroleum, on the other hand, have little attraction to each other, and the differences in charge distribution account for the fact that water and oil do not mix.

Even weaker than the bonds between non-polar molecules, however, are those between highly reactive elements, such as the noble gases and the "noble metals"—gold, silver, and copper, which resist bonding with other elements. The type of intermolecular attraction that exists in such a situation is described by the term London dispersion forces, a reference to the German-born American physicist Fritz Wolfgang London (1900-1954).

The bonding between molecules of most other metals, however, is described by the electron sea model, which depicts metal atoms as floating in a "sea" of valence electrons. These valence electrons are highly mobile within the crystalline structure of the metal, and this mobility helps explain metals' high electric conductivity. The ease with which metal crystals allow themselves to be rearranged explains not only metals' ductility (their ability to be shaped) but also their ability to form alloys, a mixture containing two or more metals.

The Crystalline Structure of Minerals

By definition, a solid is a type of matter whose particles resist attempts at compression. Because of their close proximity, solid particles are fixed in an orderly and definite pattern. Within the larger category of solids are crystalline solids, or those in which the constituent parts are arranged in a simple, definite geometric pattern that is repeated in all directions.

The term crystal is popularly associated with glass and with quartz, but only one of these is a crystalline solid. Quartz is a member of the silicates, a large group of minerals that we will discuss later in this essay. Glass, on the other hand, is an amorphous solid, meaning that its molecules are not arranged in an orderly pattern.

CRYSTAL SYSTEMS.

Elsewhere in this book (Earth, Science, and Nonscience and Planetary Science), there is considerable discussion of misconceptions originating with Aristotle (384-322 b.c.). Despite his many achievements, including significant contributions to the biological sciences, the great Greek philosopher spawned a number of erroneous concepts, which prevailed in the physical sciences until the dawn of the modern era. At least Aristotle made an attempt at scientific study, however; for instance, he dissected dead animals to observe their anatomic structures. His teacher, Plato (427?-347 b.c.), on the other hand, is hardly ever placed among the ranks of those who contributed, even ever so slightly, to progress in the sciences.

There is a reason for this. Plato, in contrast to his pupil, made virtually no attempt to draw his ideas about the universe from an actual study of it. Within Plato's worldview, the specific qualities of any item, including those in the physical world, reflected the existence of perfect and pure ideas that were more "real" than the physical objects themselves. Typical of his philosophy was his idea of the five Platonic solids, or "perfect" geometric shapes that, he claimed, formed the atomic substructure of the world.

The "perfection" of the Platonic solids lay in the fact that they are the only five three-dimensional objects in which the faces constitute a single type of polygon (a closed shape with three or more sides, all straight), while the vertices (edges) are all alike. These five are the tetrahedron, octahedron, and icosahedron, composed of equilateral triangles (four, eight, and twenty, respectively); the cube, which, of course, is made of six squares; and the dodecahedron, made up of twelve pentagons. Plato associated the latter solid with the shape of atoms in outer space, while the other four corresponded to what the Greeks believed were the elements on Earth: fire (tetrahedron), earth (cube), air (octahedron), and water (icosahedron).

All of this, of course, is nonsense from the standpoint of science, though the Platonic solids are of interest within the realm of mathematics. Yet amazingly, Plato in his unscientific way actually touched on something close to the truth, as applied to the crystalline structure of minerals. Despite the large number of minerals, there are just six crystal systems, or geometric shapes formed by crystals. For any given mineral, it is possible for a crystallographer (a type of mineralogist concerned with the study of crystal structures) to identify its crystal system by studying a good, well-formed specimen, observing the faces of the crystal and the angles at which they meet.

An isometric crystal system is the most symmetrical of all, with faces and angles that are most clearly uniform. Because of differing types of polygon that make up the faces, as well as differing numbers of vertices, these crystals appear in 15 forms, several of which are almost eerily reminiscent of Plato's solids: not just the cube (exemplified by halite crystals) but also the octahedron (typical of spinels) and even the dodecahedron (garnets).

REAL-LIFE APPLICATIONS

Mineral Groups

Before the time of the great German mineralogist Georgius Agricola (1494-1555), attempts to classify minerals were almost entirely overshadowed by the mysticism of alchemy, by other nonscientific preoccupations, or by simple lack of knowledge. Agricola's De re metallica (On minerals, 1556), published after his death, constituted the first attempt at scientific mineralogy and mineral classification, but it would be two and a half centuries before the Swedish chemist Jöns Berzelius (1779-1848) developed the basics of the classification system used today.

Berzelius's classification system was refined later in the nineteenth century by the American mineralogist James Dwight Dana (1813-1895) and simplified by the American geologists Brian Mason (1917-) and L. G. Berry (1914-). In general terms, the classification system accepted by mineralogists today is as follows:

• Class 1: Native elements
• Class 2: Sulfides
• Class 3: Oxides and hydroxides
• Class 4: Halides
• Class 5: Carbonates, nitrates, borates, iodates
• Class 6: Sulfates, chromates, molybdates, tungstates
• Class 7: Phosphates, arsenates, vanadates
• Class 8: Silicates

NATIVE ELEMENTS.

The first group, native elements, includes (among other things) metallic elements that appear in pure form somewhere on Earth: aluminum, cadmium, chromium, copper, gold, indium, iron, lead, mercury, nickel, platinum, silver, tellurium, tin, titanium, and zinc. This may seem like a great number of elements, but it is only a small portion of the 87 metallic elements listed on the periodic table.

The native elements also include certain metallic alloys, a fact that might seem strange for several reasons. First of all, an alloy is a mixture, not a compound, and, second, people tend to think of alloys as being man-made, not natural. The list of metallic alloys included among the native elements, however, is very small, and they meet certain very specific mineralogic criteria regarding consistency of composition.

The native elements class also includes native nonmetals such as carbon, in the form of graphite or its considerably more valuable alter ego, diamond, as well as elemental silicon (an extremely important building block for minerals, as we shall see) and sulfur. For a full list of native elements and an explanation of criteria for inclusion, as well as similar data for the other classes of mineral, the reader is encouraged to consult the Minerals by Name Web site, the address of which is provided in "Where to Learn More" at the end of this essay.

SULFIDES AND HALIDES.

Most important ores (a rock or mineral possessing economic value)—copper, lead, and silver—belong to the sulfides class, as does a mineral that often has been mistaken for a precious metal—iron sulfide, or pyrite. Better known by the colloquial term fool's gold, pyrite has proved valuable primarily to con artists who passed it off as the genuine article. During World War II, however, pyrite deposits near Ducktown, Tennessee, became valuable owing to the content of sulfur, which was extracted for use in defense applications.

Whereas the sulfides fit the common notion of a mineral as a hard substance, halides, which are typically soft and transparent, do not. Yet they are indeed a class of minerals, and they include one of the best-known minerals on Earth: halite, known chemically as NaCl or sodium chloride—or, in everyday language, table salt.

OXIDES.

Oxides, as their name suggests, are minerals containing oxygen; however, if all oxygen-containing minerals were lumped into just one group, that group would take up almost the entire list. For instance, under the present system, silicates account for the vast majority of minerals, but since those contain oxygen as well, a list that grouped all oxygen-based minerals together would consist of only four classes: native elements, sulfides, halides, and a swollen oxide category that would include 90% of all known minerals.

Instead, the oxides class is limited only to noncomplex minerals that contain either oxygen or hydroxide (OH). Examples of oxides include magnetite (iron oxide) and corundum (aluminum oxide.) It should be pointed out that a single chemical name, such as iron oxide or aluminum oxide, is not limited to a single mineral; for example, anatase and brookite are both titanium oxide, but they represent different combinations.

OTHER NONSILICATES.

All the mineral classes discussed to this point, as well as several others to follow, are called nonsilicates, a term that stresses the importance of silicates among mineral classes.

Like the oxides, the carbonates, or carbon-based minerals, are a varied group. This class also contains a large number of minerals, making it the most extensive group aside from silicates and phosphates. Among these are limestones and dolostones, some of the most abundant rocks on Earth.

The phosphates, despite their name, may or may not include phosphorus; in some cases, arsenic, vanadium, or antimony may appear in its place. The same is true of the sulfates, which may or may not involve sulfur; some include chromium, tungsten, selenium, tellurium, or molybdenum instead.

TWO QUESTIONABLE CLASSES.

In addition to the seven formal classes just described, there are two other somewhat questionable classes of nonsilicate that might be included in a listing of minerals. They would be included, if at all, only with major reservations, since they do not strictly fit the fourfold definition of a mineral as crystalline in structure, natural, inorganic, and identifiable by a precise chemical formula. These two questionable groups are organics and mineraloids.

Organics, as their name suggests, have organic components, but as we have observed, "organic" is not the same as "biological." This class excludes hard substances created in a biological setting—for example, bone or pearl—and includes only minerals that develop in a geologic setting yet have organic chemicals in their composition. By far the best-known example of this class, which includes only a half-dozen minerals, is amber, which is fossilized tree sap.

Amber is also among the mineraloids, which are not really "questionable" at all—they are clearly not minerals, since they do not have the necessary crystalline structure. Nevertheless, they often are listed among minerals in reference books and are likely to be sold by mineral dealers. The other four mineraloids include two other well-known substances, opal and obsidian.

Silicates

Where minerals are concerned, the silicates are the "stars of the show": the most abundant and most widely used class of minerals. That being said, it should be pointed out that there are a handful of abundant nonsilicates, most notably the iron oxides hematite, magnetite, and goethite. A few other nonsilicates, while they are less abundant, are important to the makeup of Earth's crust, examples being the carbonates calcite and dolomite; the sulfides pyrite, sphalerite, galena, and chalcopyrite; and the sulfate gypsum. Yet the nonsilicates are not nearly as important as the class of minerals built around the element silicon.

Though it was discovered by Jöns Berzelius in 1823, owing to its abundance in the planet's minerals, silicon has been in use by humans for thousands of years. Indeed, silicon may have been one of the first elements formed in the Precambrian eons (see Geologic Time). Geologists believe that Earth once was composed primarily of molten iron, oxygen, silicon, and aluminum, which, of course, are still the predominant elements in the planet's crust. But because iron has a greater atomic mass, it settled toward the center, while the more lightweight elements rose to the surface. After oxygen, silicon is the most abundant of all elements on the planet, and compounds involving the two make up about 90% of the mass of Earth's crust.

SILICON, CARBON, AND OXYGEN.

On the periodic table, silicon lies just below carbon, with which it shares an ability to form long strings of atoms. Because of this and other chemical characteristics, silicon, like carbon, is at the center of a vast array of compounds—organic in the case of carbon and inorganic in the case of silicon. Silicates, which, as noted earlier, account for nine-tenths of the mass of Earth's crust (and 30% of all minerals), are to silicon and mineralogy what hydrocarbons are to carbon and organic chemistry.

Whereas carbon forms it most important compounds with hydrogen—hydrocarbons such as petroleum—the most important silicon-containing compounds are those formed by bonds with oxygen. There is silica (SiO₂), for instance, commonly known as sand. Aside from its many applications on the beaches of the world, silica, when mixed with lime and soda (sodium carbonate) and other substances, makes glass. Like carbon, silicon has the ability to form polymers, or long, chainlike molecules. And whereas carbon polymers are built of hydrocarbons (plastics are an example), silicon polymers are made of silicon and oxygen in monomers, or strings of atoms, that form ribbons or sheets many millions of units long.

SILICATE SUBCLASSES.

There are six subclasses of silicate, differentiated by structure. Nesosilicates include some the garnet group; gadolinite, which played a significant role in the isolation of the lanthanide series of elements during the nineteenth century; and zircon. The latter may seem to be associated with the cheap diamond simulant, or substitute, called cubic zirconium, or CZ. CZ, however, is an artificial "mineral," whereas zircon is the real thing—yet it, too, has been applied as a diamond simulant.

Just as silicon's close relative, carbon, can form sheets (this is the basic composition of graphite), so silicon can appear in sheets as the phyllosilicate subclass. Included among this group are minerals known for their softness: kaolinite, talc, and various types of mica. These are used in everything from countertops to talcum powder. The kaolinite derivative known as kaolin is applied, for instance, in the manufacture of porcelain, while some people in parts of Georgia, a state noted for its kaolinite deposits, claim that it can and should be chewed as an antacid stomach remedy. (One can even find little bags of kaolin sold for this purpose at convenience stores around Columbus in southern Georgia.)

Included in another subclass, the tectosilicates, are the feldspar and quartz groups, which are the two most abundant types of mineral in Earth's crust. Note that these are both groups: to a mineralogist, feldspar and quartz refer not to single minerals but to several within a larger grouping. Feldspar, whose name comes from the Swedish words for "field" and "mineral" (a reference to the fact that miners and farmers found the same rocks in their respective areas of labor), includes a number of varieties, such as albite (sodium aluminum silicate) or sanidine (potassium aluminum silicate).

Other, more obscure silicate subclasses include sorosilicates and inosilicates. Finally, there are cyclosilicates, such as beryl or beryllium aluminum silicate.

Identifying Minerals

Mineralogists identify unknown minerals by judging them in terms of various physical properties, including hardness, color and streak, luster, cleavage and fracture, density and specific gravity, and other factors, such as crystal form. Hardness, or the ability of one mineral to scratch another, may be measured against the Mohs scale, introduced in 1812 by the German mineralogist Friedrich Mohs (1773-1839). The scale rates minerals from 1 to 10, with 10 being equivalent to the hardness of a diamond and 1 that of talc, the softest mineral. (See Economic Geology for other scales, some of which are more applicable to specific types of minerals.)

Minerals sometimes can be identified by color, but this property can be so affected by the presence of impurities that mineralogists rely instead on streak. The latter term refers to the color of the powder produced when one mineral is scratched by another, harder one. Another visual property is luster, or the appearance of a mineral when light reflects off its surface. Among the terms used in identifying luster are metallic, vitreous (glassy), and dull.

The term cleavage refers to the way in which a mineral breaks—that is, the planes across which the mineral splits into pieces. For instance, muscovite tends to cleave only in one direction, forming thin sheets, while halite cleaves in three directions, which are all perpendicular to one another, forming cubes. The cleavage of a mineral reveals its crystal system; however, minerals are more likely to fracture (break along something other than a flat surface) than they are to cleave.

DENSITY, SPECIFIC GRAVITY, AND OTHER PROPERTIES.

Density is the ratio of mass to volume, and specific gravityis the ratio between the density of a particular substance and that of water. Specific gravity almost always is measured according to the metric system, because of the convenience: since the density of water is 1 g per cubic centimeter (g/cm³), the specific gravity of a substance is identical to its density, except that specific gravity involves no units.

For example, gold has a density of 19.3 g/cm³ and a specific gravity of 19.3. Its specific gravity, incidentally, is extremely high, and, indeed, one of the few metals that comes close is lead, which has a specific gravity of 11. By comparing specific gravity values and measuring the displacement of water when an object is set down in it, it is possible to determine whether an item purported to be gold actually is gold.

In addition to these more common parameters for identifying minerals, it may be possible to identify certain ones according to other specifics. There are minerals that exhibit fluorescent or phosphorescent characteristics, for instance. The first term refers to objects that glow when viewed under ultraviolet light, while the second term describes those that continue to glow after being exposed to visible light for a short period of time. Some minerals are magnetic, while others are radioactive.

NAMING MINERALS.

Chemists long ago adopted a system for naming compounds so as to avoid the confusion of proliferating common names. The only compounds routinely referred to by their common names in the world of chemistry are water and ammonia; all others are known according to chemical nomenclature that is governed by specific rules. Thus, for instance, NaCl is never "salt," but "sodium chloride."

Geologists have not been able to develop such a consistent means of naming minerals. For one thing, as noted earlier, two minerals may be different from each other yet include the same elements. Furthermore, it is difficult (unlike the case of chemical compounds) to give minerals names that provide a great deal of information regarding their makeup. Instead, most minerals are simply named after people (usually scientists) or the locale in which they were found.

Abrasives

The physical properties of minerals, including many of the characteristics we have just discussed, have an enormous impact on their usefulness and commercial value. Some minerals, such as diamonds and corundum, are prized for their hardness, while others, ranging from marble to the "mineral" alabaster, are useful precisely because they are soft. Others, among them copper and gold, are not just soft but highly malleable, and this property makes them particularly useful in making products such as electrical wiring.

Diamonds, corundum, and other minerals valued for their hardness belong to a larger class of materials called abrasives. The latter includes sandpaper, which of course is made from one of the leading silicate derivatives, sand. Sandstone and quartz are abrasives, as are numerous variants of corundum, such as sapphire and garnets.

In 1891, American inventor Edward G. Acheson (1856-1931) created silicon carbide, later sold under the trade name Carborundum, by heating a mixture of clay and coke (almost pure carbon). For 50 years, Carborundum was the second-hardest substance known, diamonds being the hardest. Today other synthetic abrasives, made from aluminum oxide, boron carbide, and boron nitride, have supplanted Carborundum in importance.

Corundum, from the oxides class of mineral, can have numerous uses. Extremely hard, corundum, in the form of an unconsolidated rock commonly called emery, has been used as an abrasive since ancient times. Owing to its very high melting point—even higher than that of iron—corundum also is employed in making alumina, a fireproof product used in furnaces and fireplaces. Though pure corundum is colorless, when combined with trace amounts of certain elements, it can yield brilliant colors: hence, corundum with traces of chromium becomes a red ruby, while traces of iron, titanium, and other elements yield varieties of sapphire in yellow, green, and violet as well as the familiar blue.

This brings up an important point: many of the minerals named here are valued for much more than their abrasive qualities. Many of the 16 minerals used as gemstones, including corundum (source of both rubies and sapphires, as we have noted), garnet, quartz, and of course diamond, happen to be abrasives as well. (See Economic Geology for the full list of precious gems.)

DIAMONDS.

Diamonds, in fact, are so greatly prized for their beauty and their application in jewelry that their role as "working" minerals—not just decorations—should be emphasized. The diamonds used in industry look quite different from the ones that appear in jewelry. Industrial diamonds are small, dark, and cloudy in appearance, and though they have the same chemical properties as gem-quality diamonds, they are cut with functionality (rather than beauty) in mind. A diamond is hard, but brittle: in other words, it can be broken, but it is very difficult to scratch or cut a diamond—except with another diamond.

On the other hand, the cutting of fine diamonds for jewelry is an art, exemplified in the alluring qualities of such famous gems as the jewels in the British Crown or the infamous Hope Diamond in Washington, D.C.'s Smithsonian Institution. Such diamonds—as well as the diamonds on an engagement ring—are cut to refract or bend light rays and to disperse the colors of visible light.

Soft and Ductile Minerals

At the other end of the Mohs scale are an array of minerals valued not for their hardness, but for opposite qualities. Calcite, for example, is often used in cleansers because, unlike an abrasive (also used for cleaning in some situations), it will not scratch a surface to which it is applied. Calcite takes another significant form, that of marble, which is used in sculpture, flooring, and ornamentation because of its softness and ease in carving—not to mention its great beauty.

Gypsum, used in plaster of paris and wall-board, is another soft mineral with applications in building. Though, obviously, soft minerals are not much value as structural materials, when stud walls of wood provide the structural stability for gypsum sheet wall coverings, the softness of the latter can be an advantage. Gypsum wall-board makes it easy to put in tacks or nails for pictures and other decorations, or to cut out a hole for a new door, yet it is plenty sturdy if bumped. Furthermore, it is much less expensive than most materials, such as wood paneling, that might be used to cover interior walls.

GOLD.

Quite different sorts of minerals are valued not only for their softness but also their ductility or malleability. There is gold, for instance, the most ductile of all metals. A single troy ounce (31.1 g) can be hammered into a sheet just 0.00025 in. (0.00064 cm) thick, covering 68 sq. ft. (6.3 sq m), while a piece of gold weighing about as much as a raisin (0.0022 lb., or 1 g) can be pulled into the shape of a wire 1.5 mi. (2.4 km) long. This, along with its qualities as a conductor of heat and electricity, would give it a number of other applications, were it not for the high cost of gold.

Therefore, gold, if it were a person, would have to be content with being only the most prized and admired of all metallic minerals, an element for which men and whole armies have fought and sometimes died. Gold is one of the few metals that is not silver, gray, or white in appearance, and its beautifully distinctive color caught the eyes of metalsmiths and royalty from the beginning of civilization. Hence it was one of the first widely used metals.

Records from India dating back to 5000 b.c. suggest a familiarity with gold, and jewelry found in Egyptian tombs indicates the use of sophisticated techniques among the goldsmiths of Egypt as early as 2600 b.c. Likewise, the Bible mentions gold in several passages, and the Romans called it aurum ("shining dawn"), which explains its chemical symbol, Au.

COPPER.

Copper, gold, and silver are together known as coinage metals. They have all been used for making coins, a reflection not only of their attractiveness and malleability, but also of their resistance to oxidation. (Oxygen has a highly corrosive influence on metals, causing rust, tarnishing, and other effects normally associated with aging but in fact resulting from the reaction of metal and oxygen.) Of the three coinage metals, copper is by far the most versatile, widely used for electrical wiring and in making cookware. Due to the high conductivity of copper, a heated copper pan has a uniform temperature, but copper pots must be coated with tin because too much copper in food is toxic.

Its resistance to corrosion makes copper ideal for plumbing. Likewise, its use in making coins resulted from its anticorrosive qualities, combined with its beauty. These qualities led to the use of copper in decorative applications for which gold would have been much too expensive: many old buildings used copper roofs, and the Statue of Liberty is covered in 300 thick copper plates. As for why the statue and many old copper roofs are green rather than copper-colored, the reason is that copper does eventually corrode when exposed to air for long periods of time. It develops a thin layer of black copper oxide, and as the years pass, the reaction with carbon dioxide in the air leads to the formation of copper carbonate, which imparts a greenish color.

Unlike silver and gold, copper is still used as a coinage metal, though it, too, has been increasingly taken off the market for this purpose due to the high expense involved. Ironically, though most people think of pennies as containing copper, in fact the penny is the only American coin that contains no copper alloys. Because the amount of copper necessary to make a penny today costs more than one cent, a penny is actually made of zinc with a thin copper coating.

Insulation and Other Applications

Whereas copper is useful because it conducts heat and electricity well, other minerals (e.g., kyanite, and alusite, muscovite, and silimanite) are valuable for their ability not to conduct heat or electricity. Muscovite is often used for insulation in electrical devices, though its many qualities make it a mineral prized for a number of reasons.

Its cleavage and lustrous appearance, combined with its transparency and almost complete lack of color, made it useful for glass in the windowpanes of homes owned by noblemen and other wealthy Europeans of the Middle Ages. Today, muscovite is the material in furnace and stove doors: like ordinary glass, it makes it possible for one to look inside without opening the door, but unlike glass, it is an excellent insulator. The glass-like quality of muscovite also makes it a popular material in wallpaper, where ground muscovite provides a glassy sheen.

In the same vein, asbestos—which may be made of chrysotile, crocidolite, or other minerals—has been prized for a number of qualities, including its flexibility and fiber-like cleavage. These factors, combined with its great heat resistance and its resistance to flame, have made it useful for fireproofing applications, as for instance in roofing materials, insulation for heating and electrical devices, brake linings, and suits for fire-fighters and others who must work around flames and great heat. However, information linking asbestos and certain forms of cancer, which began to circulate in the 1970s, led to a sharp decline in the asbestos industry.

MINERALS FOR HEALTH OR OTHERWISE.

All sorts of other properties give minerals value. Halite, or table salt, is an important—perhaps too important!—part of the American diet. Nor is it the only consumable mineral; people also take minerals in dietary supplements, which is appropriate since the human body itself contains numerous minerals. In addition to a very high proportion of carbon, the body also contains a significant amount of iron, a critical component in red blood cells, as well as smaller amounts of minerals such as zinc. Additionally, there are trace minerals, so called because only traces of them are present in the body, that include cobalt, copper, manganese, molybdenum, nickel, selenium, silicon, and vanadium.

One mineral that does not belong in the human body is lead, which has been linked with a number of health risks. The human body can only excrete very small quantities of lead a day, and this is particularly true of children. Even in small concentrations, lead can cause elevation of blood pressure, and higher concentrations can effect the central nervous system, resulting in decreased mental functioning, hearing damage, coma, and possibly even death.

The ancient Romans, however, did not know this, and used what they called plumbum in making water pipes. (The Latin word is the root of our own term plumber. ) Many historians believe that plumbum in the Romans' water supply was one of the reasons behind the decline and fall of the Roman Empire.

Even in the early twentieth century, people did not know about the hazards associated with lead, and therefore it was applied as an ingredient in paint. In addition, it was used in water pipes, and as an antiknock agent in gasolines. Increased awareness of the health hazards involved have led to a discontinuation of these practices.

GRAPHITE.

Pencil "lead," on the other hand, is actually a mixture of clay with graphite, a form of carbon that is also useful as a dry lubricant because of its unusual cleavage. It is slippery because it is actually a series of atomic sheets, rather like a big, thick stack of carbon paper: if the stack is heavy, the sheets are likely to slide against one another.

Actually, people born after about 1980 may have little experience with carbon paper, which was gradually phased out as photocopiers became cheaper and more readily available. Today, carbon paper is most often encountered when signing a credit-card receipt: the signaturegoes through the graphite-based backing of thereceipt onto a customer copy.

In such a situation, one might notice that thecopied image of the signature looks as though itwere signed in pencil, which of course is fitting due to the application of graphite in pencil "lead." In ancient times, people did indeed uselead—which is part of the "carbon family" of elements, along with carbon and silicon—for writing, because it left gray marks on a surface. Eventoday, people still use the word "lead" in reference to pencils, much as they still refer to a galvanized steel roof with a zinc coating as a "tin roof."

(For more about minerals, see Rocks. The economic applications of both minerals and rocks are discussed in Economic Geology. In addition, Paleontology contains a discussion of fossilization, a process in which minerals eventually replace organic material in long-dead organisms.)

WHERE TO LEARN MORE

Atlas of Rocks, Minerals, and Textures (Web site). <http://www.geosci.unc.edu/Petunia/IgMetAtlas/mainmenu.html>.

Hurlbut, Cornelius, W. Edwin Sharp, and Edward Salisbury Dana. Dana's Minerals and How to Study Them. New York: John Wiley and Sons, 1997.

The Mineral and Gemstone Kingdom: Minerals A-Z (Web site). <http://www.minerals.net/mineral/>.

Minerals and Metals: A World to Discover. Natural Resources Canada (Web site). <http://www.nrcan.gc.ca/mms/school/e_mine.htm>.

Minerals by Name (Web site). <http://mineral.galleries.com/minerals/by_name.htm>.

Pough, Frederick H. A Field Guide to Rocks and Minerals. Boston: Houghton Mifflin, 1996.

Roberts, Willard Lincoln, Thomas J. Campbell, and George Robert Rapp. Encyclopedia of Minerals. New York: Van Nostrand Reinhold, 1990.

Sorrell, Charles A., and George F. Sandström. A Field Guide and Introduction to the Geology and Chemistry of Rocks and Minerals. New York: St. Martin's, 2001.

Symes, R. F. Rocks and Minerals. Illus. Colin Keates and Andreas Einsiedel. New York: Dorling Kindersley, 2000.

"USGS Minerals Statistics and Information." United States Geological Survey (Web site). <http://minerals.usgs.gov/minerals/>.

KEY TERMS

ALLOY:

A mixture of two or more metals.

ANION:

The negative ion that results when an atom or group of atoms gains one or more electrons.

ATOM:

The smallest particle of an element, consisting of protons, neutrons, and electrons. An atom can exist either alone or in combination with other atoms in a molecule.

ATOMIC NUMBER:

The number of protons in the nucleus of an atom. Since this number is different for each element, elements are listed on the periodic table in order of atomic number.

CATION:

The positive ion that results when an atom or group of atoms loses one or more electrons.

CHEMICAL BONDING:

The joining, through electromagnetic forces, of atoms representing different elements. The principal methods of combining are through covalent and ionic bonding, though few bonds are purely one or the other.

CLEAVAGE:

A term referring to the characteristic patterns by which a mineral breaks and specifically to the planes across which breaking occurs.

COMPOUND:

A substance made up of atoms of more than one element, chemically bonded to one another.

COVALENT BONDING:

A type of chemical bonding in which two atoms share valence electrons.

CRUST:

The uppermost division of the solid earth, representing less than 1% of its volume and varying in depth from 3 mi. to 37 mi. (5-60 km).

CRYSTALLINE SOLID:

A type of solid in which the constituent parts have a simple and definite geometric arrangement that is repeated in all directions.

ELECTRON:

A negatively charged particle in an atom, which spins around the nucleus.

ELECTRONEGATIVITY:

The relative ability of an atom to attract valence electrons.

ELEMENT:

A substance made up of only one kind of atom. Unlike compounds, elements cannot be broken chemically into other substances.

HARDNESS:

In mineralogy, the ability of one mineral to scratch another. This is measured by the Mohs scale.

HYDROCARBON:

Any chemical compound whose molecules are made up of nothing but carbon and hydrogen atoms.

ION:

An atom or group of atoms that has lost or gained one or more electrons and thus has a net electric charge. Positively charged ions are called cations, and negatively charged ones are called anions.

IONIC BONDING:

A form of chemical bonding that results from attractions between ions with opposite electric charges.

LUSTER:

The appearance of a mineral when light reflects off its surface. Among the terms used in identifying luster aremetallic, vitreous (glassy), and dull.

MINERAL:

A naturally occurring, typically inorganic substance with a specific chemical composition and a crystalline structure. Unknown minerals usually can be identified in terms of specific parameters, such as hardness or luster.

MINERALOGY:

The study of minerals, which includes a number of smaller sub-disciplines, such as crystallography.

MIXTURE:

A substance with a variable composition, meaning that it is composed of molecules or atoms of differing types and in variable proportions.

MOHS SCALE:

A scale, introduced in 1812 by the German mineralogist Friedrich Mohs (1773-1839), that rates the hardness of minerals from 1 to 10. Ten is equivalent to the hardness of a diamond and 1 that of talc, an extremely soft mineral.

MONOMERS:

Small, individual subunits that join together to form polymers.

NUCLEUS:

The center of an atom, a region where protons and neutrons are located and around which electrons spin.

ORE:

A rock or mineral possessing economic value.

ORGANIC:

At one time, chemists used the term organic only in reference to living things. Now the word is applied to most compounds containing carbon and hydrogen, thus excluding carbonates (which are minerals) and oxides such as carbon dioxide.

PERIODIC TABLE OF ELEMENTS:

A chart that shows the elements arranged in order of atomic number, along with the chemical symbol and the average atomic mass for that particular element.

POLYMERS:

Large, typically chainlike molecules composed of numerous smaller, repeating units known as monomers.

PROTON:

A positively charged particle in an atom.

PURE SUBSTANCE:

A substance, whether an element or compound, that has the same chemical composition throughout. Compare with mixture.

REACTIVITY:

A term referring to the ability of one element to bond with others. The higher the reactivity (and, hence, the electro negativity value), the greater the tendency to bond.

ROCK:

An aggregate of minerals.

SPECIFIC GRAVITY:

The ratio between the density of a particular substance and that of water.

STREAK:

The color of the powder produced when one mineral is scratched byanother, harder one.

VALENCE ELECTRONS:

Electrons that occupy the highest principal energy level in an atom. These are the electrons involved in chemical bonding.

MINERALS

MINERALS. Living organisms appear to selectively concentrate certain elements from the environment while rejecting others. The adult human body contains approximately thirty-five elements. Four of these (hydrogen, oxygen, carbon, and nitrogen) constitute 99 percent of the atoms in the body. As a comparison, the most abundant elements in the Earth's crust are oxygen (67 percent), silicon (28 percent), and aluminum (8 percent). The remaining 1 percent of the elements in the human body (with the exception of sulfur) are the inorganic or mineral constituents of the body and thus form the ash when the body is "burned." Seven of the remaining elements, sodium, potassium, calcium, magnesium, phosphorus, sulfur, and chloride, together represent about 0.9 percent of the body's weight. The seventeen others make up the remaining 0.1 percent, some of which, but not all, are considered nutritionally essential. These elements appear in the body at measurable concentrations but may not perform an essential biological function. Cadmium is one such example. The newborn infant is virtually free of this element, but gradually accumulates cadmium by ingestion and inhalation, such that over a lifetime an average person living in an industrial society accumulates milligrams of this element. Not only does cadmium appear to serve no essential function in the body, it is also likely to be undesirable and potentially detrimental.

Most experts agree that thirteen mineral elements are nutritionally essential. These are minerals that when deficient consistently result in an impairment of a function that is prevented or cured by supplementation. There still is some question about seven others (Table 1).

The functions of mineral elements are structural, osmotic, catalytic, and signaling. Calcium plays the most obvious role as structural component of bone but also participates in many examples of cell signaling. Sodium, chloride, and potassium constitute the majority of minerals whose function is to maintain osmotic and water balance and membrane electrical potentials. The micro-mineral elements listed in Table 1 have historically been classified as "trace" elements primarily because they occurred at levels below past methods for detection. In general, these minerals function as biocatalysts. Iron is the most prominent example because a deficiency of iron is probably the most common nutritional deficiency on earth (anemia afflicts more than 15 percent of the world's population). Copper and zinc are the prototypical biocatalysts because virtually all of their known functions involve either catalytic or structural roles in many different enzymes. Copper is unique in that all of the known deficiency symptoms in experimental animal models can be explained on the basis of failure of known enzymes. Zinc deficiency, on the other hand, presents symptoms that are not directly attributable to any of the fifty or more enzymes in which it is found. Selenium, manganese, and molybdenum are also constituents of enzymes. Deficiency symptoms for selenium and manganese have been well characterized but a nutritional deficiency of molybdenum has not been satisfactorily demonstrated. The most compelling reason to include molybdenum among the thirteen nutritionally essential elements is because of its presence (and thus function) in several important enzymes. Some microminerals serve a very narrow range of biological functions. Iodine and cobalt are exclusively constituents of thyroid hormones and vitamin B₁₂, respectively. No other role has been identified for these elements.

The remaining mineral elements are those that occur in significant concentrations in the human body and most probably serve an important biological function. However, consistent findings regarding deficiency symptoms and specific biochemical functions have not been reported. Fluorine is a unique example of a mineral that currently has no definitive biological function but because it appears beneficial to dental health, it is a recommended nutrient.

Calcium and Phosphorus

Approximately 99 and 85 percent of the total calcium and phosphorus, respectively, in the human body are found in bone. Both ions leave the bone and are deposited back each day representing normal metabolic activity or "turnover" of bone. The remaining 1 percent of calcium is found in both extracellular and intracellular pools and is absolutely critical for normal body function such as muscle contraction and nerve activity. Although very rare, a sudden drop in extracellular concentrations of calcium (<50 percent) can lead to an emergency situation such as tetany or convulsions. Nerve cells bathed in hypocalcemic fluid spontaneously "fire," leading to uncontrolled nerve activation and muscle spasm. The majority of the extracellular calcium is in chemical equilibrium with bone. Approximately 30 percent is under hormonal control by several hormones, parathyroid hormone, vitamin D, and thyrocalcitonin. As a result, the concentration of extracellular calcium is remarkably constant. Blood levels of phosphorus fluctuate much more and appear to be determined in large part by urinary excretion.

The absorption of calcium from the diet is dependent on a number of dietary and physiological factors. Vitamin D is synthesized in skin when exposed to ultra-violet irradiation [290 to 315 nanometers of ultraviolet (UV) light]. Sunscreen lotions [Sun Protection Factor (SPF) 8] can reduce this synthesis as much as 90 percent. Inadequate sunlight exposure was most likely the cause of calcium deficiency rickets observed at the turn of the century in countries at northern latitudes. A change in dietary calcium absorption in humans appears to take several weeks to accomplish but accounts for the ability of humans to tolerate diets that provide relatively little calcium (200 to 400 mg/day). This activation process becomes less potent with age and may account in part for the increased calcium requirements with age.

Dietary factors affecting the absorption of calcium are well known. They include chelating organic acids such as oxalic and phytic acid. The former is the most potent and is responsible for the markedly diminished "availability" of calcium found in spinach.The amount of calcium contained by a food is only an approximation of the amount of calcium that is ultimately "available." Estimated fractional absorption (percent of intake absorbed into the body) of calcium from these foods ranges from 5 percent for spinach to 61 percent for broccoli. Vegetables of the Brassica family such as broccoli and cabbage appear to contain little oxalate and thus contain calcium that exhibits higher bioavailability than dairy products. Milk and dairy products have relatively high calcium content as well as relatively high fractional absorption (30 percent), resulting in the highest amount of calcium per serving. Lactose in milk enhances the absorption of calcium in infants but its effect in adults is less clear. Other dietary factors affect the retention of dietary calcium but have little impact on its absorption. For example, high intakes of either sodium or protein are thought to result in increased urinary losses of calcium. Protein increases renal calcium loss by increasing acid load while sodium increases losses via shared renal transporters. Both of these conditions may affect calcium balance and ultimately the requirements for this nutrient. The bone loss associated with chronic calcium losses or negative calcium balance may ultimately lead to weakened bones or osteoporosis. Calcium supplements may adversely affect the bioavailability of iron.

Calcium deficiency occurs primarily as rickets or osteomalacia in young children. Bones are deformed (bowed legs) and weak due to inadequate calcification of the protein matrix of bone. This deficiency can arise as a result of too little dietary calcium (relatively rare) or inadequate vitamin D synthesis. Historically, the latter has been the major cause brought about primarily because of reduced exposure to sunlight. It is conceivable, however, that dietary factors such as oxalates and cultural customs (clothing) may interact to play a role in the development of rickets especially since recent cases have been reported in areas of the world near the equator where sunlight should not be limiting. Calcium deficiency does not appear to be a primary cause of osteoporosis. This condition is characterized not by inadequate bone mineralization but by a loss of total bone both protein matrix and mineral. Bones weaken and become susceptible to fracture.

Sodium and Chloride

Total body sodium is approximately one-tenth of that of calcium. One-third of body sodium is found in bone but its metabolic significance is unknown. Sodium and chloride constitute the major cation and anion, respectively, in the extracellular fluid of humans. Sodium is the primary determinant of the osmotic pressure of the extracellular fluid and as such is the main determinant of extracellular fluid volume. The sodium ion concentration changes less than 3 percent day in and day out despite dramatic fluctuations in sodium intake. This is a reflection of a very tightly controlled and highly regulated system to maintain constant osmotic pressure. Through most of human evolution, the availability of dietary salt has been very highly restricted. Much of dietary sodium (and chloride) were derived from sources such as meat and vegetables, which contain very low levels. Consequently, humans and other mammals have evolved physiological mechanisms that permit sodium conservation under extreme conditions. This physiological conservation system comprised of pressure receptors, renal renin, lung angiotensinogen, adrenal aldosterone, and vassopression all makes dietary requirements extremely difficult to assess. For example, the Yanomamo Indians in Northern Brazil have been found to excrete as little as 1 mEq/day of sodium (Na) per day. This reflects a dietary consumption of approximately 60 mg salt per day (over 100 times less than that which is normally consumed in Western populations). At the other extreme are the northern Japanese, who consume nearly 26 grams of salt each day. These regions of Japan have unusually high incidences of cerebral hemorrhage, most likely related to the high incidence of hypertension. Other areas of the world such as Northern Europe and the United States consume approximately 10 g/day or less of salt. The sodium and potassium contents of some selected foods are shown in Figure 1. It is apparent that many "unprocessed" foods contain very little sodium. Estimates of sodium intake suggest that over 85 percent of the sodium consumed in Western diets is sodium added during processing. This is clearly illustrated by the progressively higher sodium content of peas (fresh, frozen, and canned) and perhaps more important, the dramatic reduction in potassium content. The net result is a reversal of the naturally low sodium to potassium ratio found in all fresh plants.

A deficiency of sodium normally does not occur even in areas where salt is scarce. The abnormal loss of sodium and other electrolytes, however, could occur under conditions of extreme sweat loss, chronic diarrhea and vomiting, or renal disease, all of which produce an inability to retain sodium. Acute episodes of diarrhea or vomiting resulting in a loss of 5 percent of body weight could lead to shock. The most important therapy under these circumstances is to restore sodium and water or circulatory volume. Chloride deficiency has been reported in infants consuming low-sodium chloride formulas. They show signs of metabolic alkalosis, dehydration, anorexia, and growth failure. Potassium depletion most notably affects cardiac function where either elevations or reductions in serum potassium can cause arrythmias.

Magnesium

Magnesium is an important intracellular ion involved in many enzymatic reactions of food oxidation and cell constituent synthesis. Approximately 60 percent of total body magnesium is found in bone, where approximately half can be released during bone resorption. Magnesium food sources are widely distributed in plant and animal products with the highest content found in whole grains and green (high chlorophyll) leafy vegetables. Refining wheat with the removal of the germ and outer layers may remove nearly 80 percent of the magnesium from wheat. Meats and most fruits and vegetables are poor sources of magnesium. The absorption of magnesium appears to be unrelated to the absorption of calcium (that is, is independent of vitamin D) and is relatively unaffected by food constituents. Phytate and phosphates, however, may adversely affect magnesium availability by forming insoluble products although their practical significance is unclear. Experimental magnesium deficiency has been produced in humans. Urinary magnesium drops virtually to zero while plasma levels are relatively well preserved. The change in urinary excretion reflects a "urinary threshold" for magnesium. After continued deficiency, however, neuromuscular activity is affected, ultimately leading to tremors and convulsions. Serum and urinary calcium levels are profoundly reduced and not restored by parathyroid hormone administration. It was concluded that magnesium is essential for the mobilization of calcium from bone. A deficiency of magnesium under normal conditions is unlikely but may occur with the presence of other illnesses such as alcoholism or renal disease.

Iron

Over 65 percent of body iron is found in hemoglobin, the respiratory pigment used to transport oxygen within and between tissues. One-third of body iron is a "storage" form that can be mobilized during times of need. The amount of "storage" iron may vary greatly with age and gender. Food sources of iron are complicated by numerous factors that affect the bioavailability of dietary iron. Non-heme sources of iron are found in plant and vegetable products and the absorption from these sources (versus heme found in meat products) is generally lower and influenced to a greater extent by total diet composition. Vitamin C is probably the most signficant enhancer of non-heme iron absorption, while plant phenolics such as tannins found in teas and phytates found in cereals are some of the most potent inhibitors. None of these factors, however, affect the absorption of heme iron found in meats. Iron status can markedly affect the amount of iron absorbed from a meal—low status increases iron absorption. The effect is most pronounced for non-heme iron, changing over fourfold compared to 50 percent for heme iron. Although iron status can influence absorption, the most important determinant of iron availability is the composition of the diet. It is clear that non-heme iron absorption is markedly affected by the characteristics of the food with which it is eaten and that there are clear differences in the nature of absorption of heme and non-heme iron. Iron deficiency is seldom related to iron intake per se. Major causes of anemia (too little hemoglobin) include blood loss and/or diets containing either no enhancers (such as meat or ascorbic acid) or high levels of inhibitors. Infection can also change iron metabolism significantly such that much of the anemia in the world is due to chronic infection. The losses for iron for both men and women are known precisely but the amount of dietary iron requirement depends on the overall diet.

Zinc

Zinc is present in all tissues and performs both structural and catalytic functions in many different enzymes. Unfortunately, changes in the activities of these enzymes are not sufficient to explain the pathological effects of experimental zinc deficiency. Experimental animals refuse to eat experimental diets that are very low in zinc. Human zinc deficiency was demonstrated nearly two decades ago in the United States. Young children from 6 months to 5 years of age showed low amounts of zinc in the hair relative to other groups. Hair zinc and taste acuity were restored after three to five months of zinc supplementation. Earlier studies also revealed zinc deficiency in regions of Iran and Egypt. It is very difficult to assess zinc status in humans. Serum zinc is not adequate to assess nutritional status. In experimental situations, serum zinc falls remarkably (<50 percent) following a low zinc intake without immediate (or apparent) ill effects. In 1974, a Recommended Dietary Allowance (RDA) of 15 mg/day was established for zinc. (It was not until 1974 that we had enough information to estimate an RDA for zinc, at which time the value was established at 15 mg. The RDA presented in 1989 gives 15 mg per day for adults. The 2001 Institute of Medicine value is 11 mg per day.) Approximately 70 percent of zinc consumed by most people is derived from animal products. Cereals contain appreciable zinc but the availability varies considerably. Several plant compounds interfere with the absorption of zinc. The most prominent of these is phytates (inositol hexa-and pentaphosphate). These inhibitors most likely contribute to the natural incidence of dietary zinc deficiency observed in humans.

Copper

Although the importance of copper deficiency in animals has been recognized since the 1930s, it is still not possible to establish an RDA for copper in humans because of the uncertainty regarding the quantitative requirements. There is no doubt that copper is an essential nutrient for humans. Current estimates of the minimum copper requirement are between 0.4 and 0.8 mg/day. Copper is critical for the function of several enzymes, especially blood ceruloplasmin. The activity of this enzyme in blood falls dramatically in experimental animals soon after giving copper-deficient diets and is thought to be a good indicator of copper depletion even in humans. Ceruloplasmin is essential for iron absorption (it catylizes the oxidation of Fe^2× to Fe^3× required for binding of iron to the blood transport protein, transferrin) and explains the anemia observed in copper deficiency. In contrast to zinc, all of the symptoms of a copper defeciency under experimental conditions can be explained by changes in various enzymes that require copper. Two inherited diseases associated with abnormal copper metabolism have been observed—one (Menkes' disease) is associated with copper deficiency, while the other (Wilson's disease) is a disease of excessive copper accumulation. Excessive intake of zinc can precipitate a copper deficiency. An example of zinc-induced copper deficiency has been reported in humans and is attributed to a reduction in the absorption of copper. Excessive zinc may induce intestinal proteins that bind copper and thereby prevent its transfer from the intestine into the body.

Iodine

Approximately 80 percent of total body iodine (20 milligrams) is found in the thyroid gland. All of the iodine that leaves this gland does so as a component of the thyroid hormones—thyroxine and triiodothyronine. In fact, all of the functional significance of iodine is as a component of these hormones. Iodine deficiency represents the most common cause of preventable mental deficits in the world's population. Since most of the world's iodine is found in the oceans, coastal areas are not deficient. However, mountainous areas such as the Himalayas, European Alps, and the mountains of China, as well as the flooded river valleys of Asia, areas where leaching of iodine from soils has occurred for eons, produce iodine-deficient crops and plants. Iodine deficiency during pregnancy causes cretinism, a diet-related birth defect that is characterized by permanent mental retardation and severe growth stunting. In young children and adults, iodine deficiency results in enlarged thyroid glands or goiter. Although various foods such as cassava, cabbage, and turnips contain goitrogens, substances that interfer with iodine metabolism, their practical signficance is not clear. Cassava, the dietary staple in regions of Africa and other areas, may be the exception, especially when not well cooked. The cyanide released by the ingestion of this plant is transformed and ulitmately leads to an inhibition of the uptake of iodine by the thyroid. Goiter was once common in areas of the United States near the Great Lakes and westward to Washington State, but the introduction of iodized salt almost competely eliminated goiter in these areas by the 1950s. The minimum requirement for iodine to prevent goiter is approximately 1 μg/kg/day whereas the recommended intake is nearly twice this amount.

Selenium

Although selenium was first recognized as a toxic trace element for livestock, it is now clear that selenium is an essential nutrient for all animals. During the 1930s, livestock grazing in parts of the Great Plains of North America were found to contract a disease characterized by hair loss, lameness, and death by starvation. The cause of this disease was excess selenium obtained from the plants grown in soils containing high selenium concentration. In fact, selenium, more than any other essential trace element, varies greatly in its concentration in soils throughout the world. Plants accumulate selenium from soils but are not thought to require selenium for growth. Although human toxicity was not observed in affected regions in the United States, endemic selenium poisoning has been observed in high-selenium regions of China where the symptoms included loss of hair and nails. China also possesses regions of very low selenium where, in fact, humans have been diagnosed with selenium deficiency—Keshan disease (cardiomyopathy) and Keshan–Beck disease (degenerative joint disease). Although other factors may be involved, selenium deficiency is clearly a predisposing factor. Selenium functions as part of several important enzymes. The most prominent is a soluble enzyme, glutathione peroxidase, whose function is to reduce hydrogen peroxide and organic (lipid) peroxides, thus preventing the oxidative destruction of cell membranes. Selenium is incorporated into the enzyme as the amino acid selenocysteine by reactions that are unique to selenium. Together with vitamin E, selenium, as a structural component of glutathione peroxidase, forms an antioxidant defense against oxidative stress. The requirement for selenium has been estimated by various methods. On the basis of intakes in regions of China with and without deficiency disease, approximately 20 μg/day is considered an adequate amount to prevent deficiency. The estimated safe and adequate selenium intake suggested by the U.S. National Research Council ranged from 50 to 200 μg/day in 1980. An amount to maintain the highest serum glutathione peroxidase activity appears to be 70 and 55 μg/day for an average man or woman, respectively, which became the Recommended Dietary Allowance (RDA) in 1989. In 1996, the World Health Organization recommended 40 and 30 μg/day for men and women, respectively. Intakes greater that 400 μg/day are considered to be the maximum safe level. Selenium is thus an example of a nutrient that possesses a relatively narrow range of intakes that are safe and that meet requirements.

Manganese

Normal body content of manganese is very low—approximately 15 milligrams or very similar to iodine. In contrast to iodine, manganese deficiency has not been observed in humans but has occurred naturally in chickens and experimentally in many other species. Manganese is required by several enzymes, which may or may not be inolved in the symptoms of a manganese deficiency. Symptoms include impaired growth, skeletal abnormalities, and defects in lipid and carbohydrate metabolism. The role of manganese in the synthesis of the mucopolysaccharide component of bone and cartilage is the most crucial whereas mineralization of bone appears to be independent of manganese. Excessive manganese will interfere with iron absorption. Under conditions of iron deficiency, manganese absorption is increased. Both iron and manganese appear to share a common site for absorption. The recommendations for manganese intake are based on estimates of normal dietary intakes of 2 to 5 mg/day. This amount is thought to be sufficient to replace the 50 percent of body manganese that is lost every 3 to 10 weeks.

Chromium

Chromium is one of the most intriguing and potentially important trace elements because it appears to influence the action of a critical hormone, insulin. Unfortunately, the definitive role of chromium in this regard awaits further study. Decreased sensitivity of peripheral tissues to insulin appears to be the primary biochemical lesion in experimental chromium deficiency. Impaired glucose tolerance has been attributed to chromium deficiency in several experimental models. Also, several patients receiving total parenteral nutrition have responded to chromium supplementation in the predicted manner, that is, improved glucose tolerance. These findings have established chromium as an essential nutrient for humans but the specific deficiency symptoms in those who receive enteral feeding have not emerged. Overt chromium deficiency is very unlikely under normal conditions due to the small amounts of chromium needed. Moreover, a marginal deficiency is very difficult to identify due to the lack of reliable markers for diagnoses concerning chromium. Currently, there is little or no evidence that chromium supplements are either warranted or effective. Even the recommended intakes for adults (50 to 200 μg/day) are uncertain due to the lack of reliable methods for assessment.

Fluoride

Fluoride is not generally considered to be an essential element for humans. It is, however, considered beneficial in that normal intakes appear to reduce the incidence of dental caries. The mechanism of this benefit is thought to be due to incorporation of fluoride into the mineral matrix of tooth enamel, thus producing a more resistant mineral apatite crystal. Over 99 percent of the fluoride found in the body is found in bones and teeth as a component of this mineral apatite crystal. An unusually high intake of fluoride causes permanently discolored or mottled teeth, a condition identified in children drinking water with 2 to 3 parts of fluoride per million. The level of fluoride commonly maintained in municipal water supplies is 1 part per million.

Silicon and Nickel

Silicon is the most abundant mineral in the Earth's crust. It is thus surprising that a need for silicon in biological systems has not been more prominent. Limited research conducted since 1974 has indicated a role for silicon in the development of mature bones in chickens and rats. A human requirement has not been established but estimates in the range of 10 to 20 mg/day have been suggested. Most likely intakes of this magnitude occur under normal conditions. Nickel deficiency has been experimentally produced in several species. Growth depression and changes in iron metabolism have been described. Nickel has been discovered in the enzyme urease from bacteria, fungi, yeasts, algae, plants, and invertebrates. Many other enzymes exist for which nickel is apparently a component. Thus, it is likely that nickel plays an essential functional role in higher organisms, including humans.

Molybdenum

Molybdenum is an essential component of at least three important enzymes found in animals and humans. A deficiency of one of these enzymes, sulfite oxidase, can have severe consequences—seizures and severe mental retardation in infancy. This deficiency has arisen in patients with genetic mutations in cofactor synthesis but not as a primary molybdenum deficiency. The dietary requirements of molybdenum cannot be given, or even approximated, for any animal species including humans. A deficiency of molybdenum has not been observed under natural conditions for any species. Despite this, the biochemical role of molybdenum as a component of several enzymes establishes it as an essential nutrient for humans.

See also Assessment of Nutritional Status; Calcium; Dietary Assessment; Dietary Guidelines; Fluoride; Food, Composition of; Fruit; Iodine; Iron; Malnutrition; Nutrients; Nutrition; Sodium; Trace Elements; Vegetables; Vitamins.

BIBLIOGRAPHY

Brody, Tom. Nutritional Biochemistry. San Diego, Calif.: Academic Press, 1994.

da Silva, J. J. R. Frausto, and R. J. P. Williams. The Biological Chemistry of the Elements. Oxford: Oxford University Press, 1991.

Gillooly, M., T. H. Bothwell, J. D. Torrance, P. MacPhail, D. P. Derman, W. R. Bezwoda, W. Mills, and R. W. Charlton. "The Effects of Organic Acids, Phytates and Polyphenols on the Absorption of Iron from Vegetables." British Journal of Nutrition 49 (1983): 331–342.

Groff, James L., Sareen S. Gropper, and Sara M. Hunt. Advanced Nutrition and Human Metabolism. Minneapolis/St. Paul, Minn.: West, 1995.

Hallberg, L., L. Hulten, and E. Gramatkovski. "Iron Absorption from the Whole Diet in Men: How Effective Is the Regulation of Iron Absorption?" American Journal of Clinical Nutrition 66 (1997): 347–356.

Institute of Medicine. Dietary Reference Intakes. Washington D.C., National Academy Press, 2001.

Layrisse, M., C. Martinez-Torres, J. D. Cook, R. Walker, and C. A. Finch. "Iron Fortification of Food: Its Measurement by the Extrinsic Tag Method." Blood 41 (1973): 333–352.

Linder, Maria C., ed. Nutritional Biochemistry and Metabolism. New York: Elsevier, 1985.

MacGregor, Graham A., and Hugh E. de Wardner. Salt, Diet and Health. Cambridge, U.K.: Cambridge University Press, 1998.

Odell, Boyd L., and R. A. Sunde, eds. Handbook of Nutritionally Essential Mineral Elements. New York: Marcel Dekker, 1997.

Schrauzer, Gerhard N. "The Discovery of the Essential Trace Elements: An Outline of the History of Biological Trace Element Research." In Biochemistry of the Essential Ultra-trace Elements, edited by Earl Frieden, pp. 17–31. New York: Plenum, 1984.

Shils, Maurice E., James A. Olson, Moshe Shike, and A. Catherine Ross, eds. Modern Nutrition in Health and Disease, 9th ed. Baltimore: Williams and Wilkins, 1999.

Stipanuk, M. H., ed. Biochemical and Physiological Aspects of Human Nutrition. Philadelphia: W. B. Saunders, 2000.

Underwood, E. J., ed. Trace Elements in Human and Animal Nutrition, 4th ed. New York: Academic Press, 1977.

Weaver, C. M., and R. P. Heaney. "Calcium." In Modern Nutrition in Health and Disease., 9th ed., edited by M. E. Shils, J. A. Olson, M. Shike, and A. C. Ross, pp. 141–156. Baltimore: Williams and Wilkins. 1999.

Ziegler, Ekhard E., and L. J. Filer, Jr., eds. Present Knowledge in Nutrition, 7th. ed. Washington, D.C.: ILSI, 1996.

Charles Chipley W. McCormick

Calcium and Osteoporosis

The relationship between dietary calcium and osteoporosis has been studied for many years. Early indications suggested that dietary calcium intake was not correlated with bone density (a indicator of bone strength) or the bone loss that naturally occurs with aging. The complexity of the issue is illustrated by observations that many people consume relatively low calcium diets and yet show little evidence of osteoporosis. The genetic contribution to bone density is well established. Studies of identical twins demonstrate that a considerable proportion of the variation in bone density is attributable to inheritance. Mothers with osteoporosis have daughters (thirty years of age) who possess bone density that is significantly less than agematched controls. Dietary intervention with calcium has been attempted in many different studies. Those in the past decade suggest that some changes may be effected by increased calcium intake but they are relatively minor and perhaps short-lived. For example, calcium supplements of 500 mg/day over three years were found to affect bone density of some bones significantly only in older women whose habitual calcium intakes were relatively low (>400 mg/day). Supplements had no effect in older women who had higher habitual calcium intakes. This study seemed to indicate that there might be a subset of elderly women who may benefit from increased calcium intake. Because vitamin D has such a critical role in the absorption of calcium, some workers have examined both vitamin D status and calcium supplementation. Overall, the results not surprisingly support the idea that vitamin D may be a limiting factor in the absorption of dietary calcium. Many other dietary variables may also be important in optimizing the effectiveness of dietary calcium. Dietary acidity, which is promoted by protein intake and ameliorated by the consumption of fruits and vegetables, may contribute. Alkaline diets rich in potassium appear to reduce the loss of body calcium and thus preserve bones. Elevated sodium intake also appears to increase urinary calcium losses. Therefore, the development of osteoporosis is unlikely to be a simple matter of too little dietary calcium consumption, especially in the later years of life, but more of an effect of total dietary conditions superimposed on a particular genetic background.

Sodium and Potassium

In the early 1950s, scientists found that experimental animals could be selected genetically to be susceptible to dietary salt-induced hypertension. Lewis K. Dahl and colleagues established a genetic strain of rat that was sensitive to high dietary salt. These rats showed remarkably elevated blood pressure when dietary salt was increased approximately ten times above normal. The rats' kidneys appeared to have a genetically programmed sensitivity to salt-induced hypertension. However, in the absence of high dietary salt, these animals were normal. Dietary potassium was also recognized as an important factor since high concentrations could ameliorate the effect of sodium chloride. Establishing a direct link between high dietary salt intake and hypertension in humans has been difficult to prove. The problem has been that not all individuals within a population are equally sensitive. Much evidence has come from studies of populations with widely differing salt intake. Populations whose sodium intake is low (less than 100 milligrams of salt) do not appear to develop elevated blood pressure with age. Those whose intake is relatively high do show increased blood pressure with age and evidence of increased incidence of essential hypertension. Recent studies with nonhuman primates have clearly shown that changes in salt intake alone are sufficient to induce changes in blood pressure. Many other studies suggest that lower potassium intake may also be important in the etiology of elevated blood pressure. Certain individuals may be more susceptible or sensitive to sodium-induced changes in blood pressure (similar to experimental animals). All of the known mutations resulting in a phenotype of hypertension involve some aspect of sodium renal excretion and/or retention. It is likely, then, that genetic sodium sensitivity will be a prerequisite to an environmentally induced development of hypertension.

Minerals

Chemical bonding and crystal structure

Chemical bonding

Crystal structure

Mineral groups

Physical traits and mineral identification

Minerals and their uses

Resources

In ordinary usage, minerals are the natural, non-living materials that compose rocks and are mined from Earth. Examples are metals, gemstones, clays, and ores.

The scientific definition of a mineral is more limited. To be considered a mineral, a substance must be solid under ordinary conditions, thus excluding petroleum and water. Minerals must be single, homogeneous (uniform) substances. Therefore quartz is a mineral, but rocks such as granite, which contain quartz mixed together with other minerals, are not considered minerals. Minerals must have definite chemical formulas, allowing only slight variations. Therefore, a sample of a particular mineral will have essentially the same composition no matter where it is from—Earth, the moon, or beyond. Minerals must be of nonbiological, or inorganic, origin, which excludes coal and peat. Finally, the atoms of which minerals are made must be arranged in an orderly pattern; that is, minerals must be made of crystals. To summarize the scientific definition, a mineral is a naturally occurring, inorganic, homogenous solid with a definite range of chemical composition and an ordered atomic arrangement.

With these restrictions, almost 4,000 different minerals are known, with several dozen new minerals identified each year. Every mineral possesses a combination of chemical composition and crystal structure that makes it unique, and by which it is classified (grouped with similar minerals) and identified. These minerals make up the solid Earth, the moon, and even meteorites. However, only 20 or so minerals compose the bulk of Earth’s crust, that is, the part of solid Earth accessible to human beings, extending from the surface downward to a maximum depth of about 55 mi (90 km). These minerals are often called the rock-forming minerals.

Mineralogy is an important discipline for several reasons. The study of the composition of Earth’s crust gives scientists an idea of how the earth was formed. The discovery of new minerals could provide useful materials for industry. The study of the chemical properties of minerals could lead to the discovery of new uses for Earth’s mineral resources. Mining ores for their mineral components provides the materials for lasers, buildings, and jewelry. Each of the branches of mineralogy contributes to the indispensable knowledge base of minerals and their uses.

Chemical bonding and crystal structure

Mineralogists group minerals according to the chemical elements they contain. Elements are substances that cannot be broken down into simpler substances through chemical means. Over 100 of these are known, of which 88 occur naturally. However, most occur only in extremely small traces. Only 10 elements account for nearly 99% of the weight of Earth’s crust. Oxygen is the most plentiful element, accounting for 49.2% of the weight of Earth’s crust. Next is silicon, which accounts for 25.7% of its weight. Table 1 lists the 10 most abundant elements in Earth’s crust. Most minerals are compounds; that is, they contain two or more elements. Only a few minerals, known as native elements, contain atoms of just a single element.

Chemical bonding

In molecules, elements are not merely mixed together, but are joined by chemical bonds. Chemical bonds in minerals are of four types: covalent, ionic, metallic, or Van der Waals, with covalent and ionic bonds most common. Two or more of these bond types can and do coexist in most minerals.

Covalent bonds are very strong bonds formed when atoms share electrons with neighboring atoms. Sulfur, and both of carbon’s natural forms, graphite and diamond, are covalently-bonded minerals. So is quartz, which contains only silicon and oxygen.

Ionic bonds are strong bonds formed when electrons are transferred from one element to another. Since electrons carry a negative electrostatic charge, the element that acquires extra electrons becomes a negatively charged ion, an anion. The element that gave off the electron becomes a positively charged ion, a cation. The attraction between opposite charges binds anions and cations together in ionic compounds. Most metals (the elements iron, nickel, lead, aluminum, etc.) exist in nature as cations, rather than as electrically neutral atoms. Their mineral compounds are, therefore, usually ionic. This is true whether they are joined with one non-metal, as in oxides (oxygen), or with two, as in sulfates (sulfur and oxygen) and carbonates (carbon and oxygen).

Metallic bonds are generally weaker than either covalent or ionic bonds, which explains why metallically bonded minerals (true metals), like silver, gold, and copper, can be worked—for example, hammered into flat sheets or drawn into thin wires. In metallic bonds, electrons move about the crystal constantly flowing between adjacent atoms, redistributing their charge. Because of this flow of electrons, true metals are also good electrical conductors.

Van der Waals bonds are very weak bonds formed by residual charges from the other types of chemical bonds. Graphite is probably the best example of the nature of Van der Waals bonds. The atoms in graphite’s carbon layers are covalently bonded, but a weak residual charge attracts the layers to one another. Van der Waals bonds make graphite a very soft mineral, excellent for use in pencil lead.

Crystal structure

The faces and angles of natural crystals result from the orderly arrangements of the atoms and molecules that make up a crystal. The relation between crystal shape and microscopic structure was suggested in the seventeenth century by Robert Hooke and Christian Huygens. It was confirmed in the twentieth century with the development of x-ray diffraction, a technique that uses x rays to examine the atomic structures of materials.

In modern terms, a solid substance is considered to be crystalline if its atoms or molecules are arranged in an orderly pattern that repeats at regular intervals. Therefore metals are crystalline, although the individual crystals making up a lump of gold are too small to see with the naked eye. By contrast, atoms making up glass do not have any orderly atomic arrangement. Therefore, glass, even it if is carved into the shape of a crystal, is not a crystalline material. Natural glasses such as obsidian (volcanic glass) are not minerals. Non-crystalline solids are called amorphous (without form).

Although there are thousands of different minerals, the shapes of their crystals can be described using just six basic geometric forms. These are called crystal

systems. To determine what crystal system a mineral belongs to, it is necessary to obtain a well-formed specimen, and then observe the number and shape of the faces and the angles at which they meet. This task may be complicated by the fact that each crystal system includes several different forms, and a single crystal may combine several forms in its shape.

For example, consider the isometric system. This is the most symmetrical system, meaning that it has the greatest amount of sameness in its faces and angles. In fact, the basic geometric shape of the isometric system is a cube, having all sides of equal length and with all angles equal to 90^°. Halite crystals, which are cubic, are easily recognized as belonging to the isometric system. However, 15 forms are possible within the isometric system. Isometric mineral crystals include the octahedral (eight-sided) spinels, and the dodecahedral (12-sided) garnets. A single crystal combining several forms can look almost spherical.

Mineral groups

Naming mineral groups

Anions, because of their extra electrons, tend to be much larger than cations. Ionic crystals therefore are built mainly of stacks of anions, with the much smaller cations filling spaces between them. Minerals more closely resemble each other in structure or behavior if minerals with the same anions (rather than cations) are compared. That is why minerals are generally grouped according to their anions, even if the cations may be of greater practical interest. For example, ore minerals are mined for the metals (cations) they contain, which can be changed from ions to neutral atoms of pure metal by a chemical process called smelting. Nevertheless, ore minerals (for example, oxides or sulfides) are grouped according to their non-metal elements.

Silicate minerals and the role of structure

Oxygen and silicon together make up almost three fourths of the mass of Earth’s crust. The silicate minerals, a group containing silicon and oxygen atoms, are the most abundant minerals and are the major component of nearly every kind of rock. Silicate compounds make up over 90% of the weight of Earth’s crust. Most silicate minerals contain other elements in their formulas; therefore, there is a great variety of silicate minerals. In some rocks such as granite, the different silicate minerals can be seen as the small interlocking crystals of various colors. In other rocks, the mineral grains may be too small to distinguish, but they are usually silicates.

Regardless of composition, all silicates have the same basic building unit, the silica tetrahedron. This consists of a silicon atom bound covalently to four oxygen atoms. The oxygen atoms occupy the corners of a geometrical shape called a tetrahedron. The silicon atom is at its center. The entire unit bears a negative electrical charge, enabling it to form compounds with cations.

Silica tetrahedra can join together by sharing oxygen atoms. The simplest result is two tetrahedra joined at one point or six tetrahedra forming a ring. Ribbons or sheets of silica tetrahedra can be millions of units long. If all four oxygen atoms are shared with neighbors, the tetrahedra form rigid networks that extend over the entire crystal. Such large silicates are inorganic polymers, large molecules built up of a great many similar small units. (The only other element known to form polymers is carbon, and carbon-based polymers are the basis of living things.) The arrangements of tetrahedra affect the properties of the silicate minerals. Garnets are very dense and hard, because their tetrahedra stand alone, bound by strong ionic charge to nearby cations. Beryl forms long, six-sided crystals, which may be colored by traces of metal to form precious emeralds and aqua-marines. On the atomic level, beryl contains rings of six tetrahedra, the rings stacked one upon the other with their holes aligned. In muscovite, the tetrahedra are arranged in sheets, with alternating layers of aluminum and potassium atoms between them. The result is flat, flaky crystals, which can easily be separated by hand.

Non-silicate minerals

The so-called non-silicate minerals consist of a variety of different mineral groups each named for a particular anion. Only a few of these minerals contribute much volume to Earth’s crust, but many of them are very important minerals for manufacturing and other industrial uses. Most mineralogists recognize 10 or so major non-silicate groups and a variable number of lesser groups. Table 2 lists several of the major non-silicate groups.

Native elements

A few minerals, called native elements, contain only one element. These include the so-called native metals gold, silver and copper, which occur in lumps, veins, or flakes scattered in rocks. Diamond and graphite are both naturally occurring forms of pure carbon. Sulfur, a yellow non-metal, is sometimes found pure in underground deposits formed by hot springs. Although not common, these minerals are economically important in part because of their rarity.

Physical traits and mineral identification

Despite their great variety and complexity, an unknown mineral sample can often be identified by observing or testing for a few simple physical traits. A mineral’s physical traits are a direct result of its chemical composition and crystal form. Therefore, if enough physical traits are recognized, any mineral can be identified. These traits include hardness, color, streak, luster, breakage or cleavage, specific gravity, and other properties. The results of tests are compared with tables of known minerals until a match is found.

Hardness

A mineral’s hardness is defined as its ability to scratch another mineral. This is usually measured using a comparative scale devised about 200 years ago by Friedrich Mohs. The Mohs scale lists 10 common minerals, assigning to each a hardness rating, from one (talc) to 10 (diamond). A mineral can scratch all those minerals having a lower Mohs hardness number. For example, calcite (hardness three) can scratch gypsum (hardness two) and talc (hardness one), but it cannot scratch fluorite (hardness four).

Color and streak

Although some minerals can be identified by their color, this property can be misleading because mineral color is often affected by traces of impurities. Streak, however, is a very reliable identifying feature. Streak refers to the color of the powder produced when one mineral is scratched by another, harder mineral. Fluorite, for example, comes in a great range of colors, yet its streak is always white.

Luster

Luster refers to a mineral’s appearance when light reflects off its surface. There are various kinds of luster, all having descriptive names. Thus, metals have a metallic luster, quartz has a vitreous, or glassy luster, and chalk has a dull, or Earthy luster.

Cleavage and fracture

Some minerals, when struck with force, will cleanly break parallel to planes of weakness in their atomic structure. This breakage is called cleavage.

KEY TERMS

Compound— A pure substance that consists of two or more elements, in specific proportions, joined by chemical bonds. The properties of the compound may differ greatly from those of the elements it is made from.

Crystal— A solid, homogeneous body composed of a single element or compound having a fixed and regular internal atomic arrangement that may be expressed by external planar faces.

Crystal system— One of six mathematical models used to classify all mineral crystals.

Deposit— An accumulation of minerals or other Earth materials that has economic value.

Earth’s crust— The outermost layer of solid Earth, situated over the mantle and divided into continental and oceanic crust.

Element— A pure substance that can not be changed chemically into a simpler substance.

Mineral— A naturally occurring solid substance of nonbiological origin, having definite chemical composition and crystal structure.

Ore— A mineral compound that is mined for one of the elements it contains, usually a metal element.

Rock— A naturally occurring solid mixture of minerals.

Silicate— A mineral containing the elements silicon and oxygen, and usually other elements as well.

X-ray diffraction— A method using the scattering of x rays by matter to study the structure of crystals.

Muscovite cleaves in one direction only, producing thin flat sheets. Halite cleaves in three directions, all perpendicular to each other, forming cubes. A mineral’s cleavage directions may reveal the crystal system to which it belongs.

However, most minerals fracture rather than cleave. Fracture is breakage that does not follow a flat surface. Some fracture surfaces are rough and uneven. Others show smooth, concentric depressions, called conchoidal fractures. Conchoidal fracture typically occurs in glasses, which are non-crystalline solids. However, it also occurs in many common crystalline minerals, for example garnet and quartz.

Specific gravity

Two minerals can look alike, yet a piece of one may be much heavier than an identical-sized piece of the other. The heavier one has a higher specific gravity. When pure, each mineral has a predictable specific gravity. Therefore, this property is a very reliable information about a mineral’s identity. A mineral’s specific gravity can be thought of as a ratio of its weight to that of an equal volume of water. For example, the specific gravity of gold is 19.3 (19.3 times that of water), while quartz is 2.65.

To determine a mineral’s specific gravity, it is necessary to weigh a sample (using grams), then measure its volume (in cubic centimeters). The weight divided by volume is the density. To calculate specific gravity, divide the mineral’s density by that of water. Because the density of water is one gram per cubic centimeter, specific gravity and density are equal (provided all measurements are made using the metric system.)

Other identifying properties

Some minerals have unusual properties that further aid identification. Fluorescent minerals viewed under ultraviolet light glow with various colors. Phosphorescent minerals glow in the dark after exposure to ordinary light. Triboluminescent minerals give off light when crushed or hit. Several minerals containing iron, nickel, or cobalt are magnetic. Over 100 minerals contain uranium, thorium, or other radioactive elements and are therefore radioactive. These are only a few of the unique properties that can be used to identify minerals. Finally, an experienced mineralogist will take into account the location in which an unknown mineral is found. The nature of the surrounding rocks and the presence of other minerals and elements all provide clues to help in identification.

Minerals and their uses

Minerals are important materials in our technological civilization, and a single mineral may have many unrelated uses.

The mineral corundum provides a good example. Corundum is an extremely hard substance. Small bits of corundum are part of the rock called emery, which has been used since ancient times as an abrasive, to cut and grind metal and stone. Pure corundum is still used for this purpose today. Another property of corundum is that it remains solid and stable at very high temperatures, well past the melting point of iron. Therefore, masses of small corundum crystals pressed together are shaped into alumina firebricks, crucibles, and other apparatus to use in furnaces. Corundum is also the basis of several gemstones.

Pure corundum is colorless. However, as is the case with many other minerals, trace amounts of metal in the stone impart brilliant colors. Rubies are corundum colored red by traces of chromium. Sapphires come in shades of blue, yellow, green and violet; these varieties of corundum contain traces of iron, titanium or other elements.

Finally, corundum is indirectly a major source of aluminum metal. The ore of aluminum, called bauxite, is a mixture of several minerals containing aluminum together with oxygen and hydrogen. The first step in releasing the aluminum from the other elements is to convert the bauxite to corundum.

Table 2 lists a wide variety of familiar materials and the minerals that compose them. These and most other minerals will find even wider usage in the future as research in the field of materials science continues.

See also Element, chemical; Industrial minerals; Mining; Precious metals; Van der Waals forces.

Resources

BOOKS

Blatt, H., R. Tracy, and B. Owens. Petrology: Igneous, Sedimentary, and Metamorphic. New York: Freeman, 2005.

Tarbuck, E.J., F.K. Lutgens, and D. Tasa. Earth: An Introduction to Physical Geology. Upper Saddle River, NJ: Prentice Hall, 2004.

Sara G. B. Fishman

Minerals

In ordinary usage, minerals are the natural, non-living materials that compose rocks and are mined from Earth . Examples are metals, gemstones, clays, and ores.

The scientific definition of a mineral is more limited. To be considered a mineral, a substance must be solid under ordinary conditions, thus excluding petroleum and water . Minerals must be single, homogeneous (uniform) substances. Therefore quartz is a mineral, but rocks such as granite, which contain quartz mixed together with other minerals, are not considered minerals. Minerals must have definite chemical formulas, allowing only slight variations. Therefore, a sample of a particular mineral will have essentially the same composition no matter where it is from—Earth, the moon , or beyond. Minerals must be of nonbiological, or inorganic, origin, which excludes coal and peat. Finally, the atoms of which minerals are made must be arranged in orderly rows and stacks; that is, minerals must be made of crystals. Thus, to summarize the scientific definition, a mineral is a naturally occurring, inorganic, homogenous solid with a definite range of chemical composition and an ordered atomic arrangement.

With these restrictions, almost 4,000 different minerals are known, with several dozen new minerals identified each year. Every mineral possesses a combination of chemical composition and crystal structure that makes it unique, and by which it is classified (grouped with similar minerals) and identified. These minerals make up the solid Earth, the moon, and even meteorites. However, only 20 or so minerals compose the bulk of Earth's crust, that is, the part of the solid Earth accessible to human beings, extending from the surface downward to a maximum depth of about 55 mi (90 km). These minerals are often called the "rock-forming" minerals.

Chemical bonding and crystal structure

Mineralogists group minerals according to the chemical elements they contain. Elements are substances that cannot be broken down into simpler substances through chemical means. Over 100 of these are known, of which 88 occur naturally. However, most occur only in extremely small traces. Only ten elements account for nearly 99% of the weight of Earth's crust. Oxygen is the most plentiful element, accounting for 49.2% of the weight of Earth's crust. Next is silicon, which accounts for 25.7% of its weight. Table 1 lists the 10 most abundant elements in Earth's crust. Most minerals are compounds; that is, they contain two or more elements.

Chemical bonding

In molecules, elements are not merely mixed together, but are joined by chemical bonds. Chemical bonds in minerals are of four types: covalent, ionic, metallic, or Van der Waals, with covalent and ionic bonds most common. Two or more of these bond types can and do coexist in most minerals.

Covalent bonds are very strong bonds formed when atoms share electrons with neighboring atoms. Sulfur , and both of carbon's natural forms, graphite and diamond , are covalently-bonded minerals. So is quartz, which contains only silicon and oxygen.

Ionic bonds are strong bonds formed when electrons are transferred from one element to another. Since electrons carry a negative electrostatic charge, the element that acquires extra electrons becomes a negatively charged ion, an anion . The element that gave off the electron becomes a positively charged ion, a cation . The attraction between opposite charges binds anions and cations together in ionic compounds. Most metals (the

elements iron , nickel, lead , aluminum , etc.) exist in nature as cations, rather than as electrically neutral atoms. Their mineral compounds are, therefore, usually ionic. This is true whether they are joined with one non-metal, as in oxides (oxygen), or with two, as in sulfates (sulfur and oxygen) and carbonates (carbon and oxygen).

Metallic bonds are generally weaker than either covalent or ionic bonds, which explains why metallically bonded minerals (true metals), like silver, gold, and copper , can be worked—beaten into flat sheets, or drawn into thin wires. In metallic bonds, electrons move about the crystal constantly flowing between adjacent atoms, redistributing their charge. Because of this flow of electrons, true metals are also good electrical conductors.

Van der Waals bonds are very weak bonds formed by residual charges from the other types of chemical bonds. Graphite is probably the best example of the nature of Van der Waals bonds. The atoms in graphite's carbon layers are covalently bonded, but a weak residual charge attracts the layers to one another. Van der Waals bonds make graphite a very soft mineral, excellent for use in pencil lead.

Crystal structure

The faces and angles of natural crystals result from the orderly arrangements of the atoms and molecules that make up a crystal. The relation between crystal shape and microscopic structure was suggested in the seventeenth century by Robert Hooke and Christian Huygens. It was confirmed in the twentieth century with the development of x-ray diffraction , a technique that uses x rays to examine the atomic structures of materials.

In modern terms, a solid substance is considered to be crystalline if its atoms or molecules are arranged in an orderly pattern that repeats at regular intervals. Therefore metals are crystalline, although the individual crystals making up a lump of gold are too small to see with the naked eye . By contrast, atoms making up glass do not have any orderly atomic arrangement. Therefore, glass, even it if is carved into the shape of a crystal, is not a crystalline material. Natural glasses such as obsidian (volcanic glass) are not technically minerals. Non-crystalline solids are called amorphous (without form).

Although there are thousands of different minerals, the shapes of their crystals can be described using just six basic geometric forms. These are called crystal systems. To determine what crystal system a mineral belongs to, it is nesessary to obtain a well-formed specimen, then observe the number and shape of the faces and the angles at which they meet. This task may be complicated by the fact that each crystal system includes several different forms, and a single crystal may combine several forms in its shape.

For example, consider the isometric system. This is the most symmetrical system, meaning that it has the greatest amount of "sameness" in its faces and angles. In fact, the basic geometric shape of the isometric system is a cube, having all sides of equal length and with all angles equal to 90°. Halite crystals, which are cubic, are easily recognized as belonging to the isometric system. However, 15 forms are possible within the isometric system. Isometric mineral crystals include the octahedral (eight-sided) spinels, and the dodecahedral (12-sided) garnets. A single crystal combining several forms can look almost spherical.

Mineral groups

Naming mineral groups

Anions, because of their extra electrons, tend to be much larger than cations. Ionic crystals therefore are

built mainly of stacks of anions, with the much smaller cations filling spaces between them. Minerals more closely resemble each other in structure or behavior if minerals with the same anions (rather than cations) are compared. That is why minerals are generally grouped according to their anions, even if the cations may be of greater practical interest. For example, ore minerals are mined for the metals (cations) they contain, which can be changed from ions to neutral atoms of pure metal by a chemical process called smelting. Nevertheless, ore minerals (for example, oxides or sulfides) are grouped according to their non-metal elements.

Silicate minerals and the role of structure

Oxygen and silicon together make up almost three fourths of the mass of Earth's crust. The silicate minerals, a group containing silicon and oxygen atoms, are the most abundant minerals and are the major component of nearly every kind of rock. Silicate compounds make up over 90% of the weight of Earth's crust. Most silicate minerals contain other elements in their formulas; therefore, there is a great variety of silicate minerals. In some rocks such as granite, the different silicate minerals can be seen as the small interlocking crystals of various colors. In other rocks, the mineral grains may be too small to distinguish, but they are usually silicates.

Regardless of composition, all silicates have the same basic building unit, the silica tetrahedron . This consists of a silicon atom bound covalently to four oxygen atoms. The oxygen atoms occupy the corners of a geometrical shape called a tetrahedron. The silicon atom is at its center. The entire unit bears a negative electrical charge, enabling it to form compounds with cations.

Silica tetrahedra can join together by sharing oxygen atoms. The simplest result is two tetrahedra joined at one point or six tetrahedra forming a ring. Ribbons or sheets of silica tetrahedra can be millions of units long. If all four oxygen atoms are shared with neighbors, the tetrahedra form rigid networks that extend over the entire crystal. Such large silicates are inorganic polymers, large molecules built up of a great many similar small units. (The only other element known to form polymers is carbon, and carbon-based polymers are the basis of living things.) The arrangements of tetrahedra affect the properties of the silicate minerals. Garnets are very dense and hard, because their tetrahedra stand alone, bound by strong ionic charge to nearby cations. Beryl forms long, six-sided crystals, which may be colored by traces of metal to form precious emeralds and aquamarines. On the atomic level, beryl contains rings of six tetrahedra, the rings stacked one upon the other with their holes aligned. In muscovite, the tetrahedra are arranged in sheets, with alternating layers of aluminum and potassium atoms between them. The result is flat, flaky crystals, which can easily be separated by hand.

Non-silicate minerals

The so-called non-silicate minerals consist of a variety of different mineral groups each named for a particular anion. Only a few of these minerals contribute much volume to Earth's crust, but many of them are very important minerals for manufacturing and other industrial uses. Most mineralogists recognize ten or so major non-silicate groups and a variable number of lesser groups. Table 2 lists several of the major non-silicate groups.

Native elements

A few minerals, called native elements, contain only one element. These include the so-called native metals, gold, silver and copper, which occur in lumps, veins , or flakes scattered in rocks. Diamond and graphite are both naturally occurring forms of pure carbon. Sulfur, a yellow non-metal, is sometimes found pure in underground deposits formed by hot springs. Although not common, these minerals are economically important.

Physical traits and mineral identification

Despite their great variety and complexity, an unknown mineral sample can often be identified by observing or testing for a few simple physical traits. A miner al's physical traits are a direct result of its chemical composition and crystal form. Therefore, if enough physical traits are recognized, any mineral can be identified. These traits include hardness, color , streak, luster, breakage or cleavage, specific gravity, and other properties. The results of tests are compared with tables of known minerals until a match is found.

Hardness

A mineral's hardness is defined as its ability to scratch another mineral. This is usually measured using a comparative scale devised about 200 years ago by Friedrich Mohs. The Mohs scale lists ten common minerals, assigning to each a hardness from 1 (talc) to 10 (diamond). A mineral can scratch all those minerals having a lower Mohs hardness number. For example, calcite (hardness three) can scratch gypsum (hardness two) and talc (hardness one), but it cannot scratch fluorite (hardness four).

Color and streak

Although some minerals can be identified by their color, this property can be misleading because mineral color is often affected by traces of impurities. Streak, however, is a very reliable identifying feature. Streak refers to the color of the powder produced when one mineral is scratched by another, harder mineral. Fluorite, for example, comes in a great range of colors, yet its streak is always white.

Luster

Luster refers to a mineral's appearance when light reflects off its surface. There are various kinds of luster, all having descriptive names. Thus, metals have a metallic luster, quartz has a vitreous, or glassy luster, and chalk has a dull, or earthy luster.

Cleavage and fracture

Some minerals, when struck with force , will cleanly break parallel to planes of weakness in their atomic structure. This breakage is called cleavage. Muscovite cleaves in one direction only, producing thin flat sheets. Halite cleaves in three directions, all perpendicular to each other, forming cubes. A mineral's cleavage directions may reveal the crystal system to which it belongs.

However, most minerals fracture rather than cleave. Fracture is breakage that does not follow a flat surface. Some fracture surfaces are rough and uneven. Others show smooth, concentric depressions, called conchoidal fractures. Conchoidal fracture typically occurs in glasses, which are non-crystalline solids. However, it also occurs in many common crystalline minerals, for example garnet and quartz.

Specific gravity

Two minerals can look alike, yet a piece of one may be much heavier than an identical-sized piece of the other. The heavier one has a higher specific gravity. When pure, each mineral has a predictable specific gravity. Therefore, this property is a very reliable clue to a mineral's identity. A mineral's specific gravity can be thought of as a ratio of its weight to that of an equal volume of water. For example, the specific gravity of gold is 19.3 (19.3 times that of water), while quartz is 2.65.

To determine a mineral's specific gravity, it is necessary to weigh a sample (using grams), then measure its volume (in cubic centimeters). The weight divided by volume is the density . To calculate specific gravity, divide the mineral's density by that of water. Since the density of water is one gram per cubic centimeter, specific gravity and density are equal (provided all measurements are made using the metric system.)

Other identifying properties

Some minerals have unusual properties that further aid identification. Fluorescent minerals viewed under ultraviolet light glow with various colors. Phosphorescent minerals glow in the dark after exposure to ordinary light. Triboluminescent minerals give off light when crushed or hit. Several minerals containing iron, nickel, or cobalt are magnetic. Over 100 minerals contain uranium , thorium, or other radioactive elements and are therefore radioactive. These are only a few of the unique properties that can be used to identify minerals. Finally, an experienced mineralogist will take into account the location in which an unknown mineral is found. The nature of the surrounding rocks and the presence of other minerals and elements all provide clues to help in identification.

Minerals and their uses

Everything that humankind consumes, uses, or produces has its origin in minerals. Minerals are the building materials of our technological civilization, and a single mineral may have many unrelated uses.

The mineral corundum provides a good example. Corundum is an extremely hard substance. Small bits of corundum are part of the rock called "emery" which has been used since ancient times as an abrasive, to cut and grind metal and stone. Pure corundum is still used for this purpose today. Another property of corundum is that it remains solid and stable at very high temperatures, well past the melting point of iron. Therefore, masses of small corundum crystals pressed together are shaped into "alumina" firebricks, crucibles, and other apparatus to use in furnaces. Corundum is also the basis of several gemstones.

Pure corundum is colorless. However, as is the case with many other minerals, trace amounts of metal in the stone impart brilliant colors. Rubies are corundum colored red by traces of chromium. Sapphires come in shades of blue, yellow, green and violet; these varieties of corundum contain traces of iron, titanium or other elements.

Finally, corundum is indirectly a major source of aluminum metal. The ore of aluminum, called bauxite, is a mixture of several minerals containing aluminum together with oxygen and hydrogen . The first step in releasing the aluminum from the other elements is to convert the bauxite to corundum.

Table 2 lists a wide variety of familiar materials and the minerals that compose them. These and most other minerals will find even wider usage in the future as research in the field of materials science continues.

See also Element, chemical; Industrial minerals; Mining; Precious metals; Van der Waals forces.

Resources

books

Bates, Robert L. Industrial Minerals: How They Are Found and Used. Hillside, NJ: Enslow Publishers, Inc., 1988.

Hazen, Robert M. The New Alchemists: Breaking Through the Barriers of High Pressure. New York: Times Books (division of Random House), 1993.

Hochleitner, Rupert. Minerals: Identifying, Classifying and Collecting Them. 1st English language ed. Translated from German by Kathleen Luft. Hauppage, NY: Barrons Educational Series, Inc., 1994.

Holden, Martin. The Encyclopedia of Gemstones and Minerals. New York: Facts On File, A Friedman Group Book, 1991.

Klein, C. The Manual of Mineral Science. 22nd ed. New York: John Wiley & Sons, Inc., 2002.

periodicals

Ward, Fred. "Rubies and Sapphires." National Geographic 180, no. 4 (October 1991): 100-125.

Sara G. B. Fishman

KEY TERMS

Compound

—A pure substance that consists of two or more elements, in specific proportions, joined by chemical bonds. The properties of the compound may differ greatly from those of the elements it is made from.

Crystal

—A solid, homogeneous body composed of a single element or compound having a fixed and regular internal atomic arrangement that may be expressed by external planar faces.

Crystal system

—One of six mathematical models used to classify all mineral crystals.

Deposit

—An accumulation of minerals or other Earth materials that has economic value.

Earth's crust

—The outermost layer of solid Earth, situated over the mantle and divided into continental and oceanic crust.

Element

—A pure substance that can not be changed chemically into a simpler substance.

Mineral

—A naturally occurring solid substance of nonbiological origin, having definite chemical composition and crystal structure.

Ore

—A mineral compound that is mined for one of the elements it contains, usually a metal element.

Rock

—A naturally occurring solid mixture of minerals.

Silicate

—A mineral containing the elements silicon and oxygen, and usually other elements as well.

X-ray diffraction

—A method using the scattering of x rays by matter to study the structure of crystals.

Minerals

Definition

Description

Trace Minerals

Benefits

Risks

Resources

Definition

Minerals are inorganic elements that originate in the earth and cannot be made in the body. They play important roles in various bodily functions and are necessary to sustain life and maintain optimal health, and thus are essential nutrients. Most of the minerals in the human diet come directly from plants and water , or indirectly from animal foods. However, the mineral content of water and plant foods varies geographically because of variations in the mineral content of soil from region to region.

Description

The amount of minerals present in the body, and their metabolic roles, varies considerably. Minerals provide structure to bones and teeth and participate in energy production, the building of protein , blood formation, and several other metabolic processes. Minerals are categorized into major and trace minerals, depending on the amount needed per day. Major minerals are those that are required in the amounts of 100 mg (milligrams) or more, while trace minerals are required in amounts less than 100 mg per day. The terms major and trace , however, do not reflect the importance of a mineral in maintaining health, as a deficiency of either can be harmful.

Some body processes require several minerals to work together. For example, calcium, magnesium ,

Trace minerals that can be found in commercial preparations of colloidal minerals

Colloidal mineral supplements are usually liquid extracts of minerals mainly derived from humic shale deposits or from aluminosilicate-containing clays. (Illustration by GGS Information Services/Thomson Gale.)

and phosphorus are all important for the formation and maintenance of healthy bones. Some minerals compete with each other for absorption, and they interact with other nutrients as well, which can affect their bioavailability.

Mineral Bioavailability

The degree to which the amount of an ingested nutrient is absorbed and available to the body is called bioavailability. Mineral bioavailability depends on several factors. Higher absorption occurs among individuals who are deficient in a mineral, while some elements in the diet (e.g., oxalic acid or oxalate in spinach) can decrease mineral availability by chemically binding to the mineral. In addition, excess intake of one mineral can influence the absorption and metabolism of other minerals. For example, the presence of a large amount of zinc in the diet decreases the absorption of iron and copper. On the other hand, the presence of vitamins in a meal enhances the absorption of minerals in the meal. For example, vitamin C improves iron absorption, and vitamin D aids in the absorption of calcium, phosphorous, and magnesium.

In general, minerals from animal sources are absorbed better than those from plant sources as minerals are present in forms that are readily absorbed and binders that inhibit absorption, such as phytates, are absent. Vegans (those who restrict their diets to plant foods) need to be aware of the factors affecting mineral bioavailability. Careful meal planning is necessary to include foods rich in minerals and absorption-enhancing factors.

Supplementation

It is generally recommended that people eat a well-balanced diet to meet their mineral requirements, while avoiding deficiencies and chemical excesses or imbalances. However, supplements may be useful to meet dietary requirements for some minerals when dietary patterns fall short of Recommended Daily Allowances(RDAs)or Adequate Intakes(AIs) for normal healthy people.

The Food and Nutrition Board currently recommends that supplements or fortified foods be used to obtain desirable amounts of some nutrients, such as calcium and iron. The recommendations for calcium are higher than the average intake in the United States. Women, who generally consume lower energy diets than men, and individuals who do not consume dairy products can particularly benefit from calcium supplements. Because of the increased need for iron in women of childbearing age, as well as the many negative consequences of iron-deficiency anemia, iron supplementation is recommended for vulnerable groups in the United States, as well as in developing countries.

Mineral supplementation may also be appropriate for people with prolonged illnesses or extensive injuries, for those undergoing surgery, or for those being treated for alcoholism. However, extra caution must be taken to avoid intakes greater than the RDA or AI for specific nutrients because of problems related to nutrient excesses, imbalances, or adverse interactions with medical treatments. Although toxic symptoms or adverse effects from excess supplementation have been reported for various minerals (e.g., calcium, magnesium, iron, zinc, copper, and selenium ) and tolerable

KEY TERMS

Absorption —Uptake by the digestive tract.

Bioavailability —Availability to living organisms, based on chemical form.

Caries —Cavities in the teeth.

Cretinism —Arrested mental and physical development.

Fortified —Altered by addition of vitamins or minerals.

Myoglobin —Oxygen storage protein in muscle.

Neurotransmitter —Molecule released by one nerve cell to stimulate or inhibit another.

Phytate —Plant compound that binds minerals, reducing their ability to be absorbed.

upper limits set, the amounts of nutrients in supplements are not regulated by the Food and Drug Administration (FDA). Therefore, supplement users must be aware of the potential adverse effects and choose supplements with moderate amounts of nutrients.

Major Minerals

The major minerals present in the body include sodium , potassium, chloride, calcium, magnesium, phosphorus, and sulfur.

Functions. The fluid balance in the body, vital for all life processes, is maintained largely by sodium, potassium, and chloride. Fluid balance is regulated by charged sodium and chloride ions in the extracellular fluid (outside the cell) and potassium in the intra-cellular fluid (inside the cell), and by some other electrolytes across cell membranes. Tight control is critical for normal muscle contraction, nerve impulse transmission, heart function, and blood pressure. Sodium plays an important role in the absorption of other nutrients, such as glucose, amino acids, and water. Chloride is a component of hydrochloric acid, an important part of gastric juice (an acidic liquid secreted by glands in the stomach lining) and aids in food digestion. Potassium and sodium act as cofactors for certain enzymes.

Calcium, magnesium, and phosphorus are known for their structural roles, as they are essential for the development and maintenance of bones and teeth. They are also needed for maintaining cell membranes and connective tissue. Several enzymes, hormones, and proteins that regulate energy and fat metabolism require calcium, magnesium and/or phosphorus to become active. Calcium also aids in blood clotting. Sulfur is a key component of various proteins and vitamins and participates in drug-detoxifying pathways in the body.

Disease prevention and treatment. Sodium, chloride, and potassium are linked to high blood pressure (hypertension ) due to their role in the body’s fluid balance. High salt or sodium chloride intake has been linked to cardiovascular disease as well. High potassium intakes, on the other hand, have been associated with a lower risk of stroke, particularly in people with hypertension. Research also suggests a preventive role for magnesium in hypertension and cardiovascular disease, as well as a beneficial effect in the treatment of diabetes, osteoporosis , and migraine headaches.

Osteoporosis is a bone disorder in which bone strength is compromised, leading to an increased risk of fracture. Along with other lifestyle factors, intake of calcium and vitamin D plays an important role in the maintenance of bone health and the prevention and treatment of osteoporosis. Good calcium nutrition, along with low salt and high potassium intake, has been linked to prevention of hypertension and kidney stones.

Deficiency. Dietary deficiency is unlikely for most major minerals, except in starving people or those with protein-energy malnutrition in developing countries, or people on poor diets for an extended period, such as those suffering from alcoholism, anorexia, or bulimia. Most people in the world consume a lot of salt, and it is recommended that they moderate their intake to prevent chronic diseases (high salt intake has been associated with an increased risk of death from stroke and cardiovascular disease). However, certain conditions, such as severe or prolonged vomiting or diarrhea, the use of diuretics , and some forms of kidney disease, lead to an increased loss of minerals, particularly sodium, chloride, potassium, and magnesium. Calcium intakes tend to be lower in women and vegans who do not consume dairy products. Elderly people with suboptimal diets are also at risk of mineral deficiencies because of decreased absorption and increased excretion of minerals in the urine.

Toxicity. Toxicity from excessive dietary intake of major minerals rarely occurs in healthy individuals. Kidneys that are functioning normally can regulate mineral concentrations in the body by excreting the excess amounts in urine. Toxicity symptoms from excess intakes are more likely to appear with acute or chronic kidney failure.

Sodium and chloride toxicity can develop due to low intake or excess loss of water. Accumulation of excess potassium in plasma may result from the use of potassium-sparing diuretics (medications used to treat high blood pressure, which increase urine production, excreting sodium but not potassium), insufficient aldosterone secretion (a hormone that acts on the kidney to decrease sodium secretion and increase potassium secretion), or tissue damage (e.g., from severe burns). Magnesium intake from foods has no adverse effects, but a high intake from supplements when kidney function is limited increases the risk of toxicity. The most serious complication of potassium or magnesium toxicity is cardiac arrest. Adverse effects from excess calcium have been reported only with consumption of large quantities of supplements. Phosphate toxicity can occur due to absorption from phosphate salts taken by mouth or in enemas.

Trace Minerals

Trace minerals are present (and required) in very small amounts in the body. An understanding of the important roles and requirements of trace minerals in the human body is fairly recent, and research is still ongoing. The most important trace minerals are iron, zinc, copper, chromium, fluoride, iodine , selenium, manganese , and molybdenum . Some others, such as arsenic, boron, cobalt, nickel, silicon, and vanadium, are recognized as essential for some animals, while others, such as barium, bromine, cadmium, gold, silver, and aluminum, are found in the body, though little is known about their role in health.

Functions. Trace minerals have specific biological functions. They are essential in the absorption and utilization of many nutrients and aid enzymes and hormones in activities that are vital to life. Iron plays a major role in oxygen transport and storage and is a component of hemoglobin in red blood cells and myoglobin in muscle cells. Cellular energy production requires many trace minerals, including iron, copper, and zinc, which act as enzyme cofactors in the synthesis of many proteins, hormones, neurotransmitters, and genetic material.

Iron and zinc support immune function, while chromium and zinc aid insulin action. Zinc is also essential for many other bodily functions, such as growth, development of sexual organs, and reproduction. Zinc, copper and selenium prevent oxidative damage to cells. Fluoride stabilizes bone mineral and hardens tooth enamel, thus increasing resistance to tooth decay. Iodine is essential for normal thyroid function, which is critical for many aspects of growth and development, particularly brain development. Thus, trace minerals contribute to physical growth and mental development.

Benefits

In addition to clinical deficiency diseases such as anemia and goiter, research indicates that trace minerals play a role in the development, prevention, and treatment of chronic diseases. A marginal status of several trace minerals has been found to be associated with infectious diseases, disorders of the stomach, intestine, bone, heart, and liver, and cancer , although further research is necessary in many cases to understand the effect of supplementation. Iron, zinc, copper, and selenium have been associated with immune response conditions. Copper, chromium and selenium have been linked to the prevention of cardiovascular disease. Excess iron in the body, on the other hand, can increase the risk of cardiovascular disease, liver and colorectal cancer, and neurodegenerative diseases such as Alzheimer’s disease. Chromium supplementation has been found to be beneficial in many studies of impaired glucose tolerance, a metabolic state between normal glucose regulation and diabetes. Fluoride has been known to prevent dental caries and osteoporosis, while potassium iodide supplements taken immediately before or after exposure to radiation can decrease the risk of radiation-induced thyroid cancer.

Risks

With the exception of iron, dietary deficiencies are rare in the United States and other developed nations. However, malnutrition in developing countries increases the risk for trace-mineral deficiencies among children and other vulnerable groups. In overzealous supplement users, interactions among nutrients can inhibit absorption of some minerals leading to deficiencies. Patients on intravenous feedings without mineral supplements are at risk of developing deficiencies as well.

Although severe deficiencies of better-understood trace minerals are easy to recognize, diagnosis is difficult for less-understood minerals and for mild deficiencies. Even mild deficiencies of trace minerals however, can result in poor growth and development in children.

Iron deficiency is the most common nutrient deficiency worldwide, including in the United States. Iron-deficiency anemia affects hundreds of millions of people, with highest prevalence in developing countries. Infants, young children, adolescents, and pregnant and lactating women are especially vulnerable due to their high demand for iron. Menstruating women are also vulnerable due to blood loss. Vegetarians are another vulnerable group, as iron from plant foods is less bioavailable than that from animal sources.

Zinc deficiency, marked by severe growth retardation and arrested sexual development, was first reported in children and adolescent boys in Egypt, Iran, and Turkey. Diets in Middle Eastern countries are typically high in fiber and phytates, which inhibit zinc absorption. Mild zinc deficiency has been found in vulnerable groups in the United States. Copper deficiency is rare, but can be caused by excess zinc from supplementation.

Deficiencies of fluoride, iodine, and selenium mainly occur due to a low mineral content in either the water or soil in some areas of the world. Fluoride deficiency is marked by a high prevalence of dental caries and is common in geographic regions with low water-fluoride concentration, which has led to the fluoridation of water in the United States and many other parts of the world. Goiter and cretinism (a condition in which body growth and mental development are stunted) have been eliminated by iodization of salt in the United States, but still occur in parts of the world where salt manufacture and distribution are not regulated. Selenium deficiency due to low levels of the mineral in soil is found in northeast China, and it has been associated with Keshan disease, a heart disorder prevalent among people of that area.

Toxicity. Trace minerals can be toxic at higher intakes, especially for those minerals whose absorption is not regulated in the body (e.g., selenium and iodine). Thus, it is important not to habitually exceed the recommended intake levels. Although toxicity from dietary sources is unlikely, certain genetic disorders can make people vulnerable to overloads from food or supplements. One such disorder, hereditary hemochromatosis, is characterized by iron deposition in the liver and other tissues due to increased intestinal iron absorption over many years.

Chronic exposure to trace minerals through cooking or storage containers can result in overloads of iron, zinc, and copper. Fluorosis, a discoloration of the teeth, has been reported in regions where the natural content of fluoride in drinking water is high. Inhalation of manganese dust over long periods of time has been found to cause brain damage among miners and steelworkers in many parts of the world.

Resources

BOOKS

Wardlaw, Gordon M. (1999). Perspectives in Nutrition, 4th edition. Boston: WCB McGraw-Hill.

Whitney, Eleanor N., and Rolfes, Sharon R. (1996). Understanding Nutrition, 7th edition. New York: West Publishing.

OTHER

The American Dietetic Association (2002). “Position of The American Dietetic Association: Food Fortification and Dietary Supplements.” Available from <http://www.eatright.com>

The Linus Pauling Institute. “Minerals.” Available from <http://osu.orst.edu/dept/lpi>

United States Department of Agriculture (2002). “Dietary Reference Intakes (DRI) and Recommended Dietary Allowances (RDA).” Available from <http://www.nal.usda.gov/fnic>

Sunitha Jasti