French Mineralogist René Just Haüy Founds the Science of Crystallography with the Publication of Treatise of Mineralogy

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French Mineralogist René Just Haüy Founds the Science of Crystallography with the Publication of Treatise of Mineralogy

Overview

In 1801 René Haüy, a French mineralogist, described one of the first coherent theories of crystal structure, published as the Treatise of Mineralogy. From this start grew the science of crystallography, the study of crystals and their structure, growth, and form. Since that time, the science of crystallography has matured and developed new tools, including x-ray diffraction, to study the crystals from quartz to DNA. Haüy's description included an explanation for many phenomena that had been commented on, but never explained, and allowed crystallography to take its place as an undisputed scientific field rather than a simple description of shapes and forms.

Background

Although depictions of minerals appear in paintings and other artwork dating back as far as several thousand years, the first work on mineralogy was not written until the third century BC. For two thousand years, mineralogy remained more descriptive than scientific as it remained unable to predict any properties or to do more than describe the properties of a mineral. In 1669 Nicolaus Steno (1638-1686) noted that the angles between adjacent faces in quartz crystals were always the same, regardless of the crystal's size. Then, in 1783, Rome de l'Isle measured interfacial angles on a number of crystals, confirming and expanding Steno's work. Soon after, Haüy developed a mathematical theory of crystal structure that turned out to be remarkably accurate and gave crystallography a legitimate place among the sciences.

According to the most popular story, Haüy dropped a calcite crystal and, when it broke, noticed that each small piece had faces similar to those of the larger crystal. Further attempts to cleave the smaller crystals along the same lines as the larger one proved that the interfacial angles (the angles between each set of faces) remained the same no matter how small the crystals were made. Haüy deduced that this was because a "unit cell" (which Haüy called an integral molecule) existed that was the same shape as any of the visible crystals and that the crystals were all made up of these invisible unit cells that stacked together to form the larger crystals. He further suggested that the unit cells were made of small cubes that, by stacking in specific arrangements, could be made to form any of the crystal shapes seen in nature. By changing the manner in which these unit cells were stacked, all the different faces and interfacial angles of the crystals then known (and all known today) could be explained.

While, like the story of Isaac Newton (1642-1727) and the apple, this incident may or may not have actually happened, there is no doubt that Haüy's theory of crystallography gave the field the ability to explain many observations and to make predictions that were later shown to be correct. Haüy's later work dealt with further explanation of his findings and developing a mathematical framework for his theories. He also helped develop a set of conventions for describing crystal structures that allowed scientists from anywhere in the world to understand the work performed by others. This helped to put crystallography on a more formal footing and facilitated scientific collaborations.

Although the existence of atoms was still not fully accepted at this time, Haüy's theory demanded some sort of internal structure to crystals that made possible the outward forms we see. We now know that atoms are present in a precise pattern that corresponds to the outward shape of the crystal. Even on the atomic level, then, Haüy's theory remains accurate.

Work by other mineralogists followed quickly. William Wollaston (1766-1828) developed a reflecting goniometer in 1809 that made possible very rapid and precise measurements of a crystal's interfacial angles, giving accurate and much needed information on crystal structure. Measurements made with Wollaston's device and the chemical theory developed by Jöns Jakob Berzelius (1779-1848), a Swedish chemist, were the last ingredients needed for crystallography to become a full-fledged field of scientific inquiry.

Impact

Perhaps the simplest way to state the impact of Haüy's discoveries is to point out that no fewer than 20 Nobel prizes have been awarded for work based in whole or in part on principles stated by him or made possible by his work. These include the discovery of x-ray diffraction, the first description of the structure of DNA, discovery of the fullerene form of carbon, understanding of the structure and function of many biologically important compounds (including enzymes, vitamins, and many proteins), and much more.

It is important to note that, at first, Haüy's work was simply a mathematical description of what the insides of crystals might look like. The theory gave results that seemed to match what scientists saw in nature, but it was not necessarily correct. It was not until the advent of x-ray diffraction and full acceptance of the atomic theory of matter in the early twentieth century that scientists agreed that Haüy's theory of crystal structure actually provided a very good match to reality.

Another advancement was Haüy's description of the different crystal systems, or basic shapes that crystals can take. These systems are monoclinic, triclinic, orthorhombic, isometric (of which cubic symmetry is one possibility), hexagonal, and tetragonal. For example, the hexagonal system of crystals, to which corundum and quartz belong, is characterized by six faces surrounding a central axis, while the isometric system consists of cubical unit cells. Halite (salt) is an example of the isometric system and each salt crystal is either a cube or is comprised of several joined cubes.

At the time Haüy announced his discoveries they had very little immediate practical impact, and virtually no impact for anyone other than crystallographers. Scientists were delighted to have a new explanatory theory to test and use, but there was little impact on everyday life. As time went on, the interfacial angles of most crystals were measured and described, the mathematics of crystallography was refined, and the field languished for a time. Other discoveries of crystal properties were used in other branches of geology, but the very small-scale study of crystals awaited the discovery of x-ray diffraction. Once this was discovered in 1912 by Max von Laue (1879-1960), crystallography took off, becoming ever more powerful as a tool to describe the forms taken by natural substances and to predict the properties of materials based on the orderly arrangement of atoms within a substance. Even substances that are normally liquid at room temperature can be studied by freezing them or otherwise inducing them to form crystals.

In fact, x-ray diffraction is still an important tool in studying crystals, to the point that linear accelerators have been constructed for the sole purpose of generating intense x-rays for more detailed studies of crystals and crystalline solids. These studies are expected to yield valuable scientific and commercial knowledge in a number of areas.

Of the discoveries dependent on the science of crystallography, perhaps the most important was the discovery of the structure of DNA. DNA was first synthesized and crystallized in the lab by Arthur Kornberg (1918- ) in the 1950s. A few years later, the study of crystallized DNA through x-ray diffraction helped James Watson (1928- ) and Francis Crick (1916- ) deduce the structure and many of the functions of DNA, winning them the Nobel Prize and making possible all of the recent advances in genetic science that have since taken place. Similar studies have helped scientists to determine the structure of many biologically important substances, including vitamin B₁₂, certain enzymes and amino acids, some proteins, and similar compounds. In addition, crystallizing and studying some viruses and viral proteins has given researchers more information about how viruses can make people ill. This information, in turn, helps to develop more effective vaccines or treatments. Likewise, the study of some crystallized toxic agents can also lead to similar benefits.

Outside of biology, the study of crystals has proven fruitful as well. Liquid crystals are important in the displays of digital watches, calculators, and many computer monitors. Most minerals and synthetic compounds are subject to x-ray diffraction to determine their crystal structure, which can suggest possible new uses for materials as well as identifying possible weaknesses. X-ray diffraction is routinely used to identify minerals that would otherwise be impossible to identify, especially microscopic minerals such as those in soil. In fact, soil mineralogy, a field of increasing importance, depends very heavily on the study of crystals so small they can only be seen with an electron microscope. However, even at this size, x-ray diffraction reveals the interfacial angles, the spacing of planes of atoms within the crystal, and other important information. This information, in turn, can be used to determine the soil's suitability for farming, supporting buildings, or other uses.

In addition, the crystal structures of some minerals are being used to make them more useful to people. Some clays, for example, can be treated so that they will absorb oils and other organic substances, helping to decontaminate parts of the environment. Other crystalline substances have been investigated to try to better understand their electrical or structural properties, while x-ray diffraction studies have been instrumental in designing new generations of drugs and understanding the functioning of many biological compounds. In short, the field of crystallography continues to provide valuable insights into the functioning of our world and should do so for many years to come.

P. ANDREW KARAM