feldspars The most important group of rock-forming silicate minerals, including the alkali feldspars KAlSi₃O₈ to NaAlSi₃O₈ (K-feldspar to albite) and the plagioclase feldspars NaAlSi₃O₈ to CaAl₂Si₂O₈ ( albite to anorthite). The potassium, sodium, and calcium end-members can be taken to represent the three apices of an equilateral triangle into which all the alkali feldspars and the plagioclase feldspars can be plotted (usually with the plagioclase feldspars along the base of the triangle and the alkali feldspars along the left-hand side).

perthite A series of layers that occurs as intergrowths in alkali feldspars which form at high temperatures and cool slowly, unmixing as they cool. Most alkali feldspars in such slowly cooled rocks (e.g. plutonic and igneous) consist of an alkali feldspar host with an exsolved, sodium-rich, plagioclase feldspar phase that segregated from the alkali feldspar during cooling. Perthitic intergrowths have textures ranging from macroperthite (visible to the naked eye) through microperthite (visible under a microscope) to cryptoperthite (which can be detected only by X-ray diffraction and other techniques). The nature of the perthite depends upon the ordering of the silicon and aluminium atoms achieved by the mineral lattice as it cools.

microcline See ALKALI FELDSPAR.