Isomorphous substitution
This process, which is known as isomorphous substitution, results in the minerals concerned having a variable chemical composition, and the divisions between mineral species in such an isomorphous series become purely arbitrary. Usually only the most important members of such a series have separate names. The important rock-forming mineral olivine (Plate 17), for instance, forms an isomorphous series whose end members are the magnesium-rich forsterite and the iron-rich fayalite. The soda-lime feldspar series, which is collectively known as plagioclase, forms one third of the earth’s solid crust. This series has as its two end members the sodium-feldspar albite and the calcium-feldspar anorthite (Plate 3). When a mineral species contains varieties which have a different colour or crystal shape from the normal, they are usually given separate names. Amethyst (Plate 2), for instance, which has a purple hue, due to the presence of minute quantities of iron or manganese, is a variety of the mineral quartz; and emerald (Plate 25), whose green colour is due to traces of chromium, is one of the three main colour varieties of the mineral beryl. The colour varieties of the minerals classed as precious stones have been given an unnecessarily large number of separate names, which has led to much confusion. Some substances form several varieties of identical composition, but with slightly different crystalline structure, the variety formed depending on the physical conditions at the time of crystallisation. Potassium feldspar, for instance, crystallises as microcline in many deep-seated igneous rocks, as amazonite in pegmatites, as orthoclase in the cavities (druses) within granites, as sanidine in lavas and as adularia (Plate 76) in certain mineral veins.