Chemical Composition of Minerals, Crystallochemical Constraints and the Nature of Impurities

Minerals are defined as any naturally occurring crystalline substance. While over 1 million compounds have been synthesized in the laboratory, only about 3500 minerals have been recognized. Geochemical factors, e.g. element abundance, solid solution limits, mineral stability, place a limit on the composition and stability of naturally occurring compounds, hence, the relatively small number of minerals. Minerals and synthetic compounds may have identical structures. However, they differ in that minerals are rarely pure substances and typically show wide compositional variations. Minerals range in terms of their composition from pure elements (Fe,Au,Ag) through relatively simple compounds (PbS - galena, KCl - sylvite) to very complex compounds, e.g. steenstrupine - \(Na_{14} Ce_6 Mn^{2 + } Mn^{3 + } Fe_5^{2 + } \left( {Zr,Th} \right)\left( {SO_{18} } \right)_2 \left( {PO_4 } \right)_7 \cdot3H_2 O\). Chemically simple minerals (SiO2) do not necessarily have simple structures, e.g., alpha quartz. Minerals are usually named after scientists (kullerudite, gagarinite, ringwoodite) or the place of first recognition (isokaite, bytownite, atacamite). This practice leads to the introduction of names that provide no information concerning the composition or structure of the mineral.