498 MINERALS/Definition and Classification MINERALS Contents Definition and Classification Amphiboles Arsenates Borates Carbonates Chromates Feldspars Feldspathoids Glauconites Micas Molybdates Native Elements Nitrates Olivines Other Silicates Pyroxenes Quartz Sulphates Sulphides Tungstates Vanadates Zeolites Zircons Definition and Classification materials, human-influenced substances, and some biological materials that not satisfy all the criteria for the definition of a mineral species E H Nickel, CSIRO Exploration and Mining, Wembley, WA, Australia Definition of a Mineral Species ß 2005, Elsevier Ltd All Rights Reserved Introduction Minerals, the individual components comprising rocks, are generally defined in terms of chemical composition and crystal structure, and most classification systems are based on these properties Minerals are formed by geological processes and include both terrestrial and extraterrestrial materials The total number of minerals generally accepted as valid by the mineralogical community is about 4000 Mineraloids are mineral-like substances such as synthetic Minerals are substances formed by geological processes that occur on Earth or in extraterrestrial bodies A mineral is defined on the basis of its chemical composition and crystal structure, and to qualify as a mineral species, a substance must have a unique combination of these properties Some minerals have a well-defined composition (quartz, for example, is SiO2), but many have a variable composition whereby some chemical constituents are replaced by others For example, the mineral olivine can be regarded as a solid-solution series between forsterite (Mg2SiO4) and fayalite (Fe2SiO4) in which Mg2ỵ and Fe2ỵ substitute for each other in the crystal structure In such