MINERALS/Definition and Classification 501 silicates, etc., rather than as minerals of Zn, Cu, etc At this time, CS Weiss (1780–1856) introduced the seven crystal systems (1815) and Mitscherlich discovered isomorphy (1819) and polymorphy (1824) Gustav Rose (1798–1873) combined chemistry, isomorphy, and morphology to produce a chemical– morphological mineral system, and this was further developed by P von Groth (1843–1927) in his five ă bersicht der Mineralien editions of Tabellarische U nach ihrer Kristallographisch-chemischen Beziehungen JD Dana (1813–95), in his System of Mineralogy (1837), based his classification system primarily on chemistry, and this emphasis has been maintained throughout subsequent editions of the System After 1912, following the discovery of the phenomenon of X-ray diffraction by crystals by M von Laue (1879–1960) and WH Bragg (1862–1942), and the elucidation of the first crystal structure (the mineral halite) in 1914 by WL Bragg (1890–1971), the crystal structures of minerals began to be taken into account in mineral classification schemes The first classification of this type, which took into account the distribution of interatomic bonds in a structure, involved the structures of silicates, determined by Machatschki in 1928 This new field was rapidly expanded by WL Bragg in The Crystalline State (1933) and in the first edition of Atomic Structures of Minerals (1937) This combination of chemistry and structure in mineral classification was subsequently applied to many other categories of minerals, such as fluoraluminates (by Pabst; 1950), aluminosilicates (by Liebau; 1956), silicates and other minerals with tetrahedral complexes (by Zoltai; 1960), phosphates (by Liebau, in 1966, and by Corbridge, in 1971), sulphosalts (by Makovicky; 1981 and 1993), and borates (by Heller, in 1970, and by Strunz, in 1997) The classification of silicates on the basis of polymerization of corner-sharing SiO4 tetrahedra from insular groups to dimers, chains, rings, sheets, and frameworks proved to be a particularly useful scheme but, with the exception of the borates, this concept could not be comprehensively extended to other categories of minerals The polymerization of cation-centred polyhedra by sharing corners, edges, and faces to form various configurations has been applied in the classification of some minerals, such as sulphates (by Sabelli and Trosti-Ferroni; 1985), copper oxysalts (by Hawthorne; 1993), and phosphates (by Hawthorne; 1998), but such schemes have only limited applicability to minerals as a whole Other classification schemes, such as those stressing genetic aspects of mineral formation (by Kostov; 1975) or interatomic bonding (by Godovikov; 1997) have not been generally adopted by the mineralogical community Current Comprehensive Classification Systems Chemical composition and crystal structure are the two properties that define a mineral species, thus it is no surprise that systems based on one or both of these properties are the most widely used Russian mineralogists have been particularly prolific in devising crystallochemical classification schemes, e.g., Betechtin (in 1961), Povarennykh (in 1966), Lazarenko (in 1971), Godovikov (in 1983), Semenov (in 1991), and Bulakh (in 1995) Among other European mineralogists, such classification schemes appear to be less profuse, and the following few examples have been selected to illustrate the diverse approaches to mineral classification A classification based entirely on chemical composition is that of Hey (Table 1), whose Chemical Index Table Principal categories in Hey’s chemical classification system Category 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 Description Elements and Alloys (including the arsenides, antimonides, and bismuthides of Cu, Ag, and Au) Carbides, nitrides, silicides, and phosphides Sulphides, selenides, tellurides, arsenides, and bismuthides (except the arsenides, antimonides, and bismuthides of Cu, Ag, and Au) Oxysulphides Sulphosalts sulpharsenites, sulphantimonites, and sulphobismuthites Sulphosalts sulphostannates, sulphogermanates, sulpharsenates, sulphantimonates, sulphovanadates, and sulphohalides Oxides and hydroxides Halides Borates Borates with other anions Carbonates Carbonates with other anions Nitrates Silicates not containing aluminium Silicates of aluminium Silicates containing aluminium and other metals Silicates containing other anions Niobates and tantalates Phosphates Arsenates Vanadates Phosphates, arsenates, or vanadates with other anions Arsenites Antimonates and antimonites Sulphates Sulphates with halide Sulphites, chromates, molybdates, and tungstates Selenites, selenates, tellurites, and tellurates Iodates Thiocyanates Oxalates, citrates, mellitates, and acetates Hydrocarbons, resins, and other organic compounds