A SYSTEMATIC-APPROACH TO GENERAL AND STRUCTURE-TYPE FORMULAS FOR MINERALS AND OTHER INORGANIC PHASES

During the preparation of the Mineral Group List Index of the Mineral Powder Diffraction File, it was necessary to develop a consistent sche me for the representation of the structural formulas. Because the four teen letters B, C, F, H, I, K, N, O, P, S, U, V, W, and Y of the Latin (Roman) alphabet represent chemical elements (element symbols), this system uses twelve letters to represent cations, anions, and molecules occupying various sires of the structure (site symbols). The letters available as structure-site symbols represent cations with decreasing coordination numbers (CN) as follows: D, CN greater than or equal to 9 ; E, CN = 8 or 7; G, CN = 6; J, CN = 5; Q, CN = 4 planar or 2 linear; T, CN = 4 tetrahedral, and R, CN = 3 planar. These definitions leave s ufficient remaining letters for other structure-site symbols such as: A for all cations without regard to the coordination number; H,for 1on e-electron-pair cations: M for neutral molecular units; X for monatomi c anions; and Z for polyatomic anions. Structure sites of the same coo rdination type, yet distinct enough for ordered occupancy of resident cations, may be differentiated by primes (','',''', etc.). Variable co ordination numbers on a structure site and variable site occupancies a re indicated by two symbols or subscripts with the intervening symbol tt. For example, the general structure-type formula of amphibole may b e written as A(7<->8)[T4X11](2)X-2'. A specific structure-type formula also allows for a mixture of symbols to represent chemical elements a nd structure sites such as A(0<->1)(E'<->G)(2)G(3)'G(2) ''[Si4O11](2)( OH)(2) whereas an example of a chemical formula is Ca2Mg5[Si4O11](2)(O H)(2) for the amphibole tremolite.