Applications to Silicate, Carbonate, and Borate Minerals and Related Species

The most abundant materials making up the crust of the earth (i.e., the “rock-forming minerals”) can be regarded as dominated by oxyanion units; notably, the units that can be formally represented by SiO44- and AlO45- clusters of the silicate minerals, and the CO32- unit of the carbonates. less common, but geochemically interesting, oxyanion units include, for example, BO33-, BeO46- , and PO43-. in this chapter, applications of quantum-mechanical calculations and experimental techniques to such materials are considered. first, the silicates are discussed, commencing with the large amount of work undertaken on the olivines, before considering such work as has so far been done on the other silicate minerals and related materials. second, the most important of the nonsilicate rock-forming mineral groups, the carbonates, are discussed. finally, although of less petrological importance but interesting geochemically and in terms of contrast with the othergroups, the borates and related species are considered. in each case, geometric aspects of structure and the problems of calculating structural properties are considered before going on to consider electronic structures and the factors controlling stabilities and a wider range of physical properties. in all of these materials, there is considerable interest in the, bonding in the oxyanion unit and how this is affected by, and controls, the interaction with counterions or the polymeric units. the building up of the minerals by such interactions exerts the dominant control over their crystal chemistries and properties and thus forms a central theme of this chapter. the silicate minerals are, of course, characterized by the presence of the tetrahedral siO4 cluster unit and the crystal chemistry and classification of silicates dominated by the structures built up by the linking together (polymerization) of these units. in the “simplest” of the silicates, the island silicates such as the olivine minerals (dominated by the forsterite (Mg2 SiO4)-fayalite (Fe2SiO4) solid solution series), the sio4 units are isolated by counterions such as Mg2+, Fe2+, Ca2+.