- © 2000 Mineralogical Society of America
The Earth’s average mantle composition is presumed to lie between an Si:O atom ratio of 1:3 and 1:4 (e.g. Ita and Stixrude 1992). The orthosilicate group, which comprises minerals that contain isolated SiO4 tetrahedra, has thus been the subject of considerable structural investigation at elevated temperature or pressure. The group includes olivines, silicate spinels, garnets, the aluminosilicates, zircon and a few minor mineral groups such as humites and datolites (Deer et al. 1997). In addition, the silicate spinelloids are typically included here, but are not strictly orthosilicates as they contain Si2O7 dimers. Titanite (CaTiSiO5), though technically an orthosilicate, has a framework structure and is considered in a chapter by Ross (this volume), which also examines aspects of the garnet framework structure not reviewed here. Interestingly, no natural members of the group contain major amounts of monovalent (alkali) cations, although LiScSiO4 has been synthesized with the olivine structure.
The most geologically significant members of the group are those minerals of formula X2SiO4, where X is a divalent cation, typically Mg, Fe, Mn, Ca, and (more rarely) Co and Ni. These groups include the olivines, the silicate spinels and wadsleyite-type spinelloids, plus some minor phases such as phenakite (Be2SiO4), willemite (Zn2SiO4), and cadmium and chromous orthosilicates. The structures of most of the major members of the group have been studied at elevated temperatures or at elevated pressures, but there have not yet been many studies of structures at simultaneously elevated temperature and pressure.
Other members of the orthosilicate group that are of major geological and geophysical significance are the Al2SiO5 aluminosilicate polymorphs (sillimanite, andalusite and kyanite), zircon (ZrSiO4), and the garnets, which are abundant in high-pressure metamorphic rocks. The structures of these …