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...
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