New dense phases of geophysical significance

Abstract

THE major increases in density occurring at depths of 400–1,000 km in the Earth's mantle require the formation of phases in which silicon is six-coordinate. Such coordination has been demonstrated in stishovite (SiO₂ of rutile type¹), in the hollandite form of feldspar compositions², in the garnet form of (Mg,Fe)SiO₃³, in garnet-type MnSiO₃⁴, in perovskite-type CaSiO₃³ and in SiP₂O₇⁵—but examples are still few. For the two compositional types believed to provide the bulk of the mantle, ASiO₃ and A₂SiO₄ where A is Mg or Fe, high pressure phase transformations of numerous model compounds such as MnGeO₃, CaGeO₃⁶, Mn₂GeO₄, and Ca₂GeO₄⁷ have demonstrated the possibility of high pressure ilmenite, perovskite, strontium plumbate and potassium nickel fluoride forms, respectively, as ultimate high pressure silicate phases; and disproportionation into simple oxides has also been discovered^8,9. In the absence of directly observed silicate transformations it is important to delineate the systematic crystal chemistry of each of these structural classes, particularly for those members containing ions closest in size to Fe, Mg and Si, and to obtain further examples of six-coordinate silicon in a range of compounds.