POSSIBLE EXISTENCE OF A COTUNITE-LIKE PHASE OF SIO2 IN THE LOWER MANTLE

Abstract

The structure of the lower mantle (at depths greater than 1000 km) is still undetermined. The question arises whether the cotunite-like phase is the only stable phase in the lower mantle. It is quite possible, but additional research is needed in order to give a clear answer. Synthesis of a phase of SiO2 resembling Fe2N was recently reported, and this phase may be capable of existing in the lower mantle. But the occurrence of this phase does not fit well into the general succession of phases with increasing pressure in oxides of the type AX2 that has been established experimentally and by crystallochemical arguments. Our preliminary data indicate that such a phase could be an intermediate mestastable product in the transition of stishovite to SiO2 of α-PbO2 type or in the transition of the latter into SiO2 of CaF2 type.