A silica-rich sodium pyroxene phase with six-coordinated silicon

Abstract

Phase transitions in minerals and melts that involve a change in the coordination of silicon from fourfold to sixfold are generally accompanied by large changes in material properties such as density, bulk modulus and elastic moduli. When such transformations occur within the Earth, these large changes in material properties can result in seismic discontinuities. The presence of six-coordinated silicon in minerals such as majorite garnet at upper-mantle pressures is therefore of crucial importance to an understanding of the properties of this region of the Earth. The silicate pyroxenes are also believed to comprise a significant portion of the upper mantle¹, and to provide a host for large cations such as sodium and calcium. It has long been assumed that silicon is restricted to the tetrahedral sites within the pyroxene structure. Here, however, we report the high-pressure synthesis and characterization of single crystals of a pyroxene of nominal stoichiometry Na(Mg_0.5Si_0.5)Si₂O₆, which is the first pyroxene known to contain both tetrahedrally and octahedrally coordinated silicon.