Four to sixfold coordination changes in crystalline silicates at high pressure

MgSiO₃ exists in a variety of polymorphs which can be largely divided into two subclasses: (a) low pressure modifications in which Si is coordinated to four oxygens and (b) high pressure modifications in which Si is coordinated to six oxygens. The coordination change usually requires an accompanying high temperature. We present a four to six coordination change in MgSiO₃ orthoenstatite at high pressure and ambient temperature in an argon pressure medium, using Raman spectroscopic measurements in a diamond cell. CO₂ laser-heating experiments indicate that the four to sixfold coordination change is facilitated by defects in the structure which may serve as nucleation sites for the formation of the octahedrally coordinated silicate phase.