Unit cell dimensions and infra-red absorption spectra of MgSiO₃ polymorphs

Abstract

Three MgSiO₃ polymorphs were synthesized and their unit cell dimensions and infra-red absorption spectra were determined. Protoenstatite with 2 mol % Fe₂O₃ which does not transform to clinoenstatite by quenching or grinding was sucessfully synthesized.
Unit cell dimensions of three MgSiO₃ polymorphs synthesized at normal pressure are close to those synthesized under high pressure. The clinoenstatite synthesized in this study is confirmed to be the high pressure and low temperature form. The densities are found to increase in the order of proto, ortho and clinoenstatite. It is also shown that distinction between three polymorphs can be easily made by X-ray powder diffraction patterns.
Absorption spectra of these polymorphs are reasonably explained by Lazarev's empirical rule. In protoenstatite, two peaks due to the stretching of bridging Si-O are observed in 550-750 cm^-1 region, whereas in ortho and clinoenstatite, four peaks due to the stretching of bridging Si-O in non crystallographic equivalent (SiO₃) chain are observed.