High-temperature crystallography of a protopyroxene

Abstract

The structure of a synthetic protopyroxene, Mg0.7Co0.1Li0.1-Sc0.1SiO3, at high temperatures has been studied by precession photography and four-circle diffractometry using a gas flame heater as heat source. Protopyroxene, isotypic with protoenstatite, was sluggishly transformed at around 1250°C to a twinned enstatite IV with approximately retaining the c axis. Diffraction-intensity data of protopyroxene have been collected at 25°C and 1140°C and a refinement of the crystal structure at each temperature has been carried out. Comparison of the effect of temperature on the structures of protopyroxene and protoenstatite has revealed that the variation modes of M2 – O3 distances and O3 – O3 – O3 angles, which are important indicators for the structural characteristics of pyroxenes before transition, are different between protopyroxene and protoenstatite. The M2 – O3 and O3 – O3 – O3 of protoenstatite remain unchanged with increasing temperature while those of protopyroxene increase; M2 – O3 is 2.384 Å and 2.468 Å at 25°C and 1140°C, respectively and O3 – O3 – O3 is 166.64° and 172.2° at 25°C and 1140°C, respectively. Such a mode of structural variation of protopyroxene is similar to those of clinohypersthene and ferrohypersthene.