Hydrothermal crystallization of MnSiO₃ polymorphs

Abstract

Two polymorphs of MnSiO₃, pyroxmangite and rhodonite, were examined with the conventional hydrothermal technique. Pyroxmangite was synthesized in the temperature lower than about 700°C and rhodonite in the higher temperature. Synthetic and natural pyroxmangites transform into rhodonite at about 700°C as temperature rises. However, the reverse reaction from rhodonite to pyroxmangite were tried but no transformation was recognized by the X-ray diffraction technique.
The crystallization processes of MnSiO₃ were investigated using three different starting materials under a water pressure of 2kb. In the crystallization of pyroxmangite, at 650°C, a clinopyroxene-like phase appeared as an intermediate phase, changes to pyroxmangite which further transforms into rhodonite at 750°C.
Structural changes in the transformation of pyroxmangite to rhodonite with rising temperature or increasing duration time may be regarded as a decrease of the number of silica tetrahedra in a repeat unit of a single chain. There seems to be the same relationship among clinopyroxene and pyroxenoids, when clinopyroxene is regarded as a kind of pyroxenoid having a side-step after an infinite repetition of two silica tetrahedra unit along the single chain.