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The hydrothermal synthesis at 500 to 750°C and 2 kb and the dry synthesis in air at 1.200°C were made on the join MnSiO_3 -MgSiO_3. Synthesized phases are rhodonite, pyroxmangite, clinopyroxene, and t...alc+olivine. The transformation boundary between clinopyroxene and pyroxmangite was carefully examined on the temperature versus composition diagram. The obtained diagram shows that the maximum value of the magnesium contents among the pyroxmangites which crystallized under a settled water pressure is effective as a indicator of the minimum formation temperature. For instance, a calcium-free pyroxmangite from a bedded manganese ore deposit in the Ryoke metamorphic zone contains about 12 mole % MnSiO_3, and its formation temperature is estimated to be about 550°C if the pyroxmangite crystallized under water pressure of 2 kb. With increasing of magnesium content, the d-spacings of powder pattern of rhodonite synthesized by the sintering method decrease, but its infrared absorption spectra become similar to those of bustamite. The reason for this contradiction should be explained in future. It is noted that the transformation of clinopyroxene to pyroxmangite takes place when temperature rises. In pyroxenoids, there is a tendency that the elevating temperature brings about shortening of the repeat unit of Si-0 chain. If we regard pyroxene as pyroxenoid having a large number of Si-O tetrahedra in the repeat unit of single chain, this tendency in pyroxenoids may be applied to the transformation phenomenon from clinopyroxene to pyroxmangite.show more
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