Abstract
The polymorphic transformation between synthetic pyroxmangite and rhodonite of MnSiO3 composition has been reversibly bracketed in the presence of water at 3 kbar (between 425 ° and 450 ° C), 6 kbar (between 475 ° and 525 ° C), 20 kbar (between 500 ° and 900 ° C), 25 kbar (between 800 ° and 900 ° C) and 30 kbar (between 900 ° and 1,000 ° C), using standard cold-seal pressure vessels and piston cylinder apparatus. Oxygen fugacities buffered by the bomb walls and piston-cylinder cell assemblies sufficed to keep manganese in the divalent state. Pyroxmangite of MnSiO3 composition is shown to be the high-pressure, low-temperature polymorph with respect to rhodonite of the same composition. It is a stable phase at atmospheric pressure below 350–405 ° C.
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