Abstract
1. Introduction
2. Experimental methods and starting materials
3. Characterization of phases encountered
4. Previous experimental work
5. Present experimental results
6. Petrological application
Acknowledgements
References

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 ℃), 6 kbar (betwee 475゜and 525℃), 20 kbar (between 500゜and 900℃), 25 kbar (between 800 ゜and 900℃) and 30 kbar (between 900゜and 1,000℃), 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℃.

X-ray data for synthetic pyroxmangite are presented. The unit-cell parameters　(ao =6.717(2)Å, bo = 7.603(1)Å, co = 17.448(5)Å, α= 113゜50'(1'), β=82゜21'(2'), γ=94゜43'(1'); space group P1（1の頭に-）) give a unit-cell volume (807.5(0.3)ų) which, in accordance with other recent least squares lattice refinements of hydrothermally synthesized material, is slightly smaller than that obtained by single-crystal work on anhydrously synthesized material.

Application of the present results to natural rocks is severely restricted due to the great variety and extent of cationic substitutions observed in natural pyroxenoids. The univariant polymorphic transformation determined for the MnSiO3 composition is thus replaced in natural systems by a divariant field in which pyroxmangite and rhodonite of differing composition will stably coexist.

Fig. 1. Experimental results of this study. Size of symbol for our runs reflects experimental uncertainties in temperature and pressure. For equations of curves (1), (2), and (3) see text. Because the position of the solidus in the hydrous system is not known, it is possible that the high-pressure part of the transformation curve defined by the data of Akimoto and Syono (1972) may be metastable in the presence of water Maresch & Mottana(1976)による『The pyroxmangite-rhodonite transformation for the MnSiO3 composition』から