Stability of Mn-cummingtonite - an experimental study

Abstract

Stability of Mn-cummingtonite of nominal bulk composition Mg_4.65Mn_2.35Si₈-O₂₂(OH)₂ has been determined under hydrothermal conditions at 0.5, 1 and 1.5 kb (buffered by Mn₃O₄-MnO buffer) and at elevated pressure-temperature conditions using a piston-cylinder apparatus. The composition was so chosen as to closely correspond to the natural Mn-cummingtonites that occur in various hydrothermal and metamorphosed deposits of manganese in the world. Mn-cummingtonite is stable between 650°C and 675°C at 0.5, 1 and 1.5 kb and between approximately 580°C and 640°C at 3 kb. In this bulk composition, with increasing temperature talc + tephroite assemblage gives way to Mn-cummingtonite, Mn-cummingtonite + Mn-pyroxene + quartz and to Mn-pyroxene + quartz assemblages successively. The important conclusions arising out of this study are as follows. 1) Mn-cummingtonite is stable only upto 3 kb in the geologically feasible temperature range, while it coexists with Mn-pyroxene and quartz upto 5 kb, beyond which no hydrous phase is encountered. 2) The Mn-cummingtonite dehydration curve has a negative slope in P-T space. In natural occurrences, Mn-cummingtonite is associated with Mn-pyroxene, quartz with or without tephroite. The phase relationships brought about in this study can be applicable to natural occurrences.