Phase relations inferred from field data for mn pyroxenes and pyroxenoids

Electron microprobe analysis of manganese silicates from Balmat, N.Y., has helped elucidate phase relations for Mn-bearing pyroxenes and pyroxenoids. A compilation of these data along with published and unpublished analyses for phases plotting on the CaSiO₃-MgSiO₃-MnSiO₃ and CaSiO₃-FeSiO₃-MnSiO₃ faces of the RSiO₃ tetrahedron has constrained the subsolidus phase relations. For the system CaSiO₃-FeSiO₃-MnSiO₃, the compositional gaps between bustamite/hedenbergite, bustamite/ rhodonite and rhodonite/pyroxmangite are constrained for middle-upper amphibolite facies conditions and extensive solid solutions limit possible three phase fields. For the CaSiO₃-MgSiO₃-MnSiO₃ system much less data are available but it is clear that the solid solutions are much more limited for the pyroxenoid structures and a continuum of compositions is inferred for clinopyroxenes from diopside to kanoite (MnMgSi₂O₆) for amphibolite facies conditions (T=650° C). At lower temperatures, Balmat kanoites are unstable and exsolve into C2/c calciumrich (Ca_0.68Mn_0.44Mg_0.88Si₂O₆) and C2/c calciumpoor (Ca_0.12Mn_1.02Mg_0.86Si₂O₆) phases. At temperatures of 300–400° C the calcium-poor phase subsequently has undergone a transformation to a P2₁/c structure; this exsolution-inversion relationship is analogous to that relating augites and pigeonites in the traditional pyroxene quadrilateral. Rhodonite coexisting with Mn-clinopyroxenes is compositionally restricted to Mn_0.75–0.95Mg_0.0–0.15Ca_0.05–0.13SiO₃. For the original pyroxene+rhodonite assemblage, the Mg and Ca contents of the rhodonite are fixed for a specific P (6kbars)-T (650° C)-X(H₂O)-X(CO₂) by the coexistence of talc+quartz and calcite+quartz respectively.Contribution No. 363, from the Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan, Ann Arbor MI 48109, USA