Phase relations inferred from field data for mn pyroxenes and pyroxenoids |
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Authors: | Philip E. Brown Eric J. Essene Donald R. Peacor |
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Affiliation: | (1) Department of Geological Sciences, The University of Michigan, 48109 Ann Arbor, MI, USA;(2) Present address: U.S. Geological Survey, 959 National Center, 22092 Reston, VA, USA |
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Abstract: | 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 A compilation of these data along with published and unpublished analyses for phases plotting on the CaSiO3-MgSiO3-MnSiO3 and CaSiO3-FeSiO3-MnSiO3 faces of the RSiO3 tetrahedron has constrained the subsolidus phase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 (Ca0.68Mn0.44Mg0.88Si2O6) and C2/c calciumpoor (Ca0.12Mn1.02Mg0.86Si2O6) 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 Mn0.75–0.95Mg0.0–0.15Ca0.05–0.13SiO3. 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 |
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