Phase relations inferred from field data for mn pyroxenes and pyroxenoids

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 A compilation of these data along with published and unpublished analyses for phases plotting on the CaSiO 3 -MgSiO 3 -MnSiO 3 and CaSiO 3 -FeSiO 3 -MnSiO 3 faces of the RSiO 3 tetrahedron has constrained the subsolidus phase relations. For the system CaSiO 3 -FeSiO 3 -MnSiO 3 , 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 CaSiO 3 -MgSiO 3 -MnSiO 3 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 (MnMgSi 2 O 6 ) for amphibolite facies conditions ( T =650° C). At lower temperatures, Balmat kanoites are unstable and exsolve into C2 / c calciumrich (Ca 0.68 Mn 0.44 Mg 0.88 Si 2 O 6 ) and C2 / c calciumpoor (Ca 0.12 Mn 1.02 Mg 0.86 Si 2 O 6 ) phases. At temperatures of 300–400° C the calcium-poor phase subsequently has undergone a transformation to a P 2 1 / 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 0.75–0.95 Mg 0.0–0.15 Ca 0.05–0.13 SiO 3 . 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 (H 2 O)- X (CO 2 ) by the coexistence of talc+quartz and calcite+quartz respectively.