Complete isomorphic join diopside–kosmochlor CaMgSi2O6–NaCrSi2O6 in metamorphic rocks of the Sludyanka complex (southern Baikal region)

Chromium and vanadium-bearing metamorphic derivatives of siliceous-carbonate sediments in the Sludyanka granulite-facies complex, southern Baikal, contain high-Cr clinopyroxenes that belong to the diopside–kosmochlor–natalyite join (CaMgSi2O6–NaCrSi2O6–NaVSi2O6). The ternary join includes isomorphic binary joins of which the diopside–kosmochlor join with 94% kosmochlor (Kos) end-member has been studied in detail and found to be virtually complete. As indicated by signature of diopside–eskolaite reactions in the minerals, kosmochlor and high-Cr members of the join formed at the expense of metamorphic eskolaite as a result of Na input after progressive metamorphism. There is no miscibility gap along the diopside–kosmochlor join; the incompleteness of the kosmochlor formation reaction and the coexistence of its compositionally different members are rather due to kinetic factors associated with low mobility of Cr. Thus, the mechanism of Cr incorporation in clinopyroxenes does not depend on PT-conditions of metamorphism.