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

Abstract

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.

On the Chemical Composition and Possible Origin of Na–Cr-Rich Clinopyroxene in Silicocarbonatites from Samalpatti, Tamil Nadu, South India

“…However, no eskolaite was found in the samples investigated here, which could be the result of its instability in the observed paragenesis owing to Na input via metasomatism (extensive fenitization is known from the Samalpatti complex; e.g., [21]) and resulting formation of Di-Kos solid solution as proposed by Vasil'ev et al [32], or formation of Cr-rich pyroxene by another process. Eskolaite could also be the source for formation of magnesiochromite as proposed by Reznitsky et al [2] via the reaction (2):…”

“…The other two end members are jadeite (NaAlSi 2 O 6 ) + aegirine (NaFeSi 2 O 6 ) [1] and diopside (MgCaSi 2 O 6 ), and kosmochlor forms a solid solution with each of these. Reznitsky et al [2] found…”

“…Chromium-rich pyroxenes were rarely also described from compositionally exotic mantle-derived magmatic rocks like kimberlites [15] and silicocarbonatites [16]. Laboratory experiments to synthesize kosmochlor and other Cr-rich pyroxenes produced a broad range of compositions (see [2] for more details) which is similar to that observed in nature, but there is still a lot of controversy between laboratory results and natural samples. A wide compositional range and specific conditions of occurrence make Na-Cr-rich pyroxene a possibly good candidate for further petrogenetic studies although this theoretical expectation requires thorough experimental work.…”

“…Furthermore, kosmochlor may be an important component in pyroxene grains of cometary dust e.g., [7][8][9]. It was also found in a limited number of terrestrial environments, mainly in high-grade metamorphic rocks such as jadeitites and associated rocks from Myanmar, Guatemala,, peridotite xenoliths [13] and granulites [2,14]. Chromium-rich pyroxenes were rarely also described from compositionally exotic mantle-derived magmatic rocks like kimberlites [15] and silicocarbonatites [16].…”

Partitioning of chromium between garnet and clinopyroxene: first-principle modelling versus metamorphic assemblages

“…Trace and minor elements are also used to investigate diffusivity in minerals and associated timescales (John et al, 2012;Ague and Axler, 2016), constrain pressure-temperature-time paths, and study metasomatism (e.g. Hickmott and Shimizu, 1990;Rubatto and Hermann, 2003;Angiboust et al, 2014;S. Figowy et al: Partitioning of chromium between garnet and clinopyroxene Kohn, 2014).…”