Newly identified end-member kosmochlor from the Yamanobo outcrop of the Renge metamorphic belt, Itoigawa, central Japan

抄録

Kosmochlor-bearing eckermannite rock is identified in the Yamanobo outcrop of Itoigawa City, Niigata Prefecture, Japan, which is emplaced within a serpentinite body of the Renge metamorphic belt. The eckermannite rock is likely a large enclave within the serpentinite body, with an albitite zone developed between the eckermannite rock and serpentinite matrix. Kosmochlor is lens-shaped or schlieren-like domains within the eckermannite matrix, and comprises spherical aggregates of radiated kosmochloric pyroxene. Spherule cores are occasionally occupied by a corroded form of chromite. The chemical compositions of the kosmochlor exhibit a solid solution between kosmochlor (Ko₉₈Jd₀Ae₁Q₁) and jadeite (Ko₂₅Jd₆₁Ae₆Q₈). The maximum kosmochlor component is 98 mol.%, likely representing the closest known composition to the NaCrSi₂O₆ end-member, worldwide. Sodium amphiboles coexisting with kosmochloric pyroxene, and especially the cores of relatively large amphibole grains, in particular, contain high Cr₂O₃ (up to 12.2 wt.%, 1.34 apfu) and exhibit significant C-site deficiency (i.e., C-site cation < 5).