Reexamination of the pressure-composition diagram for the system NaCrSi₂O₆-NaAlSi₂O₆

Abstract

Co-existence of kosmochlor (NaCrSi₂O₆) solid solution and jadeite (NaAlSi₂O₆) solid solution in green Burmese jades has been interpreted in terms of 3d-O1 repulsion effect between the nonbonding 3d electrons of the Cr³⁺ ion and the O1 oxygens in the pyroxene structure. This interpretation predicts that kosmochlor and jadeite do not make a complete solid solution at any pressure conditions. The phase diagram proposed by Abs-Wurmbach and Neuhaus (1976) is revised by the reexamination of their experimental data.