Stability of diopside-jadeite solid solution

Paragenetic relations of natural jadeite-diopside solid solution is well explained by a phase diagram proposed by Carpenter (1980). However, his interpretation of free energy-composition relation to give rise the phase diagram is hardly accepted because it requires positive excess energy of mixing in solution with ordered phase. An alternative model giving the same phase diagram is presented. It assumes strong interaction between next neighbours with resultant stabilization of quartet of cations, which is analogous to the stabilization of NaCaMgAl quartet of omphacite. The model favors rather simple ideal solution to approximate the results of synthetic experiments over complex non-ideal model of Holland (1983). An isopleth of univariant composition in P-T is proposed.

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