THERMODYNAMIC ANALYSIS OF QUADRILATERAL PYROXENES - PART I: DERIVATION OF THE TERNARY NON-CONVERGENT SITE-DISORDER MODEL

Abstract

A thermodynamic model for quadrilateral pyroxene solutions, is developed from the following assumptions: 1) Non-convergent cation-disorder of Fe and Mg occurs over the Ml and M2 sites; 2) Ca is partitioned completely into the M2 site; and 3) Clinopyroxenes unmix to a high-Ca plus low-Ca phase. This ternary model is an extension of Thompson's (1969, 1970) binary model. Model expressions are presented that describe cation distributions, exchange equilibria and chemical potentials of both ternary and quadrilateral components.