A brief summary is presented of measurements of heats of solution of minerals belonging to the forsterite-fayalite and to the enstatite-orthoferrosilite series and of artificial ilmenite and geikielite. In the olivine and orthopyroxene series, the heat of solution in HF is found to be a linear function of the Mg: Fe ratio, indicating perfect isomorphism between the corresponding end-members. The importance and possibilities of applying calorimetry to the study of isomorphism are emphasized. On the basis of the calori-metric data available for the minerals in question, the heat and free energy are given for the reaction: olivine + fquartz → 2 pyroxene. The difference of the stabilities of MgSiO₃ and FeSiO₃ is illustrated. The stability reaction of ilmenite and geikielite in the presence of olivine or orthopyroxene is calculated from the calorimetric data. The influence of the entropy of mixing isomorphic minerals upon the stability relation is emphasized.
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Sahama et al. (1949) studied this question.