A thermochemical study of the distribution of cobalt and nickel between diopsidic pyroxene and melt

Glasses and crystals of compositions MgCaSi₂O₆, CoCaSi₂O₆ and NiCaSi₂O₆ were studied by high temperature solution calorimetry, x-ray diffraction, and pycnometry. These data were combined with trace element distribution data to obtain values for ΔAG⁰, ΔH⁰, ΔS⁰, and ΔV⁰ for M-Mg exchange reactions. Enthalpies of vitrification increase in the order Co, Mg, Ni as do volumes of vitrification and apparent melting points. The mixing of Co and Mg, Ni and Mg, and Co and Ni is almost ideal in crystals and glasses, with small negative heats of mixing and small volume changes. The exchange reactions MgCaSi₂O₆ (xl) + MCaSi₂O₆ (liq) = MgCaSi₂O₆ (liq) + MCaSi₂O₆ (xl) are exothermic for Ni and endothermic for Co. The volumes and entropies of the exchange reaction are small.