Thermochemistry of jadeite—diopside pyroxenes

The enthalpies of solution of seven synthetic clinopyroxenes on the join CaMg₂Si₂O₆ (diopside)-NaAlSi₂O₆ (jadeite), of two natural low-Fe ordered omphacites near the 1:1 composition, and of a nearly pure natural jadeite, were measured in molten Pb₂B₂O₅ at 970 K. Enthalpies of solution of the natural omphacites experimentally disordered at 1350°C and 30 kbar were also measured.The synthetic clinopyroxenes have positive excess enthalpies of mixing, which can be expressed by a symmetrical function ΔH_mix = W_HX_JdX_Di, with W_H = 7250 ±950 calories. The enthalpy of disordering of the two natural omphacites averages 1.8 kcal, which is nearly the same as the excess enthalpy of mixing of a 1:1 disordered pyroxene.The interaction parameter, W_H, can be shown to be essentially equivalent to ΔG° of the reciprocal reaction: CaMgSi₂O₆ + NaAlSi₂O₆ = CaAlSi₂O⁺₆ + NaMgSi₂O^?₆ M-site cation distribution data of natural omphacites heat-treated at 1000°C (Aldridgeet al., 1978) lead to ΔG° = 7200 cal for the above reaction, in good agreement with the calorimetric W_H. The reciprocal solution theory with ΔG° = 7200 cal predicts closely the activities of NaAlSi₂OP₆ in jadeite-diopsides found from phase equilibrium measurements at 600°C (Holland, 1979a) and is nearly equivalent to an entropically ideal two site mixing model with a (fictive) W_H of 5800 cal.Jadeite-diopside solid solutions near the 1:1 composition at temperatures of 1000–1500 K are ‘pseudoideal’; that is, they have nearly the free energies of ideal one-site mixtures (Ganguly, 1973). If the order-disorder transition is nearly first-order at about 1000 K, as suggested by Fleetet al. (1978), the pseudo-ideality holds also for ordered omphacites at least somewhat below 1000 K.