Thermochemistry of (Fe 2+, Mg)SiO 3 orthopyroxene

Abstract

Enthalpies of solution in eutectic (Li, Na) 2B 2O 4 melts at 1023 K were measured for five synthetic orthopyroxenes on the join MgSiO 3-FeSiO 3. The pyroxenes were synthesized at 1120°C and 20 kbar and thus were presumed to be highly disordered. The measurements indicate a small positive enthalpy of mixing, with W H = 950 cal/ MSiO 3. Enthalpy of solution measurements were made on a natural, well-ordered orthopyroxene near the composition En 52.5Fs 47.5 and on this material after heat-treatment at 1150°C and 20 kbar. Irreversible expansion of the unit-cell constants of the natural pyroxene after heat-treatment at various temperatures was used to characterize the degree of M-site disorder. The observed enthalpy of solution decrement of 0.85 kcal/MSiO 3 between the natural En 52.5 and the same material heated at 1150° corresponds to about half of the maximum possible disordering, or ΔX Fe M1 ≅ 0.25, which leads to a ΔH of 7.5 kcal/M 2Si 2O 6, for the exchange reaction: Fe( M2) + Mg( Ml) = Fe( Ml) + Mg( M2) if M-site interaction energy terms are ignored. This Δ H is larger than inferred from any of the analyses of site-occupancy data except that of Besancon (1981), who gave a very similar value. The measured Δ H of disorder and the W H of mixing together indicate a large Δ H as great as 3.2 kcal for the reciprocal reaction: Fe 2 Si 2 O 6 + Mg 2 Si 2 O 6 = Fe( M2) Mg( M1) Si 2 O 6 + Fe( M1) Mg( M2) Si 2 O 6 as anticipated by Sack (1980). As a consequence of the inferred magnitudes of ΔHof the exchange and reciprocal reactions, departures from ideality of Gibbs energy of mixing of orthopyroxene are very small at 700°–1000°C. Activities of MgSiO 3 and FeSiO 3 may be replaced by their mol fractions at all temperatures in most petrologic calculations.