In-situ Fe-Mg order-disorder studies and thermodynamic properties of orthopyroxene (Mg,Fe)₂Si₂O₆

A systematic in-situ high-temperature study of Fe²⁺ -Mg distribution over the two octahedral sites, M1 and M2, in synthetic orthopyroxenes (En₇₅Fs₂₅, En₆₁Fs₃₉, En₄₉Fs₅₁, En₂₅Fs₇₅, and En₁₇Fs₈₃) was undertaken at 1000, 1100, 1200, and 1300 K by the single-crystal X-ray diffraction method. The reversal measurements of cation ordering states were made on samples En₇₅Fs₂₅, En₆₁Fs₃₉, En₄₉Fs₅₁, and En₁₇Fs₈₃. Because ofthe orthopyroxeneclinopyroxene phase transition observed at ~1255 K for En₁₇Fs₈₃, no site-occupancy determinations at 1300 K were made for En₂₅Fs₇₅ and En₁₇Fs₈₃. The site occupancies in the Mg-rich sample (Fs₂₅En₇₅) at 1300 K are anomalous because of the existence of a transitional structural state prior to a phase transition from the orthopyroxene to the protopyroxene phase. The asymmetry in the distribution of Fe and Mg over M1 and M2 sites is confirmed in the range 1000–1200 K, although it is much less pronounced than that determined from quenched natural samples at lower temperatures (873–1073 K). An analysis of the site-occupancy data based on the regular solution model of Saxena and Ghose (1971) yielded: ΔG_exch = 7751 + 3.000T (±246) J/mol, W^M1 = 10230 − 2.065T (±164) J/mol, and, W^M2 = 14775 − 7.575T (±624) J/mol, where T is in kelvins. All three parameters are temperature-dependent, with ΔG_exch increasing and W^M1 and W^M2 decreasing with increasing temperature. The W^M1 – W^M2 term increases with increasing temperature, in accord with the result of Shi et al. (1992) but in contrast to most previous studies. This result suggests that the atomic configurations around the cations in M1 and M2 sites become more dissimilar, and the asymmetry of the orthopyroxene solid solution increases at elevated temperatures (1000–1200 K). The sublattice (Shi et al., 1992) and the regular solution models (Saxena and Ghose, l97l), respectively, were used to derive the microscopic and macroscopic excess thermodynamic parameters (ΔG^ex, ΔH^ex, and ΔS^ex) and the activity-composition relations. The macroscopic excess parameters show positive deviations from ideal mixing. The ΔG^ex and ΔH^ex values agree reasonably well with those determined experimentally.