The Al2O3 contents of enstatite in equilibrium with garnet in the system MgO-Al2O3-SiO2 at 15–40 kbar and 900°–1,600° C

Forty-six reversed determinations of the Al₂O₃content of enstatite in equilibrium with garnet were made in the P/T range 15–40 kbar/900–1,600° C in the MgO-Al₂O₃-SiO₂ system. Starting materials were mixtures of synthetic pyrope+Al-free enstatite and pyrope+enstatite (5–12% Al₂O₃). Al₂O₃ contents in reversal run pairs closely approached common values from both the high- and low-Al sides. Most experiments were done in a piston-cylinder device using a NaCl medium; some runs at very high temperatures were made in pyrex/NaCl or pyrex/talc assemblies. The measured enstatite compositions, expressed as mole fractions of Mg₂(MgAl)(AlSi₃)O₁₂(X _Opy ^En ) were fitted by a Monte-Carlo method to the equilibrium condition: $$\begin{gathered} \Delta H_{970}^0 - 970\Delta S_{970}^0 \hfill \\ + \mathop \smallint \limits_1^P \Delta V_{970}^0 dP - \mathop \smallint \limits_{970}^T \Delta S_T^0 dT + RT\ln X_{Opy}^{En} = 0 \hfill \\ \end{gathered}$$ where the best fit parameters of ΔH, ΔS and ΔV (1 bar, 970 K) for the reaction pyrope=opy are 2,040 cal/mol, 2.12 eu and 9.55 cc/mol. In addition to the determination of Al₂O₃ contents of enstatite, the univariant reaction pyrope+forsterite=enstatite+spinel was reversibly located in the range 1,100–1,400°C. A “best-fit” line passes through 22, 22.5 and 25 kbar at 1,040, 1,255 and 1,415°C, respectively. Our results for the univariant reaction are in agreement with previous studies of MacGregor (1974) and Haselton (1979). However, comparison of the experimentally determined curve with thermochemical calculations suggests that there may be a small error in the tabulated ΔH _f(970,1) ⁰ value for enstatite. A value of?8.32 rather than?8.81 kcal/mole (Charlu et al. 1975) is consistent with the present data. Application of garnet-enstatite-spinel-forsterite equilibria to natural materials is fraught with difficulties. The effects of nonternary components are poorly understood, and the low solubilities of Al₂O₃ in enstatite under most geologically reasonable conditions make barometric or thermometric calculations highly sensitive. More detailed studies, including reversed determinations in low-friction assemblies, are sorely needed before the effects of important diluents such as Fe, Ca and Cr can be fully understood.