Aluminous orthopyroxene: Order-disorder, thermodynamic properties, and petrologic implications

Orthopyroxene has two tetrahedral sites, designated A and B, and two octahedral sites, M1 and M2. Crystallographic studies of synthetic and natural orthopyroxenes (opx) suggest that the tetrahedral Al is ordered nearly completely in the B site, but the octahedral Al disorders between M1 and M2 sites with a preference for M1. If the 'aluminum avoidance' principle is obeyed, then the tetrahedral Si-Al ordering limits the Al substitution in opx to 25 mol%, thus leading to an end-member stoichiometry of Mg₃Al₂Si₃O₁₂ instead of MgAl₂SiO₆. The enthalpy of formation of these two components has been deduced from the available phase equilibrium data. The thermodynamic properties of the opx solid solution approximates ideal solution behavior more closely when treated in terms of the components Mg₄Si₄O₁₂(QEn)-Mg₃Al₂Si₃O₁₂(Py) than when expressed in terms of the components Mg₂Si₂O₆-MgAl₂SiO₆. A model has been developed for the octahedral disordering of Al as function of temperature and composition. These data enable calculation of the configurational entropy and molar entropy of Al-opx; distinction has been made between the cases of completely random mixing of Al and Si in the tetrahedral B site, and of random mixing without violation of the 'aluminum avoidance' principle. The second model yields entropy of the Mg₃Al₂Si₃O₁₂ end member which agrees almost exactly with the value derived from phase equilibrium data. The partial molal entropies of the Orthopyroxene components 'QEn' and 'Py' can be derived from these data; their implications with respect to the P-T slopes of Al₂O₃ isopleths for the equilibrium of Orthopyroxene with forsterite and spinel/garnet have been discussed.