Abstract
The enstatite-diopside solvus presents certain interesting thermodynamic and crystal-structural problems. The solvus may be considered as parts of two solvi one with the ortho-structure and the other with clino-structure. By assuming the standard free energy change for the two reactions (MgMgSi2O6)opx ⇋ (MgMgSi2O6)cpx and (CaMgSi2O6) opx ⇋ (CaMgSi2O6) cpx as 500 and 1 000 to 3 000 cal/mol respectively, it is possible to calculate the regular solution parameter W for orthopyroxene and clinopyroxene. These W's essentially refer to mixing on M2 sites. The expression for the equilibrium constant by assuming ideal mixing for Fe-Mg, Fe-Ca and non-ideal mixing for Ca-Mg on binary M1 and ternary M2 sites is given by
where X's are site occupancies, R is 1.987 and T is temperature in oK. Temperature of pyroxene crystallization may be estimated by substituting for T in the above equation until the equation −RT In K a=500 is satisfied. The shortcomings of this method are the incomplete standard free energy data on the end member components and the absence of site occupancy data in pyroxenes at high temperatures. The assumed free energy data do, however, show the possible extent of inaccuracy in temperature estimates resulting from the neglect of Mg-Ca non ideality.
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Saxena, S.K., Nehru, C.E. Enstatite-diopside solvus and geothermometry. Contr. Mineral. and Petrol. 49, 259–267 (1975). https://doi.org/10.1007/BF00376591
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DOI: https://doi.org/10.1007/BF00376591