Enthalpies of solution of synthetic magnesium aluminum phlogopite samples K(Mg3−XAlX)(Al1+XSi3−X)O10(OH)2 with 0.00 ≤ XEast − 0.92 were measured in molten 2PbO-B2O3 at 977 K. The enthalpy of mixing for the phlogopite-eastonite join is large, asymmetric, and endothermic. The solid solution is destabilized more by [6]Mg,[4]Si substitution into eastonite than by [6]Al,[4]Al substitution into phlogopite, reflecting the increased strain in the mica structure at high [6,4]Al content that arises from increased lateral misfit between the smaller octahedral and larger tetrahedral sheets. The entropy of mixing has been calculated assuming a random distribution of Mg and Al on one octahedral site and a short-range ordered distribution of Al and Si on four tetrahedral sites. The calculated activity-composition relations predict a large region of immiscibility up to ~ 1500 K that is consistent with observed limits of [6,4]Al substitution in naturally occurring Fe-poor phlogopite.

The thermodynamic data for phlogopite and eastonite are assessed using the metastable reaction Phl + 3 Qtz = San + 3 En + H2O, in which the breakdown of magnesium aluminum phlogopite also produces spinel. A standard enthalpy of formation of phlogopite from the elements at 298 K of −6215.0 ± 3.5 kJ/mol is consistent with both data from high-temperature solution calorimetry and results from phase equilibria experiments. The standard enthalpy of formation of eastonite from the elements at 298 K is between −6352 and −6363 kJ/mol; the large uncertainty arises from estimating the heat capacity of eastonite. The P, T stability of phlogopite increases with increasing [6,4]Al content. The equilibrium breakdown of a magnesium aluminum phlogopite with XEast = 0.20 is shifted up in temperature by 185 K at 5.0 kbar from that of the phlogopite end-member.

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