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 Mg,Si substitution into eastonite than by Al,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.