Phlogopite: High temperature solution calorimetry, thermodynamic properties, Al-Si and stacking disorder, and phase equilibria

Methods have been developed for solution calorimetry of hydrous phases in molten lead borate near 700°C. These involve thermochemical cycles using dissolution and decomposition reactions of hydrous silicates and hydroxides. Preliminary results suggest that H₂O derived from the decomposition of hydroxides dissolves in molten 2PbO-B₂O₃ with an exothermic enthalpy of solution of -5.7 ±0.7 kcal mol^-1. Hydroxyphologopite persists metastably at 714°C and its heat of solution in 2PbO·B₂O₃ has been measured. From these new data, the standard enthalpy of formation of phlogopite from the elements at 25°C is -1485.5 ±1.5 kcal mol^-1. The standard free energy of formation is -1394.6 ±1.5 kcal mol^-1, assuming complete tetrahedral Al-Si disorder. Two structural features complicate the thermodynamics of synthetic and natural micas. The first is a varying degree of tetrahedral Al-Si disorder. Raman spectroscopic study of phlogopite synthesized above 600°C suggests a disordered Al-Si distribution. Calculations of the P-T locus of the geologically important equilibrium: Phl + 3Qtz = 3En + Sa + H₂O, using our thermochemical data, agree within experimental error with the results of calculations based on the best available phase equilibrium data only if a tetrahedrally disordered phlogopite is assumed. Such calculations are very sensitive to uncertainties in ΔH° and ΔG°, and reversed phase equilibrium experiments remain essential to obtaining reliable estimates of thermodynamic properties. In contrast to these Al-Si disordered phlogopites, some biotites of low temperature parageneses (<600°C) may have substantial Al-Si order. A variable Al-Si distribution has a substantial effect on the configurational entropy and therefore on the free energy of the mica in question. Because of these and other questions, applications of biotite equilibria to determining volatile fugacities in igneous and metamorphic petrogenesis are subject to large uncertainties. The second structural complication is stacking disorder, which is present in phlogopite synthesized at 650°C but not in the 850°C sample. The enthalpy difference between these two samples, determined by solution calorimetry, is smaller than the experimental uncertainty of ±1.0 kcal mol^-1. Thus there appears to be little driving force for ordering, and micas with disordered stacking sequences may persist in many geologic environments. The effect of stacking disorder on thermodynamic properties is probably very small.