Thermochemistry of the tremolite-edenite amphiboles using fluorine analogues, and applications to amphibole-plagioclase-quartz equilibria

Heats of mixing of synthetic C2/m fluortremolite-fluoredenite amphiboles measured at 985 K show a systematic deviation from ideal mixing consistent with a subregular solution model. The deviations from ideal mixing are interpreted in terms of Na ordering in the A-site and Na-Al interactions in edenite-poor compositions. Enthalpies of 'edenite' substitution reactions in amphiboles and in SiO₂-NaAlO₂ glasses and framework silicates are comparable. Gibbs free energies of formation of fluortremolite and fluoredenite at 298K are -2,821.07±3.34 kcal mol^-1 and -2,889.59±2.40 kcal mol^-1 respectively. The former value is in good agreement with values calculated from both F-OH exchange experiments and from a natural fluortremolite-bearing metamorphic rock. Least-squares fitted sub-regular heat-of-mixing parameters are poorly constrained and unrealistically high, but estimated subregular mixing parameters consistent with 95% confidence interval uncertainties in the calorimetric data and with TEM constraints give activity-composition relations in good agreement with the A-site compositions of natural metamorphic and igneous hornblendes. These relations predict unmixing in edenite-rich compositions over a wide range of temperature, but lend no support to the existence of a hornblende-actinolite miscibility gap. Calibration of the reaction tremolite+ albite=edenite+4 quartz as a function of P, T and X_ed^amphindicates negative dP/dT slopes and a limited range of X_ed^amph(0.3 to 0.5) in equilibrium with plagioclase and quartz over a wide range of pressure and temperature, consistent with metamorphic hornblende-plagioclase assemblages. The energetics of this reaction suggest, however, that amphibole-plagioclase disequilibrium may be common.