Phlogopite: high temperature solution calorimetry, thermodynamic properties, Al-Si and stacking disorder, and phase equilibria
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/sup 0/C suggests a disordered Al-Si distribution. Calculations of the P-T locus of the geologically important equilibrium: Phl + 3Qtz = 3En + Sa + H/sub 2/O, using the authors 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 ..delta..H/sup 0/ and ..delta..HG/sup 0/, 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/sup 0/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. The second structural complication is stacking disorder, which is present in phlogopite synthesized at 650/sup 0/C but not in the 850/sup 0/C sample. The enthalpy difference between these two samples, determined by solution calorimetry, is smaller than the experimental uncertainty of +/- 1.0 kcal mol/sup -1/. Thus there appears to be little driving force for ordering, and micas with disordered stacking sequences may persist in many geologic environments.
- Research Organization:
- Arizona State Univ., Tempe (USA)
- DOE Contract Number:
- AC02-80ER10765; FG02-85ER13437
- OSTI ID:
- 7194267
- Journal Information:
- Geochim. Cosmochim. Acta; (United States), Vol. 51:9
- Country of Publication:
- United States
- Language:
- English
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