The chemical compositions of synthetic paragonite-muscovite pairs were obtained by electron probe microanalysis of run products prepared hydrothermally at 300, 400, 500, and 600 °C and 2.07 kbar. The microcrystalline run products were dispersed on polished beryllium rods, and Na, K, and Si were determined simultaneously. Compositions were determined from K/Si and Na/Si ratios referred to standards. Tentative excess thermodynamic mixing properties of paragonite-muscovite crystalline solutions, based on the two-phase composition data and on X-ray diffraction data, are represented by the following third-order Margules formulation: which leads to a critical temperature of 833 °C and a critical composition of 39.0 mole per cent Mu at 2.07 kbar (the critical phase is probably metastable with respect to alkali feldspar and corundum). These results are quantitatively in agreement with Iiyama's (1964) ion-exchange data. From relationships among the quantities (arctanh s)/s, In r2, and 1/T we obtain critical conditions which are in good agreement with the above (Tc = 829 °C, N2c = 0.396). The critical curve obtained from the above Margules parameters is given by: Tc(°C) = 768.8 + 31.00P(kbar). The above results are complicated by polymorphism and by possible lack of complete equilibrium between the two-mica synthesis products and possible substitution of hydronium for alkalies. We emphasize, therefore, that the phase diagrams and derived mixing properties should be applied with caution to natural muscovite- and paragonite-bearing assemblages.
No takes yet. Share an insight, caveat, or question.
Eugster et al. (1972) studied this question.