Abstract:
The following reactions have been reinvestigated in order to resolve conflicting evidence regarding tetrahedral AlSi disorder in muscovite, KAl2(AlSi3)O10(OH)2:muscovite + quartz = andalusite + sanidine + H2O ;muscovite = corundum + sanidine + H2O ;muscovite + H+ = 1.5 andalusite + 1.5 quartz + 1.5 H2O + K+ ;1.5 sanidine + H+ = 0.5 muscovite + 3 quartz + K+ .Results for the first reaction in the range 1.0-3.0 kbar and second reaction in the range 1.0-6.0 kbar, together with data on the compositions of 1 molal HCl-KCl solutions limited by the third and fourth reactions at 400-500°C, 1 kbar favor total tetrahedral AlSi disorder in muscovite, but this conclusion applies only to synthetic muscovite grown under relatively low-pressure conditions. The stable assemblage for the first and second reactions were based on growth and dissolution textures of muscovite observed with an SEM. The dPdT slopes of the dehydration boundaries are smaller than those determined by most previous work. Results for the fourth reaction are in good agreement with other recent work, but quench pH values for the third reaction are less acidic at 400°C, hence, the muscovite stability field in KCl-HCl solutions is narrower than previously determined. Recommended thermodynamic properties for synthetic muscovite are 306.40 J/mol . K and -5970.45 kJ/mol for the standard entropy, S°1,298.15(Ms), and enthalpy of formation from the elements, ΔfH°1,298.15(Ms), respectively. The enthalpy of formation of muscovite is based, in part, on a revised calorimetric value for sanidine ΔfH°1,298.15(Sa) = -3965.60 +/- 4.1 kJ/mol.A similar contradiction regarding tetrahedral AlSi disorder in paragonite appears to exist between structure refinements and phase equilibrium data. However, if the phase equilibrium data for the reaction paragonite = corundum + albite + H2O are omitted, almost all other evidence is consistent with near totalAlSi disorder in paragonite.