EXPERIMENTAL-STUDY OF MUSCOVITE STABILITY IN PURE H2O AND 1-MOLAL KCL-HCL SOLUTIONS

The following reactions have been reinvestigated in order to resolve c onflicting evidence regarding tetrahedral Al/Si disorder in muscovite, KAl2(AlSi3)O-10(OH)(2): muscovite + quartz = andalusite + sanidine H2O; muscovite = corundum + sanidine + H2O; muscovite + H+ = 1.5 andal usite + 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 kb ar and second reaction in the range 1.0-6.0 kbar, together with data o n the compositions of 1 molal HCl-KCl solutions limited by the third a nd fourth reactions at 400-500 degrees C, 1 kbar favor total tetrahedr al Al/Si disorder in muscovite, but this conclusion applies only to sy nthetic muscovite grown under relatively low-pressure conditions. The stable assemblage for the first and second reactions were based on gro wth and dissolution textures of muscovite observed with an SEM. The dP /dT slopes of the dehydration boundaries are smaller than those determ ined by most previous work. Results for the fourth reaction are in goo d agreement with other recent work, but quench pH values for the third reaction are less acidic at 400 degrees C, hence, the muscovite stabi lity 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 degrees(1, 298.15)(MS), and enthalpy of formation from the elements, Delta(f)H de grees,29R.15(MS), respectively. The enthalpy of formation of muscovite is based, in part, on a revised calorimetric value for sanidine Delta (f)/H degrees(1,298.15)(Sa) = -3965.60 +/- 4.1 kJ/mol. A similar contr adiction regarding tetrahedral Al/Si disorder in paragonite appears to exist between structure refinements and phase equilibrium data. Howev er, if the phase equilibrium data for the reaction paragonite = corund um + albite + H2O are omitted, almost all other evidence is consistent with near total Al/Si disorder in paragonite.