Ht. Haselton et al., EXPERIMENTAL-STUDY OF MUSCOVITE STABILITY IN PURE H2O AND 1-MOLAL KCL-HCL SOLUTIONS, Geochimica et cosmochimica acta, 59(3), 1995, pp. 429-442
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.