EXPERIMENTALLY DETERMINED ACTIVITY-COMPOSITION RELATIONS FOR CA-RICH SCAPOLITE IN THE SYSTEM CAAL2SI2O8-NAALSI3O8-CACO3 AT 7 KBAR

Equilibrium compositions of coexisting Ca-rich carbonate scapolite and plagioclase in the presence of calcite have been investigated experim entally at 7.0 kbar in the temperature range 775-850 degrees C. The Ca O-Al2O3-SiO2-CO2 univariant reaction is [GRAPHICS] with NaSi(CaAl)(-1) substitution in both plagioclase and scapolite in the Na2O-CaO-Al2O3- SiO2-CO2 divariant assemblage. The experiments were carried out in a 3 /4 in. diameter NaCl pressure cell with a hematite + magnetite buffer and a hydrous oxalic acid flux. Starting materials were a natural Cl- and S-free mizzonite (Me(63)), Baker reagent precipitated calcite, and synthetic meionite and plagioclases of composition An(100) and An(92) . Changes of compositions of the reactants were determined by micropro be analysis. The experimental results confirm the inferences based on observations in natural assemblages made by earlier workers that Na su bstitution greatly stabilizes scapolite relative to plagioclase. The m arked stabilization of mizzonite to lower temperatures is largely a co nsequence of the large amounts of atomic mixing possible in the scapol ite structure. An activity model for the meionite component in terms o f its mole fraction, X(Me), which involves complete disordering of Ca and Na and, independently, Si and Al, has been derived from the experi mental brackets a(Me) = (1 + X(Me))(6)(3 - X(Me))(6)(3X(Me) + 1)(4)/4( 10) exp [-13450/T X(Me)(1 - X(Me))(2)]. This formula may lose validity in a lower temperature range if scapolite develops greater atomic ord ering near the mizzonite composition. The disordered scapolite model i s used to calculate apparent CO2 activities for recrystallization of s ome Grenville granulites studied by Moecher and Essene (1991). We find close agreement with their predicted CO2 activities based on scapolit e + garnet + plagioclase + quartz equilibrium at 800 degrees C and 10 kbar, despite the different scapolite activity models used in the two sets of calculations. The simplest explanation of this coincidence is that their empirical scapolite model and the present experimentally ba sed model are both valid, although the latter is restricted to high te mperatures (above 750 degrees C).