Experimental determination of the reactions magnesite + quartz = enstatite + CO(sub 2) and magnesite = periclase + CO(sub 2), and enthalpies of formation of enstatite and magnesite

The equilibrium P-T curve of the reaction (1) Mg[CO.sub.3] + Si[O.sub.2] = MgSi[O.sub.3] + [CO.sub.2]/ magnesitequartzenstatite was determined by reversed reactions in well-calibrated, solid-pressure medium (NaCl) piston-cylinder apparatus. External hematite + magnetite + [H.sub.2]O [f.sub.o2] buffers were used in all experiments. A reversed point at 715 [degrees]C of the reaction (2) Mg[CO.sub.3] = MgO + [CO.sub.2]/ magnesitepericlase was obtained in an internally heated Ar pressure vessel, using a coarse-grained synthetic periclase to inhibit quenching back reaction, along with synthetic magnesite and silver oxalate. At 715 [degrees]C our pressure for Reaction 1 is 10.4 [+ or -] 0.2 kbar, and for Reaction 2 it is 590 [+ or -] 30 bars. From these two reactions we derive [Mathematical Expression Omitted] = 33.41 [+ or -] 0.83 kJ for enstatite from the oxides. Using an extrapolated heat-capacity equation for magnesite, we obtain [Mathematical Expression Omitted] = -116.68 Kj (From the oxides) and -1111.68 kJ from the elements for magnesite, somewhat less negative than the values of either the Holland and Powell (1990) or the Berman (1988) data sets. A calculated 2 kbar temperature for the magnesite + quartz reaction is 492 [degrees]C, about 25 [degrees]C lower than the data set predictions but in agreement with a prediction based on natural occurrences of magnesite, quartz, and enstatite in metacarbonates from the Alps by Trommsdorff and Connolly (1990).