ENTHALPY CHANGE OF THE REACTION 3/2 ENSTATITE + CORUNDUM = PYROPE AND THE ENTHALPY OF FORMATION OF PYROPE

The enthalpy of formation (DELTAH(f)0) of pyrope has been redetermined by solution calorimetry in lead borate solvent at 700-degrees-C (973 K) on synthetic pyrope and on mixtures of synthetic enstatite + corundum on pyrope composition. Attempts to obtain DELTAH(f)0 more directly by dissolving oxide mixtures on pyrope composition failed because of incomplete dissolution of MgO. DELTAH(f)0 (pyrope) is obtained from the reaction 3/2 Mg2SiO6 + Al2O3 = Mg3Al2SiO12 enstatite corrundum pyrope, using measured solution values for each side of the reaction combined with an enthalpy of formation of enstatite. Measured solution values for reaction (A), at 973 K, are (kJ/mole) as follows: DELTAH(sol)(3/2 En + Cor) = 139.22 +/- 3.74, n = 14 DELTAH(sol)(Py) = 111.49 +/- 2.18, n = 19. Uncertainties are twice the standard deviations on the means (delta). Using these values and the DELTAH(f,1073)0. (enstatite) from alkali borate solution calorimetry of BROUSSE et al. (1984) on synthetic enstatite, the calculated enthalpy of formation of pyrope at 298 K, corrected with measured heat content data, is DELTAH(f,298)0 (Py) = -72.01 +/- 6.98 kJ/mole. This value agrees well with the -70.96 kJ/mole value from the internally consistent dataset of HOLLAND and POWELL (1990) and with the value of BERMAN ( 1988) of -74.25 kJ/mole. This agreement indicates that major discrepancies no longer exist between thermodynamic measurements and experimental phase equilibrium data for pyrope. Anomalous solution behavior of MgO in Pb2B2O.5, Most likely incomplete dissolution, is the most probable reason for the substantially more negative DELTAH(f)0 of pyrope found by CHARLU et al. (1975).