Enthalpy change of the reaction enstatite + CORUNDUM = pyrope and the enthalpy of formation of pyrope

Details Enthalpy change of the reaction enstatite + CORUNDUM = pyrope and the enthalpy of formation of pyrope Enthalpy change of the reaction enstatite + CORUNDUM = pyrope and the enthalpy of formation of pyrope

Nov 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992gecoa..56.3971e&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 56, Issue 11, pp.3971-3977

Computer Science

1

Scientific paper

The enthalpy of formation ( H f 0 ) of pyrope has been redetermined by solution calorimetry in lead borate solvent at 700°C (973 K) on synthetic pyrope and on mixtures of synthetic enstatite + corundum on pyrope composition. Attempts to obtain H f 0 more directly by dissolving oxide mixtures on pyrope composition failed because of incomplete dissolution of MgO. H f 0 (pyrope) is obtained from the reaction 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: H sol ( Py ) = 111.49 ± 2.18, N = 19. Uncertainties are twice the standard deviations on the means ( ). Using these values and the H f ,1073 0 (enstatite) from alkali borate solution calorimetry of et al. (1984) on synthetic enstatite, the calculated enthalpy of formation of pyrope at 298 K, corrected with measured heat content data, is H 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 and (1990) and with the value of (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 Pb 2 B 2 O 5 , most likely incomplete dissolution, is the most probable reason for the substantially more negative H f 0 of pyrope found by et al. (1975).

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