Abstract:
The P-T equilibrium curve of the reaction of magnesite + enstatite = forsterite and CO2 was determined in the ranges 6-25 kbar and 700-1100 degrees C by 53 reversed experiments carried out in 1.91 cm diameter piston-cylinder apparatus with NaCl pressure medium. Silver oxalate was used as a CO2 source and charges were buffered at hematite-magnetite oxygen fugacity. Reaction progress was monitored by weight changes upon puncturing of CO2-inflated quenched charge capsules and was confirmed by comparing X-ray diffractograms of charges made before and after the experiments. The data satisfy the relation: P-EQ = -3.215 - 4.784 x 10(-3) T + 2.542 x 10(-5) T-2 with P in kbar and T in degrees C. A P-T curve calculated with the 1994 version 2.3 THERMOCALC program agrees well with our results except in the highest pressure range. Our data, together with three existing tight experimental brackets on the decarbonation reaction of magnesite to periclase and CO2, give Delta H-f(0) (298 K) (forsterite) = -61.20 +/- 0.87 kJ/mol from the oxides. This value is in agreement with the value adopted by Berman (1988). An analysis of various proposed equations of state for CO2 shows that only those of Mader and Berman (1991) and Frost and Wood (1997) meet the constraints imposed by this study near 1000 degrees C and 10-20 kbar. The modified Redlich-Kwong (MRK)-based equations of state overestimate CO2 fugacity in this pressure range.