Experimental determination of the reaction; magnesite + enstatite = forsterite + CO ₂ in the ranges 6-25 kbar and 700-1100 degrees C

University of Dayton, Department of Geology, Dayton, OH, United States
University of Chicago, United States

The P-T equilibrium curve of the reaction of magnesite + enstatite = forsterite and CO ₂ 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 CO ₂ source and charges were buffered at hematite-magnetite oxygen fugacity. Reaction progress was monitored by weight changes upon puncturing of CO ₂ -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 to 4.784X10 (super -3) T+2.542X10 (super -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 CO ₂ , give Delta H ⁰ _t (298 K) (forsterite) = -61.20+ or -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 CO ₂ 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 CO ₂ fugacity in this pressure range.

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