Enthalpy of formation of some lime silicates by high-temperature solution calorimetry, with discussion of high pressure phase equilibria

Details Enthalpy of formation of some lime silicates by high-temperature solution calorimetry, with discussion of high pressure phase equilibria Enthalpy of formation of some lime silicates by high-temperature solution calorimetry, with discussion of high pressure phase equilibria

Apr 1978

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1978gecoa..42..367c&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 42, Issue 4, pp.367-375

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Scientific paper

The enthalpies of solution of anorthite, grossular, the three CaSiO 3 polymorphs and CaO were measured at 970 K in a lead borate melt. These data, combined with previously published work, determine the enthalpies of formation (kcal/gfw) of the above silicates and diopside and CaAl 2 SiO 6 pyroxene from the oxides at 970 K: CaAl 2 Si 2 O 8 -anorthite (synthetic) -23.14 ± 0.45 CaAl 2 Si 2 O 8 -anorthite (natural) -23.93 ± 0.48 Ca 3 Al 2 Si 3 O 12 -grossular (synthetic) -77.91 ± 0.67 CaMgSi 2 O 6 -diopside (synthetic and natural) -34.99 ± 0.41 CaAl 2 SiO 6 -pyroxene (synthetic) -18.23 ± 0.39 CaSiO 3 -wollastonite (synthetic and natural) -21.48 ± 0.36 CaSiO 3 -pseudowollastonite (synthetic) -19.92 ± 0.31 CaSiO 3 -high pressure polymorph (synthetic) -19.84 ± 0.34 Natural high temperature anorthite has a significantly more negative enthalpy of formation than synthetic anorthite. The enthalpy of formation of synthetic anorthite agrees with that given by the acid calorimetry of (1962, U.S. Bur. Mines Kept. Inv. 5900 ). Synthetic grossulars prepared both hydrothermally at 1000°C and 28 kbar and in the dry way at 1250°C and 35 kbar have the same enthalpy of solution. The enthalpy of formation of grossular from the oxides is more positive by 2-4 kcal/gfw than estimated by other workers from high pressure equilibrium diagrams. The enthalpy of formation of diopside agrees with that reported by and (1976, Geochim. Cosmochim. Acta 40 , 1281-1288) and is about two kcal less negative than given by and (1968, U.S. Geol. Surv. Bull. 1259 ). The enthalpy of formation and entropy of CaAl 2 SiO 6 pyroxene agree well with the parameters derived from high temperature, high pressure phase equilibria by and (1968). The enthalpies of formation of wollastonite and pseudowollastonite agree very well with those resulting from HF solution calorimetry. The dense high pressure CaSiO 3 polymorph has an anomalously high entropy. The dP / dT slope of the stability boundary relative to wollastonite is indeed negative, as found by Essene (1974, Contrib. Mineral. Petrol. 45 , 247-250) and and (1975, Am. Mineralogist 60 , 213-217).

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