Physics
Scientific paper
Dec 1967
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1967gecoa..31.2399s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 31, Issue 12, pp.2399-2411
Physics
10
Scientific paper
The equilibrium curve for the reaction: calcite + quartz + anatase 2 d sphene + CO 2 , has been experimentally determined between 340 and 450°C. Solubility measurements of rutile and anatase at temperatures of 200 and 300°C showed that the free energy of the reaction: anatase rutile, is of the order of -0.2 to -0.3 kcal, which means that an equilibrium curve for the formation of sphene from calcite, quartz and rutile will be shifted only slightly towards higher temperatures or lower CO 2 -pressures relative to the same equilibrium with anatase instead of rutile. Experiments with perovskite and various silicates show that perovskite cannot stably coexist with quartz, enstatite, albite and sanidine, and thus is restricted to very silica-under-saturated rocks (carbonatites, ultramafic rocks and phonolitic extrusives). The value for the free energy of the reaction: perovskite + quartz sphene was found to be between -4.75 and -8.6 kcal, which checks well with the calorimetrically determined value for this reaction of -7.05 kcal ( and , 1956). As sphene in almost all cases seems to be stable under natural conditions relative to the association rutile + quartz + calcite, this sets rather severe upper limits to the CO 2 -pressures which can be reached in nature during epizonal metamorphism.
Schuiling Reolof D.
Vink B. W.
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