Stability relations of some titanium-minerals (sphene, perovskite, rutile, anatase)

Abstract

The equilibrium curve for the reaction: calcite + quartz + anatase 2d sphene + CO2, 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 CO2-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 CO2-pressures which can be reached in nature during epizonal metamorphism.