The Calcite-Aragonite Transition Measured in the System CaO-CO_2-H_2O

The study of melting reactions in the system CaO-CO_2-H_2O included redetermination of the calcite-aragonite transition boundary. The morphology of aragonite in equilibrium with the ternary liquid is distinct from that of calcite; the original shape of calcite or aragonite becomes frozen into the liquid during the quench and is preserved for microscopic examination regardless of the polymorphic changes that occur in quenching. The shape of the carbonate crystals in the quenched assemblage CaCO_3 + liquid + vapor was therefore used to bracket the calcite-aragonite transition boundary. Some original aragonite crystals inverted to biaxial calcite during the quench. The kinetics of the inversion are strongly influenced by crystal size. The transition boundary was extended below the solidus temperature (580° C. at 12.1 kb.) on the basis of microscopic examination of morphology of the quenched crystals, supplemented by X-ray diffraction patterns. The reaction was reversed at several points. The transition boundary changes slope at 480° C. and 9.4 kb., the triple point for aragonite + calcite I + calcite II. This change in slope is probably caused by ΔS for calcite I - calcite II, because ΔV is insignificant. The lowest point measured on the aragonite-calcite I curve is 400° C. at 8.3 kb., and the highest point measured on the aragonite-calcite II curve is 800° C. at 19.8 kb. These results are compared with a detailed review of previous experimental determinations. Geological applications of the calcite-aragonite geobarometer are discussed, as well as the possible use of biaxial calcite as a criterion for recognizing inverted aragonite