Biaxial calcite inverted from aragonite

Abstract

We have recently extended the phase relationships involving a liquid phase in the system CaO-CO_2-H_2O from 4 kilobars pressure (Wyllie and Tuttle, 1960; Wyllie and Raynor, 1965) to 40 kilobars pressure (to be published). Under appropriate conditions the phase assemblage calcite+liquid+vapor undergoes a transition to aragonite+liquid+vapor. Published data indicate that the calcite-aragonite transition curve should intersect the ternary univariant solidus curve at about 580°C at a pressure between 11.5 and 13 kilobars. At temperatures above 600°C, BeIl and England (1964) concluded that the aragonite which formed stably within a wide pressure interval inverted to calcite during the quench. Thus, quenching problems have apparently contributed to the conflicting reports for the position of the calcite-aragonite phase transition. Examination of the morphology of the carbonate in the quenched assemblage CaCO_3+liquid+vapor suggested a method for locating the calcite-aragonite transition boundary regardless of the polymorphic changes that may occur during the quench.