Revision of the Calcite–Aragonite Transition, with the Location of a Triple Point between Calcite I, Calcite II and Aragonite

Abstract

CONFLICTING reports have been published for the position of the calcite–aragonite phase transition, and two factors contributing to this are the slow solid state reaction kinetics¹ and the problems of quenching the stable assemblage². Bell and England² concluded from their experiments that aragonite which formed stably within a wide pressure range (above 600° C) inverted to calcite during the quench. We have recently extended the phase relationships involving a liquid phase in the system calcium oxide–carbon dioxide–water from 4 kbars pressure^3,4 to 40 kbars⁵ using a piston-and-cylinder high-pressure apparatus. The liquids in this system are very reactive media, and no problems have been encountered in reaching equilibrium in this and a variety of related systems⁶. Across the calcite–aragonite boundary, the ternary phase assemblage calcite + liquid + vapour undergoes a transition to aragonite + liquid + vapour. In runs completed at pressures well above and well below the phase boundary, aragonite and calcite crystals in the quenched assemblage calcium carbonate + liquid + vapour were positively identified by X-ray and optical properties. The morphology of aragonite crystals coexisting with the ternary liquid is distinct from that of the calcite crystals, and the original shape of the equilibrium phase, aragonite or calcite, becomes frozen into the surrounding liquid during the quench and is thus preserved for microscopic examination. The shape of the carbonate crystals in the quenched assemblage calcium carbonate + liquid + vapour was therefore used to bracket the calcite–aragonite transition boundary down to the solidus temperature at 580° C. X-ray powder diffraction and refractive-index measurements demonstrate that crystals with the morphology of aragonite, in runs close to the transition boundary, have inverted to complexly twinned, biaxial calcite with optic axial angles ranging from near zero up to an estimated 20° (2V for aragonite is 18°). The implications of this observation have been discussed elsewhere⁷.