Synthetic polycrystalline aragonite to calcite transformation kinetics: experiments at pressures close to the equilibrium boundary

Abstract

¶This study experimentally investigated the transformation kinetics of synthetic polycrystalline aragonite to calcite at four temperature/pressure conditions (330 °C/200 MPa, 380 °C/325 MPa, 430 °C/580 MPa, and 480 °C/875 MPa), close to the calcite-aragonite equilibrium phase boundary. The extents of transformation measured as a function of time in a synthetic system, using in-situ annealed, high purity samples, are consistent with the kinetic model for grain-boundary nucleation and interface-controlled growth. The growth rates are slightly lower than those previously determined for a natural polycrystalline sample at 330 and 380 °C. The activation energy (158 kJ/mol) for calcite growth from synthetic samples is lower than that (247 kJ/mol) from natural samples, but is close to the previously reported value (163 kJ/mol) from a single crystal aragonite. The extrapolation of our experimental data to natural conditions reveals unusually fast transformation rates, in contrast to those of natural samples. The presence of deformational strain, fractures, defects or impurities in natural samples, and other factors may account for the discrepancy. This study suggests that the retrograde metamorphism of aragonite to calcite may proceed in a wide range of rates also depending on other geological factors than temperature and pressure.