TRANSFORMATION KINETICS OF SYNTHETIC POLYCRYSTALLINE CALCITE TO ARAGONITE

The kinetics of calcite to aragonite transformation has been re-investigated using polycrystalline calcite aggregate synthesized in-situ during kinetic experiments. The in-situ annealed sample minimizes the residual stress and fractures, which otherwise may be induced by the sample deformation during the run-up. The synthesis of the starting sample may also allow adding the known amounts of mineral impurities in the sample. The polycrystalline calcite was re-crystallized at 800 deg.C and 1 GPa for 9 hours using ultra-pure CaCO3 powder in the first stage, immediately followed by the second-stage in the same experiment at conditions ranging from 500 to 700 deg. C and 1.75 to 2.7 GPa to determine the transformation rate. The percents of transformation as a function of time were determined at four temperature-pressure conditions. The data were fitted to the Cahn's model for grain-boundary nucleation and growth (Rubie et al., 1990, JGR v. 95, p.15829-15844). The computed growth rates at these temperature-pressure conditions were used to calculate the activation energy (Q) for growth. Alternatively, the Q calculated directly from the extent of reaction method shows a slightly higher value. Both values, however, are significantly lower than that determined by directly measuring growth rates from experimental run products using natural marble as starting material (Hacker et al., 1992, Science, v. 258, p. 110-112.). Extrapolation of the experimental rates to geological conditions using the high Q value suggests an extremely sluggish rate at geological conditions (< 200 deg. C; Hacker et al., 1992), whereas the extrapolation using the Q values determined in this study shows a very rapid transformation rate at similar conditions. The results also reveal that the Mg-calcite transforms to aragonite at much slower rate than the pure calcite, due to higher equilibrium transformation pressure for the Mg-calcite. In contrast, Sr-bearing calcite transforms at a considerably higher rate than the pure calcite due to lower equilibrium transformation pressure for the Sr-calcite. The rate enhancement has also been observed in experiments with polycrystalline aggregate containing calcite and quartz, probably due to the stress on calcite induced by quartz grains during experiments.