TRANSFORMATION KINETICS OF POLYCRYSTALLINE ARAGONITE TO CALCITE - NEWEXPERIMENTAL-DATA, MODELING, AND IMPLICATIONS

Experimental data are used to model the transformation rate of polycry stalline aragonite (grain diameter approximately 80 mum) to calcite. O ptimized values for nucleation and growth rates were obtained by numer ically fitting the overall transformation rates from 280-degrees to 38 0-degrees-C and 0.10 MPa to an expression for a grain-boundary-nucleat ed and interface-controlled transformation. The nucleation rate is app roximately 4-5 orders of magnitude faster than for calcite nucleated w ithin aragonite grains, and the growth rate is slower below 300-degree s-C than for calcite growing in aragonite single crystals. The activat ion enthalpy for growth in polycrystalline aggregate is approximately 247 kJ/mol compared to 163 kJ/mol for growth in single crystals. Perma nent deformation of the phases limits the elastic strain energy due to the approximately 7% volume change and reduces the coherency of the c alcite/aragonite interace. Theoretical expressions are used to extrapo late the data for nucleation and growth to other temperatures, and dat a from 0.10 to 400 MPa are used to evaluate the effect of pressure on the grain-boundary nucleation rate. Because of permanent deformation o f the phases, the effective strain energy for nucleation is less-than- or-equal-to 0.55 kJ/mol, which is less than a quarter of the value for purely elastic deformation. These data are used to predict the percen t transformation for various P-T-t paths; without heating during uplif t partial preservation of aragonite in dry blueschist facies rocks can occur if the calcite stability field is entered at approximately 235- degrees-C, and the kinetic data are also consistent with published P-T -t paths which include heating during uplift. The predicted percent tr ansformation is relatively insensitive to variations in the initial gr ain size of the aragonite, but strongly dependent on the effective str ain energy.