Temperature Dependence of the Structural Parameters in the Transformation of Aragonite to Calcite, as Determined From in Situ Synchrotron Powder X-Ray-Diffraction Data

The temperature dependency of the crystal structure and the polymorphic transition of CaCOsub 3 from aragonite to calcite were studied using Rietveld structure refinement and high-temperature in situ synchrotron powder X-ray-diffraction data at ambient pressure, P. The orthorhombic metastable aragonite at room P, space group Pmcn, transforms to trigonal calcite, space group Rbar 3c, at about Tsub c = 468 C. This transformation occurs rapidly; it starts at about 420 C and is completed by 500 C, an 80 C interval that took about 10 minutes using a heating rate of 8 C/min. Structurally, from aragonite to calcite, the distribution of the Ca atom changes from approximately hexagonal to cubic close-packing. A 5. 76% discontinuous increase in volume accompanies the reconstructive first-order transition. Besides the change in coordination of the Ca atom from nine to six from aragonite to calcite, the COsub 3 groups change by a 30sup o rotation across the transition.