Effect of impurities on grain growth in synthetic calcite aggregates

We investigated grain growth of calcite aggregates fabricated from crushed natural single crystals with different impurity content. The total trace-element concentration of the starting powders varied from about 170 ppm to more than 930 ppm with Mn as the major component. Samples were produced by hot-isostatic pressing of the different powders at 300 MPa confining pressure at 600 degreesC for 2 h. The starting material for the anneals was dry and had a uniform microstructure with an average grain size of about 5 gin and a porosity of < 2.1 %. Three disks with Mn concentrations of 10, 350, and 670 ppm, respectively, were annealed in the same run at a confining pressure of 300 MPa, and temperatures between 700 and 900 degreesC for up to 20 h. Grain growth was fastest in samples with the highest Mn concentrations. A multivariable fit to the data yields grain-growth exponents of 2.0 +/- 0.3 for samples with 10 ppm Mn and 2.3 +/- 0.2 for those with 670 ppm Mn. The activation energies for grain growth vary from 99 +/- 12 kJ mol(-1) to 147 +/- 14 kJ mol(-1) for the respective calcite compositions.