EFFECT OF IMPURITIES ON GRAIN GROWTH IN SYNTHETIC CALCITE AGGREGATES

Abstract

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 °C for 2 h. The starting material for the anneals was dry and had a uniform microstructure with an average grain size of about 5 μm 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 °C 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-1 147 ± 14 kJ mol-1r the respective calcite compositions.