The normal grain growth behaviour of nominally pure calcitic aggregates

Abstract

The normal grain growth behaviour of four different, but all nominally pure, calcite powders (99%+ analytic grade calcite, 99.7% chalk, 99.97% crushed Iceland Spar, 99.95%+ chelometric grade calcite) has been investigated as a function of temperature (550, 600, 650, 700 °C) and confining pressure (100, 190 MPa) under both “dry” and hydrostatic (P fluid = P total) conditions. The initial particle size of both the analytic grade and chelometric grade calcite was about 5 μm, and that of the chalk was about 3 μm, while the experiments on the Iceland Spar were conducted on powders of three different initial particle sizes (3.4, 7.5, 38.5 μm). On each material, at each pressure/temperature condition 6 to 15 experiments, equally spaced in log time from 15 minutes to 50 days, were conducted. Under dry conditions all four materials recrystallized to aggregates which contained less than 2% porosity and which had a grain size of between 4 and 20 μm (depending on the initial particle size). Subsequently the aggregates coarsened by normal grain growth, with the kinetics of the growth process being controlled by the rate at which the grain boundaries could drag the residual pores with them as they migrated. Under nominally identical conditions both the mechanism and rates of pore drag differed greatly for the different materials, implying that this process is highly sensitive to trace solute impurity concentrations. This sensitivity renders the task of providing a systematic account of dry calcite grain growth kinetics highly problematic. Under hydrostatic conditions all the powders followed the same normal grain growth kinetics in which the growth process was rate-controlled by diffusion through the pore fluid on the grain boundaries. An activation enthalpy of 162.6 kJ mol−1 and an activation volume of 34.35 cm3 mol−1 was obtained for this process.