Grain growth kinetics of dolomite, magnesite and calcite: a comparative study

The rates of grain growth of stoichiometric dolomite [CaMg(CO3)(2)] and magnesite (MgCO3) have been measured at temperatures T of 700-800A degrees C at a confining pressure P (c) of 300 MPa, and compared with growth rates of calcite (CaCO3). Dry, fine-grained aggregates of the three carbonates were synthesized from high purity powders by hot isostatic pressing (HIP); initial mean grain sizes of HIP-synthesized carbonates were 1.4, 1.1, and 17 mu m, respectively, for CaMg(CO3)(2), MgCO3, and CaCO3, with porosities of 2, 28, and 0.04% by volume. Grain sizes of all carbonates coarsened during subsequent isostatic annealing, with mean values reaching 3.9, 5.1, and 27 mu m for CaMg(CO3)(2), MgCO3, and CaCO3, respectively, in 1 week. Grain growth of dolomite is much slower than the growth rates of magnesite or calcite; assuming normal grain growth and n = 3 for all three carbonates, the rate constant K for dolomite (a parts per thousand integral 5 x 10(-5) mu m(3)/s) at T = 800A degrees C is less than that for magnesite by a factor of similar to 30 and less than that for calcite by three orders of magnitude. Variations in carbonate grain growth may be affected by differences in cation composition and densities of pores at grain boundaries that decrease grain boundary mobility. However, rates of coarsening correlate best with the extent of solid solution; K is the largest for calcite with extensive Mg substitution for Ca, while K is the smallest for dolomite with negligible solid solution. Secondary phases may nucleate at advancing dolomite grain boundaries, with implications for deformation processes, rheology, and reaction kinetics of carbonates.