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The rates of grain coarsening are under investigation for stoichiometric dolomite (CaMg(CO3)2) and magnesite (MgCO3) annealed hydrostatically at temperatures to 800 C and pressures Pc to 400 MPa, and results are compared with rates of grain coarsening for calcite (CaCO3). Dense, fine-grained aggregates of the three carbonates were produced by first cold pressing (Pc = 300 MPa) and then hot isostatically pressing (HIP) powders at T = 600 C, Pc = 300 MPa for durations of 5 days (for dolomite and magnesite) and 9 hours (for calcite), following procedures similar to those of Olgaard and Evans (1988). The initial dolomite powder consisted of crushed and sized (< 2 mm) natural material while reagent grade calcium and magnesium carbonates were used, respectively, to produce calcite and magnesite specimens. Sequential heat treatments at T = 300 C, PCO2 = 0.1 MPa and T = 600 C, PCO2 = 20MPa were required to dehydrate the magnesium carbonate and eliminate |
oxides remaining after dehydration. Initial grain sizes of the HIP specimens were 1.4 mm, 1.1 mm, and 17 mm for CaMg(CO3)2, MgCO3, and CaCO3, respectively. Grain growth of dolomite is much slower than either the rates of calcite or magnesite; assuming normal grain growth, its rate constant K at T = 800 C, Pc = 300 MPa is 3 orders of magnitude smaller than that of calcite and smaller than that of magnesite by a factor of 30. Self-diffusion of Mg across magnesite grain boundaries is apparently slower than diffusion of Ca across calcite grain boundaries, and combined rates of Mg and Ca diffusion across dolomite grain boundaries are slower yet. Given that Mg and Ca of dolomite are fully ordered ahead and behind of advancing grain boundaries, the low grain boundary mobility of dolomite may be explained by larger diffusional jump distances than are involved in grain growth of the end-member carbonates. |