Cation disorder in dolomite, CaMg[(C[O.sub.3]).sub.2], and its influence on the aragonite + magnesite [left and right arrow] dolomite reaction boundary

The structure of dolomite, CaMg[(C[O.sub.3]).sub.2], was determined from 298 to 1466 K at a constant pressure of about 3 GPa using in situ synchrotron X-ray diffraction data to investigate the state of disorder. An order parameter s, defined as 2 [x.sub.Ca] - 1, varies from s = 1 (where [x.sub.Ca] = 1) for a completely ordered dolomite to s = 0 (where [x.sub.Ca] = 0.5) for a completely disordered dolomite. On heating, there is no measured change in s until the temperature is high enough to cause exchange of [Ca.sup.2+] and [Mg.sup.2+] cations. Significant disorder began to occur at about 1234 K [s = 0.83(1)] and increases along a smooth pathway to T = 1466 K [s = 0.12(5)]. The R[bar.3] [left and right arrow] R[bar.3]c transition in dolomite is described by a modified Bragg-Williams thermodynamic model with the following molar free energy of disorder, [[bar.G].sub.d] (T; s) = R[T.sub.c][1 - [s.sup.2] + 1/2 a([s.sup.4] - 1) - (T/[T.sub.c]) {2 ln2 - (1+s)ln(1 + s) - (1 - s) ln(1 - s)}]. Using [T.sub.c] = 1466 K and a = -0.29, this model provides an excellent agreement with experimental data. Moreover, the maximum enthalpy of disorder, [[bar.H].sub.d](s = 0) = R[T.sub.c](1 - 1/2 a) ~ 14 kJ/mol, agrees with published calorimetric data. A thermodynamic description of the aragonite + magnesite [left and right arrow] dolomite reaction boundary is also presented and it reproduces the main qualitative features correctly.