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The structure of dolomite, CaMg(CO3)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 xCa - 1, varies from s = 1 (where xCa = 1) for a completely ordered dolomite to s = 0 (where xCa = 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 2+  and Mg 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 R3 - ↔ R3 - c transition in dolomite is described by a modified Bragg-Williams thermodynamic model with the following molar free energy of disorder, G̅ d  (T; s) = RT c [1 - s 2  + 1/2 a(s 4  - 1) - (T/T c ) {2 ln2 - (1+s) ln(1 + s) - (1 - s) ln(1 - s)}]. Using T c  = 1466 K and a = -0.29, this model provides an excellent agreement with experimental data. Moreover, the maximum enthalpy of disorder, H̅ d (s = 0) = RTc(1 - 1/2 a) ~ 14 kJ/mol, agrees with published calorimetric data. A thermodynamic description of the aragonite + magnesite ↔ dolomite reaction boundary is also presented and it reproduces the main qualitative features correctly.

Cation disorder in dolomite, CaMg(CO 3 ) 2 , and its influence on the aragonite + magnesite ↔ dolomite reaction boundary