Cation disorder in dolomite, CaMg(CO₃)₂, and its influence on the aragonite + magnesite ↔ dolomite reaction boundary

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 x_Ca - 1, varies from s = 1 (where x_Ca = 1) for a completely ordered dolomite to s = 0 (where x_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 Ca²⁺ and Mg²⁺ cations. Significant disorder began to occur at about 1234 K [s = 0.83(l)] and increases along a smooth pathway to T = 1466 K [s = 0.12(5)]. The R3̄ ↔ R3̄c transition in dolomite is described by a modified Bragg-Williams thermodynamic model with the following molar free energy of disorder, Ḡ_d(T; s) = RT_c[1 - s² + 1/2 a(s⁴ - 1) - (T/T_c) 2 1n2 - (1 + s) 1n(1 + s) - (1 - s) 1n(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, H̄_d(s = 0) = RT_c(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.