Ab initio study of cation disorder in dolomite

Ab initio calculations at the athermal limit were carried out in order to study the disordering mechanism which drives the non-ideal cation distribution over dolomite crystallographic sites and to analyse the thermodynamic stability of differently disordered dolomite structures. The configurational entropy contribution was taken into account by means of a quantum-statistical thermodynamic approach. Intra- and inter-layering disordered structures were simulated. Total static energy calculations showed that ordered dolomite is always the most stable configuration at the athermal limit being the energy differences between ordered and disordered configurations lower than - 7 kJ/mol. In order to quantify disordering conditions by taking into account the two types of disorder, some parameters were proposed such as a weighted s parameter (s(w)) referring to the intra-layer type disorder and the frequency of stacking disorder f together with the distance of stacking inversion d as regards the inter-layer type. Those parameters were summarized in a total ordering parameter (s(t)) having values between 1 (fully order) and 0 (fully disorder). Quite wide disordering conditions were analysed as the s(r) parameter for the simulated structures ranges from 1 to 0.25. An intra-layered type disorder was suggested as the main process driving the thermally induced cation distribution over crystallographic sites. In addition, a configurational disorder parameter (s(c)) was proposed in order to take into account the probability of existence of each disordered configuration as a function of temperature (T). At very low T a fully ordered dolomite configuration is the most stable one. As temperature increases, cations disordering arises in the crystal structure and at room temperature s(c) counts 0.83. Such parameter rapidly goes down and it is reduced to 0.62 at 750 K; it asymptotically approaches 0.54 at higher T. These temperatures are significantly lower than 1200 K which is found experimentally to be the temperature of first appearance of disorder (Hammouda et al., 2011[1]), likely due to kinetic reasons. (C) 2012 Elsevier Ltd. All rights reserved.