THE EFFECT OF ORIENTATIONAL DISORDER OF THE ANION IN CALCITE ON THE ARAGONITE (Pmcn) CALCITE (R3c) EQUILIBRIUMvr

Abstract

A thermodynamic analysis of the order–disorder transition in calcite, based on a modified Bragg–Williams model, has been extended to include the first-order transition from aragonite to calcite, with a view to developing a quantitative understanding of the relative stability of calcite ( R 3 c ) and aragonite and, by extension, a mathematical description of the aragonite–calcite coexistence curve in the phase diagram for CaCO 3 . The thermodynamic model necessarily relies on some estimated values of thermal and calorimetric properties of the two structures. The predictions of our model depend largely on thermodynamically consistent extrapolations of known properties of calcite and (metastable) aragonite at 1 bar pressure and an estimated value for the temperature coefficient of the difference in standard molar heat capacities. Agreement with experimental results is good up to T c = 1240 K, the critical temperature for orientational disorder of the anion in calcite, which we assume to be independent of pressure. The effect of CO 3 orientational disorder is shown as an expansion of the stability field of calcite. The R 3 c → R 3 m second-order transition in calcite is also analyzed in light of this thermodynamic model. Assuming the existence of new calcite phases, or invoking CO 3 disorder, is not necessary to account for the observed curvature of the coexistence line between 600 and 800 K.