THE EFFECT OF ORIENTATIONAL DISORDER OF THE ANION IN CALCITE ON THE ARAGONITE (Pmcn) < - > CALCITE (R3c) EQUILIBRIUM

摘要

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 (Ric) 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.