Lattice energies of calcite-structure metal carbonates III. Theoretical excess energies for solid solutions (Ca,M)CO 3 [M=Cd, Mn, or Fe

Abstract

A theoretical method for calculating excess energy values ( H EX or G EX ) of calcite-structure solid solutions has been devised by extending the lattice energy calculation technique for pure carbonate minerals [3,5]. By determining Δ U as a function of composition (χ) for Ca χCd 1 - χCO 3 (ss), Ca χMn 1-χCO 3 (ss), and Ca χFe 1-χCO 3(ss), excess energies are found through the use of standard thermodynamic relations and the regular-solution model. Input data for these solid-solution, lattice-energy calculations includes experimental lattice parameters (as a function of χ), and end-member refractive indices. Computation of the refractive indices for intermediate compositions (i.e., 0<χ<1) is performed by the following two distinct methods: Lorentz/Lorenz and Gladstone/Dale [9]. Hence, two sets of theoretical excess energy values are obtained for each of the three solid solutions under investigation. These results, when compared with experimentally reported excess energy values, generally reproduce the sign and compositional dependence of the appropriate excess energy function, except for one set of theoretical excess Gibbs energies for the (Ca,Fe)CO 3 system.