The lattice energy of forsterite. Charge distribution and formation enthalpy of the SiO ⁴⁻₄ ion

Abstract

In the lattice energy expression of forsterite, based on a Born-Mayer (electrostatic+repulsive+dispersive) potential, the oxygen charge z _o, the hardness parameter ρ and the repulsive radii r _Mg and r _Si appear as unknown parameters. These were determined by calculating the first and second partial derivatives of the energy with respect to the cell edges, and equalizing them to quantities related to the crystal elastic constants; the overdetermined system of equations was solved numerically, minimizing the root-mean-square deviation. To test the results obtained, the SiO ⁴⁻₄ ion was assumed to move in the unit-cell, and the least-energy configuration was sought and compared with the experimental one. By combining the two methods, the optimum set of parameters was: z _o=−1.34, ρ=0.27 Å, r _Mg=0.72 Å, r _Si=0.64 Å. The values −8565.12 and −8927.28 kJ mol⁻¹ were obtained, respectively, for the lattice energy E _Land for its ionic component E ⁰_L ,which accounts for interactions between Mg²⁺ and SiO ⁴⁻₄ ions only. The charge distribution calculated on the SiO ⁴⁻₄ ion was discussed and compared with other results. Using appropriate thermochemical cycles, the formation enthalpy and the binding energy of SiO ⁴⁻₄ were estimated to be: ΔH _f(SiO ⁴⁻₄ )=2117.6 and E(SiO ⁴⁻₄ )=708.6 kJ mol⁻¹, respectively.