Crystal field electronic effects on the thermodynamic properties of Fe2+ minerals.

Although the free energies of silicates cannot be accurately predicted, their heat capacies and entropies obey simple relationships. For Fe²⁺, however, the sixth d electron is disordered over three atomic orbitals in FeO, but due to crystal field splitting in distorted sites, is essentially localized in one low-lying level in, for example, fayalite. The observed splitting of the d orbital energy levels of Fe²⁺ may be used to calculate the entropy and heat capacity terms arising from the crystal field effect in distorted sites. The calculated crystal-field corrected entropies of almandine and ferrosilite agree within 1% with the values obtained from phase equilibrium experiments.R.A.H.