Anomalous birefringence in andradite-grossular solid solutions: a quantum-mechanical approach

The static linear optical properties (refractive indices, birefringence and axial angle) of andradite-grossular (Ca3Fe2Si3O12-Ca3Al2Si3O12) solid solutions have been computed at the ab initio quantum-mechanical level through the Coupled Perturbed Kohn-Sham scheme, using an all-electron Gaussian-type basis set. Geometry relaxation after substitution of 1-8 Al for Fe atoms in the primitive cell of andradite yields 23 non-equivalent configurations ranging from cubic to triclinic symmetry. Refractive indices vary quite regularly between the andradite (1.860) and grossular (1.671) end-members; the birefringence delta and the axial angle 2V at intermediate compositions can be as large as 0.02A degrees and 89A degrees, respectively. Comparison with experiments suffers from inhomogeneities and impurities of natural samples; however, semi-quantitative agreement is observed.