Prediction of refractive indices in minerals from crystallographic data: Applications and limitations of the point-dipole model

A point-dipole calculation which incorporates an expression for the Lorentz-factor tensor has been used to model the effect of chemical substitution on refractive index and optic axial angle (2V). Isotropic electronic polarizabilities for the sodium D line are refined by least-squared methods from lattice dipole sums and observed refractive indices. Although the assumption of isotropic polarizability is unrealistic for many mineral structures, the model is useful in interpreting the relationship between structure, chemical composition, and optical properties.