The power of crystal optics

It is demonstrated here that refractive-index measurements can be used to determine details of chemical compositions and physical properties of crystals. For example, the H2O content of hydrous minerals can be determined from their mean refractive indices with high accuracy as shown for 157 zeolite-type minerals and compounds (Fischer et al., 2020). This is especially important when only small crystals are available not suitable for thermal analyses or for other reliable methods of measuring the amount of H2O. The physical basis for these predictions is the additivity of electronic polarizabilities of cations and anions. If the chemical composition and the molar volume of a compound are known, the total electronic polarizability can be calculated from the sum of the individual polarizabilities of the elements. Using the Anderson-Eggleton equation, the mean refractive index can be calculated. Deviations between observed and calculated mean refractive indices can be used, e.g., to identify ion conductors showing unusually high discrepancies. We have evaluated more than 3000 datasets of minerals and compounds to determine the electronic polarizabilities of four anions and 76 cations in various coordinations. Based on these parameters, the mean refractive index for more than 2000 minerals and compounds could be calculated with an error <2%. Some groups of compounds are identified showing systematic deviations. The results are compared with the Gladstone-Dale approach after Mandarino (1976).

Fischer, R.X., Burianek, M., Shannon, R.D. (2020) Eur. J. Mineral. 32, 27.

Mandarino, J.A. (1976) Can. Mineral. 14, 498.

Shannon, R.D., Fischer, R.X. (2016) Amer. Mineral. 101, 2288.