The elastic tensor of natural topaz: the effect of OH/F ratio

Topaz Al2SiO4(OHx,F1-x)2 is a crustal mineral, typical of magmatic and hydrothermal environments, but also present in metamorphic orogenic settings. It is characterized by high density and mechanical properties that make it a popular precious stone. Although apparently simple, the chemical composition of topaz shows a wide variability in the ratio OH/F, which has attracted researchers interested in connecting it to their geological settings, and for provenance analysis to identify the sources of commercial stones. We present a study of the full elastic tensor of a set of four, well characterized, natural Topaz crystals from different geological settings and with different OH/F ratios. The substitution of F with OH produces a general softening of the elastic coefficients of topaz. We present and interpret the compositional dependence of the individual tensor coefficients and the aggregate moduli in connection with the consequences of OH/F substitution in the crystal structure of topaz.