Influence of OH Content on Elastic Constants of Single-Crystal Topaz Studied via Sphere-Resonance Method

Abstract:

Transport of ocean floor sediments by descending plates might play an important role in the circulation of materials within the Earth. Imaging subducted sediments through seismological observations requires a thorough understanding of elastic properties of sediment origin hydrous minerals. Topaz is a hydrous mineral with ideal chemical formula Al₂SiO₄(F_2-x,OH_x), which can be formed from subducted sediment at high pressures. Although elastic constants of topaz have been studied both experimentally and theoretically, the influence of OH content on elasticity has been poorly understood. In order to understand the influence of OH content on elastic constants, we have determined elastic constants of 5 single-crystals of topaz with different OH content (x = 0.28 ~ 0.72) by the sphere-resonance method.

Sphere samples (D=2.6~6.5 mm) were made from single crystals of natural topaz by the two-pipe method. Lattice parameters were determined and orthorhombic symmetry was confirmed by XRD. Resonant frequencies were measured at ambient temperature with different specimen-holding forces. Extrapolating to the specimen-holding force of zero, we obtained frequencies of "free" oscillation. Elastic constants were determined by comparing measured and calculated resonant frequencies. The xyz algorithm (Visscher et al., 1991) was employed to calculate resonant frequencies of sphere samples. Our determined elastic constants C₁₁, C₂₂, C₄₄, C₆₆, C₁₂, C₂₃ and C₃₁ increase with increasing OH content, while C₃₃ and C₅₅ decrease. These changes in elastic constants are similar to those in theoretically determined elastic constants (Ulian and Valdrè, 2017) except for C₁₁, though theoretical values are distinctly lower than our experimental values. Lower elastic constants must be attributed to larger lattice parameters used in calculations. The influence of OH content on elasticity will be discussed on the basis of the crystal structure of topaz.