High-temperature X-ray diffraction, Raman and IR spectroscopy on serandite

We collected in situ high-temperature powder X-ray diffraction (XRD) patterns, as well as Raman and Fourier transform infrared (FTIR) spectra on a natural serandite sample. The volumetric thermal expansion coefficient αV (K−1) is determined as a linear function of T (K): 37.6(5) × 10−9 × T + 11.1(3) × 10−6, with an averaged value of 31.7(10) × 10−6 K−1, while the anisotropy of axial thermal expansivities shows the order of αa > αb > αc. The isobaric Grüneisen parameters γiP are constrained as: 0.2–1.6 for most of the lattice vibrations below 500 cm−1; while 0–0.6 for the O–Si–O bending and Si–O stretching modes inside the Si3O9 chains above 500 cm−1. As compared with common silicate phases in the upper mantle and the transition zone, the γiP parameters for the internal vibrations are significantly smaller in silicate chains (serandite, enstatite) than those in Si2O7 groups (wadsleyite) and isolated SiO4 units (forsterite, pyrope), since the rotation of the bridging O atoms (in the direction perpendicular to Si–O–Si) could relax the SiO4 tetrahedra at elevated temperature. The OH-bending vibration of serandite is observed in both Raman-active (1378 cm−1) and IR-active (1389 cm−1) modes, with a temperature-dependence of − 0.07 and − 0.09 cm−1/K, respectively. The red-shift of these OH-bending modes with increasing temperature indicates that the hydrogen bond gets weaker as the O3…O4 distance becomes larger during thermal expansion. Additionally, our polarized FTIR spectra confirm that the stretching mode of the very strong hydrogen bond, a broad absorption region (1400–3300 cm−1), is polarized in the direction of E//b, as reported by Hammer et al. (Am Mineral 83:569–576, 1998).