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

Abstract

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 α_V (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 α_a > α_b > α_c. The isobaric Grüneisen parameters γ_iP 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 γ_iP 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).

Appendix

See Tables 2, 3.

Table 2 Variation of the unit-cell parameters of serandite with temperature, as well as the principal axes for thermal expansion, which are normalized to those at 300 K

Table 3 The vibrational modes for serandite (v_i) at ambient condition, as compared with those for pectolite (numbers in italics below), the temperature-dependence of the modes (∂vi/∂T)_P and the isobaric mode Grüneisen parameters (γ_iT)