Thermo-structural investigations on Beryl using Vibrational Spectroscopy and X-Ray Diffraction techniques

Molecular orientations and structural distortions are essential properties of minerals that help us understand their response under varying thermodynamic conditions of temperature (T) and pressure (P). Understanding such properties under varying thermodynamic conditions is helpful in material and geological science. The study of Beryl (Be₃Al₂(SiO₃)₆) under high temperature (high-T) aids in understanding its effect on atomic vibrations, bonding environment, and overall structural integrity. Here, we present the detailed molecular response of Beryl at high-T from 25 °C to 1200 °C probed using Raman, infrared (IR), and Terahertz-Time Domain (THz-TD) spectroscopic techniques in the 16-4000 cm^-1 wide spectral range. Upon heating above 900 °C, minor changes in the full width half maxima (FWHM) of symmetric stretch of BeO and SiO Raman active modes were observed in the 200-1200 cm^-1 spectral range. Signatures of two types of water (H₂O) molecules were observed in the IR spectra, type-I around 3699 cm^-1 and type-II at around 3596 cm^-1 and 1632 cm^-1, which disappeared upon heating around 900 °C. These subtle changes in vibrational spectra indicate minor distortions at the molecular level in Beryl at high-Ts. However, no changes were observed in the lattice region in the 16-66 cm^-1 spectral range when probed using THz-TDS. This fact is well supported by high-T X-Ray Diffraction (XRD) studies, which suggest a stable structure up to 1200 °C.