Equation of state of elbaite at high pressure up to 21.1 GPa and room temperature

The equation of the state of a natural elbaite sample has been investigated at room temperature and up to 21.1 GPa for the first time using in situ synchrotron X-ray diffraction in this study. No phase transition is observed on elbaite over the experimental pressure range. The pressure–volume data were fitted by the third-order Birch-Murnaghan equation of state (EoS) with the zero-pressure unit-cell volume V0 = 1540.7 (6) Å3, the zero-pressure bulk modulus KT0 = 114.7 (7) GPa, and its pressure derivative K'T0 = 4.2 (1), while obtained V0 = 1540.1 (4) Å3 and KT0 = 116.4 (4) GPa when fixed K'T0 = 4. Furthermore, the axial compressional behavior of elbaite was also fitted with a linearized third-order Birch-Murnaghan EoS, the obtained axial moduli for a-axis and c-axis are Ka0 = 201 (4) GPa and Kc0 = 60 (1) GPa, respectively. The axial compressibilities of a-axis and c-axis are βa = 1.66 × 10–3 GPa−1 and βc = 5.56 × 10–3 GPa−1 with an anisotropic ratio of βa: βc = 0.30: 1.00, which shows an intense axial compression anisotropy. The potential influencing factors on the bulk moduli and the anisotropic linear compressibilities of tourmalines were further discussed.