Thermal equation of state of Li-rich schorl up to 15.5 GPa and 673 K: Implications for lithium and boron transport in slab subduction.

The thermal equation of state (EoS) of a natural schorl has been determined at high temperatures up to 673 K and high pressures up to 15.5 GPa using in situ synchrotron X-ray diffraction combined with a diamond-anvil cell. The pressure-volume (P-V) data were fitted to a third-order Birch-Murnaghan EoS with V₀ = 1581.45 ± 0.25 ų, K₀ = 111.6 ± 0.9 GPa, and K 0 ′ = 4.4 ± 0.2; additionally, when K 0 ′ was fixed at a value of 4, V₀ = 1581.04 ± 0.20 ų, and K₀ = 113.6 ± 0.3 GPa. The V₀ (1581.45 ± 0.25 ų) obtained by the third-order Birch-Murnaghan EoS agrees well with the V₀ (1581.45 ± 0.05 ų) measured at ambient conditions. Furthermore, the axial compression data of schorl at room temperature were fitted to a "linearized" third-order Birch-Murnaghan EoS, and the obtained axial moduli for the a- and c-axes are K_a = 621 ± 9 GPa and K_c = 174 ± 2 GPa, respectively. Consequently, the axial compressibilities are β_a = 1.61 × 10^–3 GPa^–1 and β_c = 5.75 × 10^–3 GPa^–1 with an anisotropic ratio of β_a:β_c = 0.28:1.00, indicating axial compression anisotropy. In addition, the compositional effect on the axial compressibilities of tourmalines was discussed. Fitting our pressure-volume-temperature (P-V-T) data to a high-temperature third-order Birch-Murnaghan EoS yielded the following thermal EoS parameters: V₀ = 1581.2 ± 0.2 ų, K₀ = 110.5 ± 0.6 GPa, K 0 ′ = 4.6 ± 0.2, (∂K_T/∂T)_P = –0.012 ± 0.003 GPa K^–1 and α_V0 = (2.4 ± 0.2) × 10^–5 K^–1. These parameters were compared with those of previous studies on other tourmalines, and the potential factors influencing the thermal EoS parameters of tourmalines were further discussed. [ABSTRACT FROM AUTHOR]