Compressibility of the 23 Å phase under high pressure and high temperature

A new Al-bearing high-pressure hydrous Mg-silicate named 23 angstrom phase with a hexagonal structure, which is stable in chlorite composition and could be an important water carrier in the subducting slab, was reported by Cai et al. (2015a). Here, for the first time, we determined the equation of state of the 23 angstrom phase up to 10 GPa and 1073 K by energy-dispersive in situ X-ray diffraction. Fitting the P-V data to the room temperature BirchMurnaghan equation of state yielded V-0 = 538.0(3) angstrom(3) and K-0 = 111(1) GPa by fixing K' at 4. The high temperature 3rd order Birch-Murnaghan equation of state was used to fit the P-V-T data, yielding V-0 = 538.0(3) angstrom(3), K-0 = 109(1) GPa, partial derivative K/partial derivative T = -0.012(5) GPa/K, and a(0) = 3.0(4) x 10(-5)/K by fixing K' at 4. Very little anisotropy was observed, and the axial compressibilities were 2.54(2) 10(-3)/GPa for the a axis and 2.68(5) 10(-3)/GPa for the c axis. The 23 angstrom phase has similar volume compressibility and density as phase A, indicating that the stability region (or the depth in the subduction zone) of these phases are close to each other. This is consistent with the observations of Cai et al. (2015a). Based on the present results, the potential effect of hydration on the physical properties of the subduction slab were discussed, and the possible formation enthalpy and entropy of the 23 angstrom phase were calculated.