Thermal expansivities and compressibilities of hydrous phases in the system MgO–SiO₂–H₂O: talc, phase A and 10-Å phase

Abstract

 We have measured the thermal expansivity of talc, Mg₃Si₄O₁₀(OH)₂, and phase A, Mg₇Si₂O₈(OH)₆, and the compressibility of talc, phase A and 10-Å phase, Mg₃Si₄O₁₀(OH)₂ ⋅ xH₂O, using powder X-ray diffraction. The thermal expansivity of talc and phase A were measured at temperatures up to 810° C and 600° C, respectively. Volumes of both phases increase linearly with temperature, and can be described as follows: Talc: V/V ₀=1+2.15 (±0.05)×10^-5 (T−298), V ₀=136.52 (±0.03) cm³ mol^-1; Phase A: V/V ₀=1+4.86 (±0.18)×10^-5 (T– 298), V ₀=154.42 (±0.09) cm³ mol^-1. Compressibility measurements of talc, 10-Å phase and phase A were made at pressures up to 6.05, 8.52 and 9.85 GPa, respectively. Values of the isothermal bulk modulus K ₂₉₈ and its pressure derivative K′, obtained by fitting the compressibility data to the Murnaghan equation, are as follows: Talc: K ₂₉₈=41.6±0.9 GPa, K′=6.5±0.4; 10-Å phase: K ₂₉₈=32.2±5.5 GPa, K′=9.2±2.8; Phase A: K ₂₉₈=145±5 GPa (assuming that K′=4). Combining the new talc data with existing thermodynamic data provides a more accurate thermodynamic description of talc than previously available, enabling its high-pressure, high-temperature phase relations to be calculated. The data for 10-Å phase are consistent with a positive slope for its dehydration reaction, making 10-Å phase a good candidate for H₂O storage in subducting slabs. The measurements of the thermal expansivity and compressibility of phase A allow its enthalpy of formation and entropy to be derived from the results of phase equilibrium experiments on phase A.