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

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/V₀ = 1 + 2.15 (± 0.05) × 10^-5 (T - 298), V₀ = 136.52 (± 0.03) cm³ mol^-1; Phase A: V/ V₀ = 1 + 4.86 (± 0.18) × 10^-5 (T - 298), V₀ = 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 K₂₉₈ and its pressure derivative K′, obtained by fitting the compressibility data to the Murnaghan equation, are as follows: Talc: K₂₉₈ = 41.6 ± 0.9 GPa, K′ = 6.5 ± 0.4; 10-Å phase: K₂₉₈ = 32.2 ± 5.5 GPa, K′ = 9.2 ± 2.8; Phase A: K₂₉₈ = 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.