THERMAL EXPANSIVITIES AND COMPRESSIBILITIES OF HYDROUS PHASES IN THE SYSTEM MGO-SIO2-H2O - TALC, PHASE-A AND 10-ANGSTROM PHASE

We have measured the thermal expansivity of talc, Mg3Si4O10(OH)(2), an d phase A, Mg7Si2O8(OH)(6), and the compressibility of talc, phase A a nd 10-Angstrom phase, Mg3Si4O10(OH)(2) . xH(2)O, using powder X-ray di ffraction. The thermal expansivity of talc and phase A were measured a t temperatures up to 810 degrees C and 600 degrees C, respectively. Vo lumes of both phases increase linearly with temperature, and can be de scribed as follows: Talc: Y/V-0 = 1 + 2.15 (+/- 0.05) x 10(-5) (T - 29 8), V-0 = 136.52 (+/- 0.03) cm(3) mol(-1); Phase A: V/V-0 = 1 + 4.86 ( +/- 0.18)x 10(-5) (T - 298), V-0 = 154.42 (+/- 0.09) cm(3) mol(-1). Co mpressibility measurements of talc, 10-Angstrom 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 equatio n, are as follows: Talc: K-298 = 41.6 +/- 0.9 GPa, K ' = 6.5 +/- 0.4; 10-Angstrom 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 ta lc data with existing thermodynamic data provides a more accurate ther modynamic description of talc than previously available, enabling its high-pressure, high-temperature phase relations to be calculated. The data for 10-Angstrom phase are consistent with a positive slope for it s dehydration reaction, making 10-Angstrom phase a good candidate for H2O storage in subducting slabs. The measurements of the thermal expan sivity and compressibility of phase A allow its enthalpy of formation and entropy to be derived from the results of phase equilibrium experi ments on phase A.