Elastic behaviour and phase stability of pyrophyllite and talc at high pressure and temperature

The compressional behaviour of (triclinic) pyrophyllite-1Tc was investigated by means of in situ synchrotron single-crystal diffraction up to 6.2 GPa (at room temperature) using a diamond anvil cell. Its thermal behaviour was investigated by in situ synchrotron powder diffraction up to 923 K (at room pressure) with a furnace. No evidence of phase transition has been observed within the pressure range investigated. The alpha angle decreases whereas the beta and gamma angles increase with P, with the following linear trends: alpha(P) = alpha (0) - 0.203(9)center dot Delta P, beta(P) = beta (0) + 0.126(8)center dot Delta P, and gamma(P) = gamma (0) + 0.109(5)center dot Delta P (angles in A degrees and P in GPa). P-V data fits with isothermal Murnaghan and third-order Birch-Murnaghan Equations of State yield: K (T0) = 47(3) GPa and K' = 6.6(14) for the M-EoS fit, K (T0) = 47(4) GPa and K' = 7.3(19) for a III-BM-EoS fit, with the following anisotropic compressional scheme: beta (a) :beta (b) :beta (c) = 1.06:1:4.00. The evolution of the "Eulerian finite strain" versus "normalized stress" leads to: Fe(0) = 47(3) GPa as intercept value and regression line slope with K' = 7.1(18). A drastic and irreversible change of the thermal behaviour of pyrophyllite-1Tc was observed at 700 < T < 850 K, likely ascribable to the first stage of the T-induced de-hydroxylation. Between 298 and 700 K, the alpha angle shows a slight decrease whereas the beta and gamma angles tend to be unaffected in response to the applied temperature; all the unit-cell edges show a monotonic increase. The axial and volume thermal expansion coefficients of pyrophyllite were modelled between 298 and 773 K following the equation alpha (V)(T) = alpha (0)(1 - 10T (-1/2)), with alpha (V298 K) = 2.2(2) x 10(-5) K-1 [with V (0) = 424.2(1) (3) and alpha (0) = 5.5(3) x 10(-5) K-1] and thermal anisotropic scheme alpha (a) :alpha (b) :alpha (c) = 1.20:1:2.72. By linear regression, we obtained: V(T)/V (0) = 1 + alpha (0V)center dot T = 1 + 3.1(2) x 10(-5) (T - T (0)). The thermal behaviour of talc-1Tc was investigated by in situ synchrotron powder diffraction up to 1,173 K (at room-P) with a furnace. At 423 K, the diffraction pattern was indexable with a monoclinic unit-cell but with a doubling of the c-axis (as expected for the 2M-polytype). At T > 1,123 K, an irreversible transformation occurs, likely ascribable to the first stage of the T-induced de-hydroxylation. Between 423 and 1,123 K, the beta angle decreases in response to the applied temperature; all the unit-cell edges show a monotonic increase. The volume expansion coefficient of talc was modelled between 423 and 1,123 K by the linear regression, yielding: V(T)/V (0) = 1 + alpha (0V)center dot T = 1 + 2.15(3) x 10(-5) (T - T (0)). The comparative elastic analysis of pyrophyllite and talc, using the data obtained in this and in previous studies, shows that pyrophyllite is more compressible and more expandable than talc.