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. Theα angle decreases whereas the β and γ angles increase with P, with the following linear trends: α(P) = α0 − 0.203(9)·ΔP, β(P) = β0 + 0.126(8)·ΔP, and γ(P) = γ0 + 0.109(5)·ΔP (angles in ° and P in GPa). P–V data fits with isothermal Murnaghan and third-order Birch-Murnaghan Equations of State yield: KT0 = 47(3) GPa and K′ = 6.6(14) for the M-EoS fit, KT0 = 47(4) GPa and K′ = 7.3(19) for a III-BM-EoS fit,with the following anisotropic compressional scheme: βa:βb:βc = 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 α angle shows a slight decrease whereas the β and γ 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 αV(T) = α0(1 − 10T−1/2), with αV298 K = 2.2(2) × 10−5 K−1 [with V0 = 424.2(1) Å3and α0 = 5.5(3) × 10−5 K−1] and thermal anisotropic scheme αa:αb:αc = 1.20:1:2.72. By linear regression, we obtained: V(T)/V0 = 1 + α0V·T = 1 + 3.1(2) × 10−5 (T − T0). 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 firststage of the T-induced de-hydroxylation. Between 423 and 1,123 K, the β angle decreases in response to the applied temperature;all the unit-cell edges show a monotonic increase. The volume expansion coefficient of talc was modelledbetween 423 and 1,123 K by the linear regression, yielding: V(T)/V0 = 1 + α0V·T = 1 + 2.15(3) × 10−5 (T − T0). Thecomparative elastic analysis of pyrophyllite and talc, using the data obtained in this and in previous studies, shows thatpyrophyllite is more compressible and more expandable than talc