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

Abstract

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 α angle decreases whereas the β and γ angles increase with P, with the following linear trends: α(P) = α ₀ − 0.203(9)·ΔP, β(P) = β ₀ + 0.126(8)·ΔP, and γ(P) = γ ₀ + 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: 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: β _a :β _b :β _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 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 α _V(T) = α ₀(1 − 10T ^−1/2), with α _V298 K = 2.2(2) × 10⁻⁵ K⁻¹ [with V ₀ = 424.2(1) Å³ and α ₀ = 5.5(3) × 10⁻⁵ K⁻¹] and thermal anisotropic scheme α _a :α _b :α _c  = 1.20:1:2.72. By linear regression, we obtained: V(T)/V ₀ = 1 + α _0V·T = 1 + 3.1(2) × 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 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 modelled between 423 and 1,123 K by the linear regression, yielding: V(T)/V ₀ = 1 + α _0V·T = 1 + 2.15(3) × 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.