Behavior of epidote at high pressure and high temperature: a powder diffraction study up to 10 GPa and 1,200 K

The thermo-elastic behavior of a natural epidote [Ca1.925 Fe0.745Al2.265Ti0.004Si3.037O12(OH)] has been investigated up to 1,200 K (at 0.0001 GPa) and 10 GPa (at 298 K) by means of in situ synchrotron powder diffraction. No phase transition has been observed within the temperature and pressure range investigated. P-V data fitted with a third-order Birch-Murnaghan equation of state (BM-EoS) give V0 = 458.8(1)Å3, KT0 = 111(3) GPa, and K′ = 7.6(7). The confidence ellipse from the variance-covariance matrix of KT0 and K′ from the least-square procedure is strongly elongated with negative slope. The evolution of the "Eulerian finite strain" vs "normalized stress" yields Fe(0) = 114(1) GPa as intercept values, and the slope of the regression line gives K′ = 7.0(4). The evolution of the lattice parameters with pressure is slightly anisotropic. The elastic parameters calculated with a linearized BM-EoS are: a0 = 8.8877(7) Å, KT0(a) = 117(2) GPa, and K′(a) = 3.7(4) for the a-axis; b0 = 5.6271(7) Å, KT0(b) = 126(3) GPa, and K′(b) = 12(1) for the b-axis; and c0 = 10.1527(7) Å, KT0(c) = 90(1) GPa, and K'(c) = 8.1(4) for the c-axis [KT0(a):KT0(b):KT0(c) = 1.30:1.40:1]. The β angle decreases with pressure, βP(°) = βP0 -0.0286(9)P +0.00134(9)P2 (P in GPa). The evolution of axial and volume thermal expansion coefficient, α, with T was described by the polynomial function: α(T) = α0 + α1T-1/2. The refined parameters for epidote are: α0 = 5.1(2) × 10-5 K-1 and α1 = -5.1(6) × 10-4 K1/2 for the unit-cell volume, α0(a) = 1.21(7) × 10-5 K-1 and α1(a) = -1.2(2) × 10-4 K1/2 for the a-axis, α0(b) = 1.88(7) × 10-5 K-1 and α1(b) = -1.7(2) × 10-4 K1/2 for the b-axis, and α0(c) = 2.14(9) × 10-5 K-1 and α1(c) = -2.0(2) × 10-4 K1/2 for the c-axis. The thermo-elastic anisotropy can be described, at a ry α0(a): α0(b): α0(c) =1'G.D:1.55:1.77. The β angle increases continuously with T, with βT(°) = βT0 + 2.5(1) × 10-4T + 1.3(7) × 10-8T2. A comparison between the thermo-elastic parameters of epidote and clinozoisite is carried out.