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 [Ca(1.925) Fe(0.745)Al(2.265)Ti(0.004)Si(3.037)O(12)(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 V (0) = 458.8(1)angstrom(3), K (T0) = 111(3) GPa, and K' = 7.6(7). The confidence ellipse from the variance-covariance matrix of K (T0) 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: a (0) = 8.8877(7)angstrom, K (T0)(a) = 117(2) GPa, and K'(a) = 3.7(4) for the a-axis; b (0) = 5.6271(7)angstrom, K (T0)(b) = 126(3) GPa, and K'(b) = 12(1) for the b-axis; and c (0) = 10.1527(7) angstrom, K (T0)(c) = 90(1) GPa, and K'(c) = 8.1(4) for the c-axis [K (T0)(a):K (T0)(b):K (T0)(c) = 1.30:1.40:1]. The beta angle decreases with pressure, beta(P)(A degrees) = beta(P0) -0.0286(9)P +0.00134(9)P (2) (P in GPa). The evolution of axial and volume thermal expansion coefficient, alpha, with T was described by the polynomial function: alpha(T) = alpha(0) + alpha(1) T (-1/2). The refined parameters for epidote are: alpha(0) = 5.1(2) x 10(-5) K(-1) and alpha(1) = -5.1(6) x 10(-4) K(1/2) for the unit-cell volume, alpha(0)(a) = 1.21(7) x 10(-5) K(-1) and alpha(1)(a) = -1.2(2) x 10(-4) K(1/2) for the a-axis, alpha(0)(b) = 1.88(7) x 10(-5) K(-1) and alpha(1)(b) = -1.7(2) x 10(-4) K(1/2) for the b-axis, and alpha(0)(c) = 2.14(9) x 10(-5) K(-1) and alpha(1)(c) = -2.0(2) x 10(-4) K(1/2) for the c-axis. The thermo-elastic anisotropy can be described, at a first approximation, by alpha(0)(a): alpha(0)(b): alpha(0)(c) = 1 : 1.55 : 1.77. The beta angle increases continuously with T, with beta(T)(A degrees) = beta(T0) + 2.5(1) x 10(-4) T + 1.3(7) x 10(-8) T (2). A comparison between the thermo-elastic parameters of epidote and clinozoisite is carried out.