High-pressure behavior of natural single-crystal epidote and clinozoisite up to 40 GPa

The comparative compressibility and high-pressure stability of a natural epidote (0.79 Fe-total per formula unit, Fe-tot pfu) and clinozoisite (0.40 Fe-tot pfu) were investigated by single-crystal X-ray diffraction and Raman spectroscopy. The lattice parameters of both phases exhibit continuous compression behavior up to 30 GPa without evidence of phase transformation. Pressure-volume data for both phases were fitted to a third-order Birch-Murnaghan equation of state with V (0) = 461.1(1) (3), K (0) = 115(2) GPa, and = 3.7(2) for epidote and V (0) = 457.8(1) (3), K (0) = 142(3) GPa, and = 5.2(4) for clinozoisite. In both epidote and clinozoisite, the b-axis is the stiffest direction, and the ratios of axial compressibility are 1.19:1.00:1.15 for epidote and 1.82:1.00:1.19 for clinozoisite. Whereas the compressibility of the a-axis is nearly the same for both phases, the b- and c-axes of the epidote are about 1.5 times more compressible than in clinozoisite, consistent with epidote having a lower bulk modulus. Raman spectra collected up to 40.4 GPa also show no indication of phase transformation and were used to obtain mode Gruneisen parameters (gamma (i)) for Si-O vibrations, which were found to be 0.5-0.8, typical for hydrous silicate minerals. The average pressure coefficient of Raman frequency shifts for M-O modes in epidote, 2.61(6) cm(-1)/GPa, is larger than found for clinozoisite, 2.40(6) cm(-1)/GPa, mainly due to the different compressibility of FeO6 and AlO6 octahedra in M3 sites. Epidote and clinozoisite contain about 2 wt% H2O are thus potentially important carriers of water in subducted slabs.National Natural Science Foundation of China [U1232204, 41473056]; Chinese Scholarship Council; US National Science Foundation [EAR-1452344]; David and Lucile Packard Foundation; COMPRES under NSF [EAR 11-57758]; GSECARS through NSF Grant [EAR-1128799]; DOE [DE-FG02-94ER14466]SCI(E)ARTICLExiang.wu@pku.edu.cn9649-6594