1 3 Phys Chem Minerals (2016) 43:649–659 DOI 10.1007/s00269-016-0824-7 ORIGINAL PAPER High-pressure behavior of natural single-crystal epidote and clinozoisite up to 40 GPa Fei Qin 1,4 · Xiang Wu 1 · Ying Wang 1 · Dawei Fan 2 · Shan Qin 1 · Ke Yang 3 · Joshua P. Townsend 4 · Steven D. Jacobsen 4 Received: 25 February 2016 / Accepted: 13 June 2016 / Published online: 27 June 2016 © Springer-Verlag Berlin Heidelberg 2016 be 0.5–0.8, typical for hydrous silicate minerals. The aver- age 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 FeO 6 and AlO 6 octahedra in M3 sites. Epidote and clinozoisite contain about 2 wt% H 2 O are thus potentially important carriers of water in sub- ducted slabs. Keywords Epidote · Clinozoisite · Synchrotron single- crystal X-ray diffraction · Compressibility Introduction Epidote-group minerals are common hydrothermal altera- tion minerals in basaltic rocks and potentially important carriers of H 2 O in subducted oceanic crust. Epidote is also a common secondary mineral in marbles and schists and occurs as a pervasive replacement in geothermal sys- tems experiencing multiple stages of epidote formation and dissolution (Bird and Spieler 2004). Monoclinic epi- dote-group minerals along the join between epidote [end member: Ca 2 Al 2 (Fe 3+ )(SiO 4 )(Si 2 O 7 )O(OH)] and clinozo- isite [end member: Ca 2 Al 3 (SiO 4 )(Si 2 O 7 )O(OH)] are typi- cally associated with low-grade metamorphism and hydro- thermal activity at 250–400 °C and 1–2 kbars. Poli and Schmidt (1998, 2004) showed that epidote and clinozoisite are stable over a wide pressure and temperature range in both continental and ocean crust in continental collisions and subduction zones (Enami et al. 2004). The composition of epidote is variable, with Fe 3+ content (i.e., mol fraction of epidote end member) depending on bulk rock and fluid composition (Bird and Spieler 2004; Shikazono 1984). Monoclinic epidote with less than 50 % mol fraction of the Abstract The comparative compressibility and high-pres- sure stability of a natural epidote (0.79 Fe-total per for- mula 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–Murna- ghan equation of state with V 0 = 461.1(1) Å 3 , K 0 = 115(2) GPa, and K ′ 0 = 3.7(2) for epidote and V 0 = 457.8(1) Å 3 , K 0 = 142(3) GPa, and K ′ 0 = 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 Grüneisen parameters (γ i ) for Si–O vibrations, which were found to * Xiang Wu xiang.wu@pku.edu.cn 1 Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, Peking University and School of Earth and Space Sciences, Peking University, Beijing 100871, China 2 Key Laboratory of High Temperature and High Pressure Study of the Earth’s Interior, Institute of Geochemistry, Guiyang 550081, China 3 Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China 4 Department of Earth and Planetary Sciences, Northwestern University, Evanston, IL 60208, USA