ORIGINAL PAPER Li Zhang H. Ahsbahs A. Kutoglu C.A. Geiger Single-crystal hydrostatic compression of synthetic pyrope, almandine, spessartine, grossular and andradite garnets at high pressures Received: 29 July 1998 / Revised, accepted: 7 April 1999 Abstract The compression of synthetic pyrope Mg 3 Al 2 (SiO 4 3 , almandine Fe 3 Al 2 (SiO 4 3 , spessartine Mn 3 Al 2 SiO 4 3 grossular Ca 3 Al 2 SiO 4 3 and andradite Ca 3 Fe 2 SiO 4 3 was studied by loading the crystals to- gether in a diamond anvil cell. The unit-cell parameters were determined as a function of pressure by X-ray diraction up to 15 GPa using neon as a pressure transmitting medium. The unit-cell parameters of py- rope and almandine were measured up to 33 and 21 GPa, respectively, using helium as a pressure medium. The bulk moduli, K T0 , and their ®rst pressure deriva- tives, K 0 T0 , were simultaneously determined for all ®ve garnets by ®tting the volume data to a third order Birch- Murnaghan equation of state. Both parameters can be further constrained through a comparison of volume compressions between pairs of garnets, giving for K T0 and K 0 T0 171(2) GPa and 4.4(2) for pyrope, 185(3) GPa and 4.2(3) for almandine, 189(1) GPa and 4.2 for spessartine, 175(1) GPa and 4.4 for grossular and 157(1) GPa and 5.1 for andradite, where the K 0 T0 are ®xed in the case of spessartine, grossular and andradite. Direct comparisons of the unit-cell volumes determined at high pressures between pairs of garnets reveal anomalous compression behavior for Mg 2 in the 8-fold coordi- nated triangular dodecahedron in pyrope. This agrees with previous studies concerning the compression be- haviors of Mg 2 in 6-fold coordinated polyhedra at high pressures. The results show that simple bulk modulus± volume systematics are not obeyed by garnets. Key words High pressure Single-crystal diraction Garnet á Bulk modulus Introduction Garnet is one of the major phases of Earth's upper mantle and transition zone. The high pressure garnets that are stable at mantle conditions are magnesium-rich solid solutions. It is known, however, that a variety of dierent cations can be incorporated in the garnet structure and that changes in composition aect the elastic properties. It is necessary, therefore, to study composition±bulk modulus relationships. Extensive studies have been carried out in this area, but reliable experimental results have, however, only been obtained at ambient conditions or at relatively low pressures (Babuska et al. 1978; O'Neill et al. 1989; Bass 1986; Webb 1989). An extrapolation of these data to higher pressures is required for modeling, for example, mantle densities. This is dicult because of disagreements over the ®rst pressure derivatives of the bulk moduli K 0 T 0 determined on various garnet compositions. Moreover, until recently most measurements at pressures exceeding 10 GPa were performed under nonhydrostatic condi- tions (Takahashi and Liu 1970; Weaver et al. 1976; Leger et al. 1990; Olijnyk et al. 1991). We have shown that such conditions may severely bias the measured parameters (Zhang et al. 1998; Zhang and Ahsbahs 1998). Anderson and Anderson (1970) proposed a simple bulk modulus±volume systematics, where K 0 V 0 = con- stant. It has been shown in dierent studies (Hazen et al. 1994 and references therein), however, that this simple systematics is not obeyed in the silicate garnets. Several studies (e.g. Weaver et al. 1976; Olijnyk et al. 1991) have attempted to rationalize the bulk modulus±volume ob- servations based on considerations of crystal ®eld ef- fects, bonding type of the X 2 dodecahedral cations and other structural properties. Recently, it was found in dierent mineral phases that the magnesium endmem- Phys Chem Minerals (1999) 27: 52±58 Ó Springer-Verlag 1999 Li Zhang (&) H. Ahsbahs A. Kutoglu Institute of Mineralogy, University of Marburg, Hans-Meerweinstrasse, 35032 Marburg, Germany Fax: 0049-6421-161973 C.A. Geiger Institute of Mineralogy, University of Kiel, Olshausenstrasse 40, 24098 Kiel, Germany