Single-crystal hydrostatic compression of synthetic pyrope, almandine, spessartine, grossular and andradite garnets at high pressures

The compression of synthetic pyrope Mg3Al2 (SiO4)(3), almandine Fe3Al2 (SiO 4)(3), spessartine Mn3Al2 (SiO4)(3) grossular Ca3Al2 (SiO4)(3) and andradit e Ca3Fe2 (SiO4)(3) was studied by loading the crystals together in a diamon d anvil cell. The unit-cell parameters were determined as a function of pre ssure by X-ray diffraction up to 15 GPa using neon as a pressure transmitti ng medium. The unit-cell parameters of pyrope and almandine were measured u p to 33 and 21 GPa, respectively, using helium as a pressure medium. The bu lk moduli, K-T0, and their first pressure derivatives, K'(T0). were simulta neously determined for all five garnets by fitting the volume data to a thi rd order Birch-Murnaghan equation of state. Both parameters can be further constrained through a comparison of volume compressions between pairs of ga rnets, giving for K-T0 and K'(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) GP a and 4.4 for grossular and 157(1) GPa and 5.1 for andradite, where the K'( T0) are fixed in the case of spessartine, grossular and andradite. Direct c omparisons of the unit-cell volumes determined at high pressures between pa irs of garnets reveal anomalous compression behavior for Mg2+ in the 8-fold coordinated triangular dodecahedron in pyrope. This agrees with previous s tudies concerning the compression behaviors of Mg2+ in 6-fold coordinated p olyhedra at high pressures. The results show that simple bulk modulus-volum e systematics are not obeyed by garnets.