Isothermal compression of grossular garnets to 250 Kbar and the effect of calcium on the bulk modulus

The effect of pressure on the lattice parameter, and hence on the volume, of two samples of grossularite garnet (Ca3Al2Si3O12) has been measured at room temperature up to 250 kbar with the use of a diamond anvil highpressure X ray diffraction camera. The pressure to which the sample was subjected was measured by means of the NaCl internal pressure standard. A least squares fit of the pressure-volume to the Birch-Murnaghan equation yields a bulk modulus value of KO = 1.74 ± 0.04 Mbar when a (∂K/∂P)∣P=0 value of 4.25 (Halleck, 1973) is used. This KO value is in agreement with the value of 1.704 Mbar obtained by Halleck [1973], using ultrasonic techniques. A comparison of the bulk moduli of grossularite, pyrope, and almandite shows that KO∝ Vo−1 for Ca and Mg garnets as expected from static lattice models. On the other hand, KO∝ Vo−5 for the Mg-Fe garnet solid solutions. This behavior has not been explained by available theories.