Equations of state for Fe32+Fe23+Si3O12 "skiagite" garnet and Fe2SiO4-Fe3O4 spinel solid solutions

The equations of state of Fe32+Fe23+Si3O12, or "skiagite" garnet, and two F e2SiO4-Fe3O4 spinel solid solutions were determined from room temperature h ydrostatic compression experiments up to 11 GPa performed at the European S ynchrotron Radiation Facility. These data indicate that there are no first- order phase transitions or changes in compression mechanism in skiagite gar net or Fe3+-bearing silicate spinel solid solutions up to Il GPa at room te mperature. Skiagite garnet has an isothermal bulk modulus K-0T= 157.4(3.0) Cpa and a pressure derivative of the isothermal bulk modulus K'(0T)=6.7(8), where numbers in parentheses represent 1 estimated standard deviation of t hese values. Combination of this result with those for other silicate garne ts reveals an inverse linear relationship between the octahedral cation-oxy gen (Y-O) bond length and the bulk modulus. The dominating effect of the oc tahedral site on the bulk compression of garnet can be understood by consid ering that the octahedral-tetrahedral network has no rigid unit modes and t hat no cation substitution occurs on the tetrahedral sites in these garnets . It is apparent that the incorporation of Fe3+ and Cr acts to lower the bu lk modulus of aluminosilicate garnets. Consideration of the effect of compr ession on the molar volume results in nonsystematic changes in calculated o xygen fugacity for garnet-bearing mantle peridotites that equilibrated at h igh pressures. The pressure-volume data obtained from two Fe3+-bearing sili cate spinel solid solutions with compositions X-fay = 0.45 and X-fay = 0.57 (fay, fayalite component) yielded similar enough; values of K-0T and K'(0T ) to warrant a combined refinement with the data points scaled to the appro priate zero-pressure volumes. This gave K-0T = 168.9(1.2) GPa and K'(0T) = 5.7(1.2) with chi(W)(2) = 4.6 (weighted chi(2)) and K-0T = 175.5(1.4) GPa w hen K'(0T) is fixed equal to 4 (chi(W)(2) = 4.9), Our solid solutions have significantly lower bulk moduli than either magnetite or Fe2SiO4 spinel end -members or indeed many other spinels.