Effect of isovalent Si,Ti substitution on the bulk moduli of Ca(Ti1-xSix)SiO5 titanites

The equations of state of A2/a titanite phases of CaTiSiO5, Ca(Ti0.5Si0.5)S iO5, and CaSi2O5 have been determined from high-pressure X-ray diffraction measurements. The isothermal bulk moduli are K-0,K-T = 131.4(7) GPa (for P > 3.6 GPa), 151.9(1.6) GPa, and 178.2(7) GPa, respectively, for a second or der Birch-Murnaghan equation-of-state (i.e., with K' = partial derivative K /partial derivative P = 4). Refinements of third order equations-of-state y ielded values of K' that did not differ significantly from 4. The complete substitution of Si for Ti in the octahedral sites of the titanite structure , therefore, results in approximately a 30% increase in bulk modulus and a 13% increase in density. The large stiffening of the structure can be attri buted to the absence of rigid-unit modes from the structure and the direct involvement of cation-cation interactions in the compression of the structu re.