Pressure-enforced Cr substitution in Cr1+xAl1−xO(SiO4), synthetic analogues of kyanite

Abstract

Cr 3+ can substitute for Al 3+ in the crystal structure of kyanite, Al 2 O(SiO 4). Cr 3+-rich solid solutions along the binary Al 2 O(SiO 4)-Cr 2 O(SiO 4) joint were synthesized under distinct high-pressure conditions. Sample crystals of Cr 1+x Al 1−x O(SiO 4) with x = 0.025(12) and 0.188(8) have been synthesized at pressures of 7 GPa and temperatures up to 1200 °C. Unit-cell dimensions and single-crystal X-ray diffraction of the new Cr 3+-rich kyanite-type phases show a linear increase of lattice parameters a, b and c with increasing Cr 3+ content. Cr 3+ replaces Al 3+ on the octahedrally coordinated M-sites with Cr 3+ preferentially occupying large [MO 6 ]-octahedra. Cr 3+ substitution for Al 3+ is highest on the M3 site in a sequence of Cr 3+ (M3) > Cr 3+ (M2) > Cr 3+ (M1) > Cr 3+ (M4) following the polyhedral volumes V(M3O 6) > V(M2O 6) > V(M1O 6) > V(M4 O 6). The compressibility of Cr 1.19 Al 0.81 O(SiO 4) has been determined via precise lattice parameters up to 6.00(4) GPa. The pressure-volume data fitted to the third-order Birch-Murnaghan equation of state yielded an isothermal bulk modulus (K T0) of 196(16) GPa and a pressure derivative K′ T0 of 2(4) at V 0 = 310.3(1) Å 3. The value of K T0 is in accordance to the Anderson-Anderson relation but it is slightly smaller than the respective value for natural Cr 3+-free kyanite.