Pressure-enforced Cr substitution in Cr1+x Al1−x O(SiO4), synthetic analogues of kyanite

Cr3+ can substitute for Al3+ in the crystal structure of kyanite, Al2O(SiO4). Cr3+-rich solid solutions along the binary Al2O(SiO4)–Cr2O(SiO4) joint were synthesized under distinct high-pressure conditions. Sample crystals of Cr1+xAl1−xO(SiO4) 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 Cr3+-rich kyanite-type phases show a linear increase of lattice parameters a, b and c with increasing Cr3+ content. Cr3+ replaces Al3+ on the octahedrally coordinated M-sites with Cr3+ preferentially occupying large [MO6]-octahedra. Cr3+ substitution for Al3+ is highest on the M3 site in a sequence of Cr3+(M3) > Cr3+(M2) > Cr3+(M1) > Cr3+(M4) following the polyhedral volumes V(M3O6) > V(M2O6) > V(M1O6) > V(M4O6). The compressibility of Cr1.19Al0.81O(SiO4) 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 (KT0) of 196(16) GPa and a pressure derivative K′T0 of 2(4) at V0 = 310.3(1) Å3. The value of KT0 is in accordance to the Anderson–Anderson relation but it is slightly smaller than the respective value for natural Cr3+-free kyanite.