High Pressure — High Temperature — Synthesis and Properties of Chromium Kyanite, (Al,Cr)2SiO5

At pressures up to 30 kb and temperatures of 1000–1500°C up to 25% of the Al₃₊ in the Al₂SiO₂-polymorph kyanite could be replaced by Cr₃₊ (composition (Al_1.5Cr_0.5)SiO₅). The lattice constants a₀, b₀, c₀ increase by this substitution, whereas the triclinic angles α, β, γ remain virtually unaffected. Cr₃₊ substitutes for Al in the chains of (AlO₃₊)-octahedra parallel to c₀. The reflectance spectrum (240-1000 nm) exhibits the spin-forbidden ruby lines in addition to the spin-allowed transitions in Cr₃₊ in octahedral coordination. The half width (～3.8 kK), position, and shape of the 10 Dq band indicates a symmetry not higher than D_4h for the Cr₃₊-containing octahedra which is in accordance with structure refinement data for pure Kyanite (Burnham 1963).