Vanadium- and chromium-bearing andalusite; occurrence and optical-absorption spectroscopy

A graphitic quartz-muscovite schist in the Llano uplift of central Texas bears euhedral porphyroblasts of andalusite with unusual yellow-to-colorless pleochroism. They contain, by weight, up to 0.81% V₂O₃ and 0.12% Cr₂O₃, but less than 0.07% Fe₂O₃, 0.22% TiO₂, and 0.004% Mn₂O₃. Optical absorption spectra in all principal vibration directions are dominated by an intense charge-transfer band in the ultraviolet, on which are superimposed broad absorption maxima at 420 and 565 nm (intense in α but weaker in β and γ, and broad maxima at 745 and 370 nm (without strong polarization dependence). The intensity of each of these absorption bands is quantitatively correlated with V + Cr content. Weak absorption is also observed in the γ spectrum near 320 nm. Assignment of the bands at 320, 420, and 565 nm to V³⁺ and Cr³⁺ in the octahedral site gives good agreement with data from other vanadian minerals for the relation of the absorption peak position to the mean bond length for the sites. The absorption peak at 745 nm probably results from octahedral V⁴⁺, but the origin of the 370-nm band is uncertain. The distinctive pleochroism results from the strong polarization dependence of the 420-nm absorption band.