Vanadium- and Chromium-Bearing Andalusite: 

Occurrence and Optical-Absorption Spectroscopy

William D. Carlson
Department of Geological Sciences
University of Texas at Austin, Austin, Texas 78713, USA

George R. Rossman
Division of Geological and Planetary Sciences
 California Institute of Technolgy, Pasadena, CA 91125, USA


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 maxma 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 656 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 positions 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.

American Mineralogist 72, 1366-1369