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
ABSTRACT
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