Single-crystal
optical spectra of corundum (Al2O3)
and the Al2SiO5
polymorphs andalusite, kyanite, and sillimanite, containing both Fe2+
– Fe3+ and Fe2+ – Ti4+
intervalence charge transfer (IVCT) absorption bands were measured at
temperatures up to 1000 °C. Upon heating, thermally equilibrated IVCT
bands significantly decreased in intensity and recovered fully on
cooling. These trends contrast with the behavior of crystal field bands
at temperature for Fe, Cr and V in corundum, kyanite, and spinel. The
effects of cation diffusion and aggregation, as well as the
redistribution of band intensity at temperature, are also discussed.
The loss of absorption intensity in the visible and near-infrared
regions of the spectrum of these phases may point to a more general
behavior of IVCT in minerals at temperatures within the Earth with
implications for radiative conductivity within the Earth.