The colors of sillimanite

Abstract

Sillimanite occurs in three colored varieties: yellow, brown, and blue. The yellow color is characteristic of unaltered single crystals of sillimanite from high-grade metamorphic rocks and associated pegmatites. Such sillimanites contain up to 1.8 wt.% Fe_2O_3 and up to 0.3% Cr_2O_3. The yellow color is due to Fe^(3+), or in a few cases, to Cr^(3+). The ion dominant in the optical spectrum of sillimanite containing negligible Cr^(3+) is Fe^(3+) in tetrahedral coordination. However, Mössbauer data suggest that about 80% of the iron is in octahedral coordination and only 20% in tetrahedral coordination. The salient features in the optical spectrum from Fe^(3+) are absorption bands at 462, 440, and 412 nm in ɑ, 616, 474, and 438 nm in y, and 361 nm in ɑ and y. No evidence for Fe^(2+) was found. Absorption from Cr^(3+) occurs near 620 and 423 nm. The brown variety is also formed in high-grade metamorphic rocks and associated pegmatites; some brown sillimanite is chatoyant from abundant acicular inclusions oriented parallel to c. Brown sillimanite generally contains 1 or more wt.% Fe_2O_3. Optical absorption spectra of brown sillimanite include some of the iron features characteristic of yellow sillimanite and prominent bands at ~452 nm and 542 nm. These last two bands may be due to incipient exsolution of an iron-rich phase, which appears to constitute the inclusions. Blue sillimanite has been documented from crustal xenoliths in basalt at two localities and from alluvial deposits at two other localities. Blue sillimanite contains no more than 1 wt.% Fe_2O_3. The prominent absorption features, restricted to y, consist of two bands at 595 and ~836 nm. The spectra are similar to those of blue kyanite. As with kyanite, it is likely that the coloration is the result of intervalence charge transfer although the intensity of the absorption bands is not correlated with either Fe or Ti content.