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The colors of sillimanite

Rossman, George R. and Grew, Edward S. and Dollase, W. A. (1982) The colors of sillimanite. American Mineralogist, 67 (7-8). pp. 749-761. ISSN 0003-004X. http://resolver.caltech.edu/CaltechAUTHORS:20130514-084541722

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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.


Item Type:Article
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http://www.minsocam.org/msa/collectors_corner/amtoc/toc1982.htmPublisherUNSPECIFIED
Additional Information:© 1982 Mineralogical Society of America. Manuscript received, January 6, 1982; accepted for publication, March 18, 1982. Grew thanks members of the 18th and 19th Soviet Antarctic Expeditions (1973-74) and of the 1977-78 and 1979-80 Australian National Antarctica Research Expeditions for providing logistic support for field work in Antarctica. Grew also thanks P. C. Grew for assistance in collecting xenoliths at Kilbourne Hole and Bournac. We acknowledge the following people and institutions for their generosity in providing us with samples for this study: C. Frondel-Harvard University; J. S. White, Jr.-The U. S. National Museum (Smithsonian Institution); J. P. Fuller-The British Museum; J. Fabries-The Museum d'Histoire Naturelle (Minéralogie); G. E. Harlow-the American Museum of Natural History; U. Hálenius, E. R. Padovani and F. H. Pough. David Veblen provided electron microscopy results; K. Langer and R. G. Burns provided helpful discussion of our results. This work was funded in part by National Science Foundation grants DPP 75-17390 and DPP 76-80957 to UCLA, and EAR 79-04801 to Caltech; electron microscopy studies were funded by grant EAR 81-15790 to Arizona State University.
Funders:
Funding AgencyGrant Number
NSFDPP 75-17390
NSFDPP 76-80957
NSFEAR 79-04801
NSFEAR 81-15790
Record Number:CaltechAUTHORS:20130514-084541722
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20130514-084541722
Official Citation:The colors of sillimanite George R. Rossman, Edward S. Grew and W. A. Dollase 749-761, 1982
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:38479
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:14 May 2013 16:14
Last Modified:14 May 2013 16:14

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