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The origin of color in “fire” obsidian

Ma, Chi and Rossman, George R. and Miller, James A. (2007) The origin of color in “fire” obsidian. Canadian Mineralogist, 45 (3). pp. 551-557. ISSN 0008-4476.

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A variety of obsidian from Glass Buttes, Oregon, known as “fire” obsidian and named for thin layers showing various colors, was investigated with field-emission scanning electron microscopy, X-ray energy-dispersive spectroscopy, electron back-scatter diffraction, and optical spectroscopy methods. Our study reveals that the thin layers mainly consist of concentrated nanometric crystals of magnetite. The thin layers, which have a thickness of 300 to 700 nm, give rise to brilliant colors in reflection. The color is caused by thin-film optical interference, in which the thin layers have a higher calculated index of refraction (1.496 < n < 1.519) than that of the host glass (n = 1.481).

Item Type:Article
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Ma, Chi0000-0002-1828-7033
Rossman, George R.0000-0002-4571-6884
Additional Information:© 2007 Mineralogical Association of Canada. Received January 6, 2006; revised manuscript accepted September 15, 2006. FE–SEM, EDS and EBSD analyses were done at the Caltech GPS analytical facility, which is supported in part by the MRSEC Program of the NSF under DMR–0080065. Additional support was provided to G.R.R. through NSF grant EAR–0337816. A special “thank you” is extended to Nadine and Don Carey, owners of the fire obsidian claim, who granted permission to collect samples. The photos of fire obsidian shown in Figure 1 were supplied by Steve Sayer for this publication.
Subject Keywords:“fire” obsidian, nanometric magnetite, thin-film interference, Glass Buttes, Oregon
Issue or Number:3
Record Number:CaltechAUTHORS:MACcm07
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:9084
Deposited By: Archive Administrator
Deposited On:26 Oct 2007
Last Modified:09 Mar 2020 13:19

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