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THE ORIGIN OF COLOR IN "FIRE" OBSIDIAN

Chi Ma^1,, George R. Rossman¹ and James A. Miller²

¹ Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.
² GeoEngineers, Inc., Redmond, Washington 98052, U.S.A.

E-mail address: chi{at}gps.caltech.edu

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).

Keywords: "fire" obsidian, nanometric magnetite, thin-film interference, Glass Buttes, Oregon.