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Published May 1986 | public
Journal Article

The color of meteoritic hibonite: an indicator of oxygen fugacity


Hibonites similar in composition to those found in Ca-Al-rich inclusions change color from blue, to green, to orange, to nearly colorless as oxygen fugacity is increased at high temperature from below the iron-wustite buffer up to air. The development of the blue color is correlated with the growth of an absorption band at 715 nm in the optical spectra of the hibonites as the oxygen fugacity is reduced. The growth of this band is attributed to the increasing concentration of Ti^(3+) in these hibonites with decreasing oxygen fugacity. The blue hibonites in meteorites reflect equilibration under reducing conditions; we estimate, based on the intensity of the 715 nm band, that the hibonite in the Blue Angel inclusion indicates an oxygen fugacity 4–5 orders of magnitude more oxidizing than that expected in the early solar nebula. This may be due to formation in an anomalously oxidizing region of the nebula or to oxidation during cooling or later alteration. The orange hibonites in Allende reflect oxygen fugacities approximately ten or more orders of magnitude more oxidizing than the expected primitive nebula; this color probably indicates alteration of initially more reduced (blue?) hibonites. The colorless hibonite in the HAL inclusion reflects highly oxidizing conditions and/or its low Ti content.

Additional Information

© 1986 Elsevier Science Publishers B.V. Received 9 September 1985; Revised 18 February 1986. The idea of examining the origin of color in meteoritic hibonites and its relation to oxygen fugacity came out of a discussion with Professor G.R. Rossman and Dr. J.T. Armstrong. The encouragement and comments of these two scientists are greatly appreciated. Information obtained from J. Beckett, W.V. Boynton, B. Fegley, I.D. Hutcheon, D. Live, G. MacPherson, D. McIntyre, and J. Paque has been very helpful. Reviews by J. Beckett, T.L. Grove, S. Haggerty, and G. MacPherson are appreciated. Thin sections were generously loaned to us by L. Grossman and G.J. Wasserburg. This work was begun under an Undergraduate Summer Internship in the Division of Geological and Planetary Sciences, Caltech, and formed the basis of an Honors Thesis submitted to Pomona College. This work was supported by NASA grants NAGW-257 and NAG 9-105. Caltech Division of Geological and Planetary Sciences Contribution Number 4270.

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