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