The Constituent Minerals in Calcium-Aluminum Inclusions: Spectral Reflectance Properties and Implications for CO Carbonaceous Chondrites and Asteroids

Abstract

The reflectance spectra of the major constituent minerals of calcium aluminum inclusions (CAIs) have been measured to determine which minerals contribute to CAI reflectance spectra. Spinel appears to be the most significant phase because of its widespread presence in different classes of inclusions and chondrules and its higher ferrous iron content compared to other inclusion phases. Spectra of CAI phases such as perovskite and grossular are nearly featureless spectrally, while melilite, fassaite, and plagioclase feldspar are more weakly featured than spinel. Measured reflectance spectra of CAIs indicate a widespread and generally intense absorption feature in the 2-μm region attributable to minerals containing ferrous iron in tetrahedrally coordinated sites, primarily spinel. CO carbonaceous chondrite reflectance spectra exhibit the clearest spectral evidence for CAI absorption features, suggesting that asteroids with CO type surface mineralogies (or ∼10% CAIs on their surfaces) may be spectrally detectable if the CAIs are optically isolated from the dark matrix. Telescopic reflectance spectra of the K class asteroid 221 Eos show no clear evidence for CAI absorption features. Telescopic reflectance spectra of the anomalous S class asteroids 387 Aquitania and 980 Anacostia can be interpreted as indicating the presence of a chondritic assemblage enriched in minerals which constitute CAIs, such as spinel, or as achondritic chromite-rich assemblages similar to some unique meteorites.