The ungrouped achondrite Northwest Africa (NWA) 7325: Spectral reflectance properties and implications for parent body identification

摘要

We have measured reflectance spectra (0.35-25.0 mu m) of different size powders of the ungrouped achondrite NWA 7325 in order to facilitate spectroscopic identification of its parent body. Previous work has suggested that the meteorite may have come from the planet Mercury based on its oxidation state. The 0.35-2.5 mu m reflectance spectra of NWA 7325 exhibit absorption bands that can be attributed to the presence of chromium-bearing diopside and possibly to Ca-rich plagioclase. Spectral evidence for olivine is generally lacking, likely due to interference from stronger diopside absorption bands. With increasing grain size, albedo decreases while absorption band depths increase. The absorption bands are unique in the sense that they allow for the identification of the Cr-rich diopside in NWA 7325. The mid-infrared spectra are similar to those measured by previous investigators, and enable detection of the major silicates in NWA 7325, including more robust identification of olivine and plagioclase feldspar. We find no spectroscopic or compositional evidence supporting a link to Mercury as a possible parent body, even accounting for plausible spectrum-altering processes. In terms of a link to an asteroidal parent body, the most confident link would be made based on the unique Cr-diopside-related absorption bands in the 0.65, 1.05, and 2.3 mu m regions. At present, the closest spectral match we have found is with asteroid 10,537 (1991 RY16). (C) 2018 Elsevier Inc. All rights reserved.