The M-/X-asteroid menagerie: Results of an NIR spectral survey of 45 main-belt asteroids

Diagnostic mineral absorption features for pyroxene(s), olivine, phyllosilicates, and hydroxides have been detected in the near-infrared (NIR: approximately 0.75-2.50μm) spectra for 60% of the Tholen-classified (Tholen 1984, 1989) M-/X-asteroids observed in this study. Nineteen asteroids (42%) exhibit weak Band I (approximately 0.9μm) ± Band II (approximately 1.9μm) absorptions, three asteroids (7%) exhibit a weak Band I (approximately 1.05-1.08μm) olivine absorption, four asteroids (9%) display multiple absorptions suggesting phyllosilicate±oxide/hydroxide minerals, one (1) asteroid exhibits an S-asteroid type NIR spectrum, and 18 asteroids (40%) are spectrally featureless in the NIR, but have widely varying slopes. Tholen M-asteroids are defined as asteroids exhibiting featureless visible-wavelength (λ) spectra with moderate albedos (Tholen 1989). Tholen X-asteroids are also defined using the same spectral criterion, but without albedo information. Previous work has suggested spectral and mineralogical diversity in the M-asteroid population (Rivkin et al. 1995, 2000; Busarev 2002; Clark et al. 2004; Hardersen et al. 2005; Birlan et al. 2007; Ockert-Bell et al. 2008, 2010; Shepard et al. 2008, 2010; Fornasier et al. 2010). The pyroxene-bearing asteroids are dominated by orthopyroxene with several likely to include higher-Ca clinopyroxene components. Potential meteorite analogs include mesosiderites, CB/CH chondrites, and silicate-bearing NiFe meteorites. The Eos family, olivine-bearing asteroids are most consistent with a CO chondrite analog. The aqueously altered asteroids display multiple, weak absorptions (0.85, 0.92, 0.97, 1.10, 1.40, and 2.30-2.50μm) indicative of phyllosilicate ± hydroxide minerals. The spectrally featureless asteroids range from metal-rich to metal-poor with meteorite analogs including NiFe meteorites, enstatite chondrites, and stony-iron meteorites.