Spectroscopy of B-type asteroids: Subgroups and meteorite analogs

B-type asteroids have a negative slope from similar to 0.5 to similar to 1.1 mm and beyond. What causes this? Visible to near-infrared reflectance spectra (0.4-2.5 mm) are assembled for 22 B-type asteroids. The spectra fall naturally into three groups: (1) those with negative (blue) spectral shapes like 2 Pallas (7 objects), (2) those with concave curve shapes like 24 Themis (11 objects), and (3) everything else (4 objects). The asteroid spectra are compared to mineral and meteorite spectra from the Reflectance Experiment Laboratory library of 15,000 samples, in a least squares search for particulate analogs, constrained by spectral brightness. The Pallas group objects show a trend of analogs from the CV, CO, and CK meteorite groups. Only three of the seven Pallas-like objects are determined to be dynamically related (2, 1508, and 6411). The Themis group objects show a trend of analogs from the CI, CM, CR, CI-Unusual, and CM-Unusual meteorites (as expected from the work of Hiroi et al. (1996)). Seven of the 11 Themis-like objects are dynamically related (24, 62, 222, 316, 379, 383, and 431). Allowing for reasonable uncertainties in the spectral matches, we find no need to invoke mineralogies that do not exist in the meteorite collection to explain B-type spectra or their negative slopes. Our Themis group results are as expected and are consistent with previous work, but our Pallas group results are new and, in some cases, in conflict with previous work.