The 2.5-5.1 μm reflectance spectra of HED meteorites and their constituent minerals: Implications for Dawn

For all of the HEDs, with the exception of PRA 04401, a CM xenolith-bearing howardite, the <45 μm fraction has the highest overall reflectance. The <45 μm HED spectra generally have the shallowest absorption bands compared to the larger grain sizes, with the exception of the diogenites. Relative band depths change with grain size, likely due to different bands saturating at different grain sizes, leading to merging of adjacent absorption bands (causing movement of apparent band minima) and changes from resolvable absorption bands to inflections. The number of bands that appear in the HED reflectance spectra varies with both composition and grain size. In general, weak absorption bands become more resolvable in slab and coarse grain spectra, but multiple diagnostic absorption bands are present for all classes of HED meteorites regardless of grain size. Because these 4-5 μm region absorption bands are due to different mechanisms than those responsible for shorter wavelength absorption bands (e.g., Fe2+ crystal field transitions), they provide complementary information on the composition of HEDs and the surface of Vesta. Importantly, these longer wavelength bands can provide direct evidence for the presence of plagioclase feldspar, which is difficult to derive from shorter wavelength observations. Finally, we have found that HEDs and their constituent minerals exhibit even stronger absorption bands just beyond the range of the Dawn VIR spectrometer, from 5.15 to 5.60 μm, that are also attributable to Si-(Al-)O overtones.