Mid-infrared reflectance spectroscopy of aubrite components

Aubrites Pena Blanca Spring and Norton County were studied in the mid-infrared reflectance as part of a database for the MERTIS (Mercury Radiometer and Thermal Infrared Spectrometer) instrument on the ESA/JAXA BepiColombo mission to Mercury. Spectra of bulk powder size fractions from Pena Blanca Spring show enstatite Reststrahlen bands (RB) at 9 mu m, 9.3 mu m, 9.9 mu m, 10.4 mu m, and 11.6 mu m. The transparency feature (TF) is at 12.7 mu m, the Christiansen feature (CF) at 8.1-8.4 mu m. Micro-FTIR of spots with enstatite composition in Norton County and Pena Blanca Spring shows four types: Types I and II are similar to the bulk powder spectra but vary in band shape and probably display axis orientation. Type III has characteristic strong RB at 9.2 mu m, 10.4 mu m, and 10.5 mu m, and at 11.3 mu m. Type IV is characterized by a strong RB at 10.8-11.1 mu m. Types III and IV could show signs of incipient shock metamorphism. Bulk results of this study confirm earlier spectral studies of aubrites that indicate a high degree of homogeneity and probably make the results of this study representative for spectral studies of an aubrite parent body. Spectral types I and II occur in all mineralogical settings (mineral clasts, matrix, melt, fragments in melt vein), while spectral type III was only observed among the clasts, and type IV in the melt. Comparison with surface spectra of Mercury does not obtain a suitable fit, only type IV spectra from quenched impact glass show similarity, in particular the 11 mu m feature. Results of this study will be available upon request or via the IRIS database (Munster) and the Berlin Emissivity Database (BED).