ABUNDANT AMORPHOUS SILICATES IN PRIMITIVECHONDRITES:IMPLICATIONSFORASTEROIDBENNU. D. L. Schrader1, T. J. McCoy1, L. P. Keller2, H. C.
Connolly Jr.3,4, K. Nakamura-Messenger2, D. S. Lauretta4, andthe OSIRIS-REx Team. 1Smithsonian Institution, Washington,
DC 20560-0119; 2ARES, NASA JSC, Houston, TX 77058, USA.
3
Kingsborough Community College, Brooklyn NY 11235 &Graduate Center of CUNY, New York, NY, USA; 4LPL, University of Arizona, Tucson, AZ 85721, USA. (schraderd@si.edu)
Introduction: Type C and B asteroids have been linked tocarbonaceous chondrites because of their similar VIS-NIR spectral properties [e.g., 1,2]. Asteroid mineralogy is inferred fromcharacteristic features in reflectance spectra obtained over a wide
spectral range [e.g., 1–4]. Identifying mineral phases and theirabundances will be critical to characterize the Type B asteroid
101955 Bennu using remote sensing data obtained during theOSIRIS-REx mission, especially for sample-site selection. To
this end, we have begun a blind study to evaluate spectral deconvolution methods of complex mixtures prepared from wellcharacterized minerals likely to occur on Bennu. However, amorphous silicates are a major component of primitive meteoritic
materials including comets and a few of the least altered carbonaceous chondrites [5–9]. High abundances of amorphous silicates
may prove key to identifying a sample site on Bennu of high science value. However, the detection of amorphous silicates inspectra is complicated because they show only a broad, featureless spectrum around 10 µm [e.g., 10] overlain by numerous
sharp bands from crystalline silicates.
Discussion: Amorphous silicates are abundant in comets andthe least thermally and aqueously altered carbonaceous chondrites (up to ~40%) [e.g., 5–9]. They are most abundant in meteorites that contain the most pristine organics [5,11]. Amorphoussilicates within these chondrites are believed to be nonequilibrium nebular condensates [6,8] and are known to be highly
susceptible to aqueous and thermal alteration [5,6]. A further
complication to spectral studies is that even in the least altered
CRs these amorphous silicates are likely hydrated [6,8,12,13].
Progressive aqueous alteration converts these hydrated amorphous silicates to phyllosilicates [5]. Despite their high abundance [5] amorphous silicates were not detected in a thermal IR
study of the least altered CRs, leading [14] to consider that theirfeatures were masked by those of crystalline phases.
We propose to identify an appropriate analog for amorphoussilicates to include in our analog mixtures in an attempt to further
our ability to identify amorphous silicates on Bennu. This may
also provide a useful parameter for identifying the degree ofaqueous alteration via spectroscopy. Testing this hypothesis requires analysis of the returned sample, which will provide
ground-truth for remote sensing data; a major goal of theOSIRIS-REx mission.
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