Saponite+carbonaceous mixtures as spectral-compositional analogues for dark asteroids

Spectral (and compositional) analogues of hydrated carbonaceous chondrite (CCs) meteorites are an important material for furthering our exploration of dark/carbonaceous asteroids and possible CC parent bodies. The scientific importance of CCs is underscored by the fact that the target asteroids of the Hayabusa2, OSIRIS-REx, and Dawn missions are believed to be CC-like. To attempt to reproduce the spectral reflectance properties and spectral reflectance variations of dark blue-sloped asteroids, we produced and developed a series of analogues. Our initial results focus on simple two-component mixtures of an Mg-rich saponite (containing ~25 wt.% dolomite) and two forms of carbon (graphite and lampblack). We produced a series of mixture spanning a range of carbonaceous material abundances (-10 wt.%). We used Mg-rich saponite as the primary phyllosilicate because it is the most abundant phyllosilicate in the most hydrated CCs (e.g., Browning et al., 1993; Buseck and Hua, 1993; Zolensky et al., 1993; Howard et al., 2011). We used fine-grained amorphous carbon and graphite because both are known to induce a bluing (reflectance decreasing toward longer wavelengths) in mixtures with phyllosilicates (Cloutis et al., 2011a, 2011b). We produced a series of saponite+lampblack and saponite+graphite mixtures with carbonaceous phase abundances that encompasses (and exceeds) the range of carbonaceous phase abundances in CI1 and CM1-2 carbonaceous chondrites (Pearson et al., 2006). Our mixtures included a natural saponite, a fine-grained synthetic lampblack, and a synthetic graphite. SAP105 is a saponite sourced from Amargosa Valley, CA-NV, USA. It was provided by IMV Minerals (Lhoist North America), and is marketed under the trade name Imvite. It was supplied as a fine-grained beige powder. For opaque carbonaceous materials, we used either a fine-grained sample of synthetic carbon black (lampblack; our sample #LCA101; Johnson Matthey, #14237A; <0.021µm particle size) or a synthetic graphite (our sample #GRP102: Johnson Matthey, #10130A, -300 mesh, 99.5% pure). SAP105 was also found to contain 3.43 wt.% carbon; equivalent to ~26 wt.% dolomite if all C is present in dolomite (which was detected by XRD). Both GRP102 and LCA101 are high-purity samples. In order to produce samples with intimately-mixed phyllosilicates+opaques, we adapted a procedure developed by Hildebrand et al. (2015) for their Bennu analogues. The end members were all fine-grained (<45 µm), so no additional sample crushing was required. Approximately 50 grams of each mixture was produced. The end members were weighed out and placed into an alumina mortar and pestle and ground together for one minute to reduce clumping. The powders were then mixed with reverse osmosis (RO) water at a volumetric ratio of roughly 2:1 water:powder in a stainless steel cup with agitators. The resulting slurries were mixed together with a commercial grade drink mixer for roughly 10 minutes and then poured into aluminum pie trays with crenulated bottoms. The mixtures were then heated to 150°C in air and kept at that temperature for 4 days using a drying oven. The slurries were initially ~10 cm thick, and the heating process resulted in a very large volume loss and formation of mostly small chunks due to desiccation cracking about 1 cm thick. The resulting sample had a rough upper surface with a coating of light-colored precipitate (likely halite). The sample was separated into dry chunks (with rough upper surfaces and rough/crenulated lower surfaces). The upper portions were scraped with a razor blade to remove the salt crust and then sanded with 60 grit aluminum oxide sandpaper to produce a matte surface. Other portions of the sample were ground by hand in the alumina mortar and pestle and dry-sieved to produce <1000 and <45 µm powders after removing the salt crusts. This resulted in four different types of samples for spectral analysis: slabs with flat-rough and flat-matte surfaces, <1000 µm powders, and <45 µm powders. SEM and microscopy indicated that the lampblack was not fully dispersed, with opaque aggregates with sizes up to a few tens of microns. This is similar to the sizes of carbonaceous materials in CM chondrites (e.g., Croat et al., 2003; Amari et al., 2005); therefore that incomplete disaggregation of the lampblack more closely reproduces CC matrix textures. Results: The mixtures containing >5 wt.% carbonaceous material show the greatest similarities to dark presumed carbonaceous asteroids, exhibiting low reflectance and a variety of spectral slopes that are a function of physical properties. The most blue-sloped spectra are associated with solid surfaces. Acknowledgements: We thank Dave Rachford and IMV Minerals for providing the SAP105 sample, and Dr. Stan Mertzman of Franklin and Marshall College for the SAP105 analysis. This study was supported by CSA, NSERC, MRIF, CFI, and UWinnipeg. References Amari, C.E., et al. (2005) The micro-distribution of carbonaceous matter in the Murchison meteorite as investigated by Raman imaging. Spectrochimica Acta A, 61, 2049-2056. Browning, L.B., et al. (1993) Correlated alteration effects in CM carbonaceous chondrites. Geochimica et Cosmochimica Acta, 60, 2621-2633. Buseck, P.R., and X. Hua (1993) Matrices of carbonaceous chondrite meteorites. Annual Reviews of Earth and Planetary Science, 21, 255-305. Cloutis, E.A., et al. (2011a) Spectral reflectance properties of carbonaceous chondrites: 1. CI chondrites. Icarus, 212, 180-209. Cloutis, E.A., et al. (2011b) Spectral reflectance properties of carbonaceous chondrites: 2. CM chondrites. Icarus, 216, 309-346. Croat, T.K., et al. (2003) Structural, chemical, and isotopic microanalytical investigations of graphite from supernovae. Geochimica et Cosmochimica Acta, 67, 4705-4725. Hildebrand, A.R., et al. (2015) An asteroid regolith simulant for hydrated carbonaceous chondrite lithologies (HCCL-1). 78th Meteoritical Society Meeting; abstract #5368. Howard, K.T., et al. (2011) Modal mineralogy of CM chondrites by X-ray diffraction (PSD-XRD): Part 2. Degree, nature and settings of aqueous alteration. Geochimica et Cosmochimica Acta, 75, 2735-2751. Pearson, V.K., et al. (2006) Carbon and nitrogen in carbonaceous chondrites: Elemental abundances and stable isotopic compositions. Meteoritics and Planetary Science, 41, 1899-1918. Zolensky M. E., et al. (1993) Mineralogy and composition of matrix and chondrule rims in carbonaceous chondrites. Geochimica et Cosmochimica Acta 57, 3123-3148. Below: LCA101+SAP105 mixtures: 2 and 5 wt.% LCA101 mixtures for different sample types.

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