The Reflectance Spectrum of Troilite and the T-Type Asteroids

Details The Reflectance Spectrum of Troilite and the T-Type Asteroids The Reflectance Spectrum of Troilite and the T-Type Asteroids

Jul 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.207b&link_type=abstract

Meteoritics, vol. 27, no. 3, volume 27, page 207

Computer Science

8

Scientific paper

Troilite (stoichiometric FeS) is a common mineral in most meteorites, but meteorite spectroscopists have neglected to measure its spectral properties and consider its possible role in interpretation of asteroid spectra. Ordinary chondrites are typically 5-6 wt% troilite and this mineral is present in almost all iron meteorites in amounts up to 60%. Troilite's occurrence in meteorites is typically as 0.1-1.0 mm-sized blebs in stony meteorites and cm-sized nodules in iron and stony-iron meteorites [1]. Meteoritical and theoretical evidence strongly suggests that there should be troilite-rich zones in the interiors of differentiated asteroids. The cores of differentiated asteroids probably contained a few wt% S which was largely concentrated in the final 5-10 vol% of eutectic liquid. This liquid crystallized as troilite (90 vol%) and metal. An asteroid derived from a metal core would probably display sections of crystallized eutectic liquid [2] provided that the asteroid is not covered with regolith. The distribution of troilite on the surface of metallic asteroids may therefore provide information about the crystallization history of the core. A fraction of the S may, however, become trapped in the dendrites during crystallization [3]. This would account for the abundance of troilite nodules in iron meteorites. The troilite distribution in the core may also affect the way the core breaks up during impacts. Fractures will preferentially propagate through the much more friable troilite and large core fragments may therefore have dendritic shapes. Regolith present on metal cores may also be enriched in the more friable troilite. Material of the expected eutectic composition would be very fragile, and collisional and/or atmospheric disruption may account for its absence among meteorites. Measurements: The bidirectional reflectance spectrum of troilite was measured from a sample of the Mundrabilla iron meteorite held in the collection of the University of Hawaii. This meteorite has an unusually high sulfur content (8 wt%) and total troilite content is estimated at 25-35 vol%. Average troilite composition in weight % is as follows: 63% Fe, 0.5% Cr, 0.3% Zn, and 36.2% S [4]. The sample was crushed in a clean ceramic mortar and pestle to a bulk powder and dry sieved to a particle size of <250 micrometers. Six additional particle size separates were dry sieved from this bulk sample. Shown in Figure 1 are the spectra of the bulk sample and the particle size separates of Mundrabilla troilite. The spectrum of the bulk material is dark, always less than 10% reflective, and strongly red sloped. The rapid increase in reflectance at the green and red wavelengths (0.4-0.5 microns) is probably responsible for the overall bronze color of hand sample troilite. Since Mundrabilla is a find, the depth of the UV-visible absorption may have been increased by small amounts of Fe3+ from terrestrial rust. Additional samples of troilite from fresh fall need to be measured to confirm this result. The bulk sample has a reflectance between the smallest and largest particle size separates suggesting that its reflectance is dominated by small particles coating larger grains. Previous work with spectral mixture modelling shows that small particle size troilite and metal can dominate the spectra of ordinary chondrite meteorites, producing a dark, subdued and reddened spectrum similar to some dark asteroids [5]. Implications for Asteroids: The strong red slopes and low reflectances of the troilite spectra are similar to the spectral characteristics of the T and possibly some M-class asteroids. Shown in Figure 2 are the spectra of bulk troilite (solid lines) and four T-class asteroids (boxes and error bars). The IR spectra of 96 Aegle, 114 Kassandra, and 233 Asterope are strongly similar to the spectrum of bulk troilite. The deeper W absorption in troilite may be due to terrestrial rust. The spectrum of 308 Polyxo is substantially different, but Polyxo is also the only T-class asteroid that has been shown to have strong water of hydration features at 3.0 micrometers. The heliocentric distribution of T-asteroids are similar to that of the M-class, but their albedos are substantially less with an average IRAS albedo for T-asteroids of 0.066. T-class asteroids may represent a stage in the collisional evolution of the metallic cores of differentiated asteroids during which the upper, troilite-rich zones of the core are exposed and the friable troilite dominates the regolith. The abundant troilite would produce the dark, red- sloped reflectance spectrum characteristic of T-asteroids. Later stages of collisional evolution may also produce troilite-rich regoliths if cores fracture along zones of dendrite formation. Some M-class asteroids are almost certainly rich in FeNi metal, and troilite is almost certainly present in their regoliths. Variations in troilite abundance and particle size may account for some of the variations in slope and albedo seen in this class. Finally, the S-class asteroids may contain significant amounts of troilite which may account for some of the red slope common in that class. References: [1] Dodd R.T. (1981) Meteorites. [2] Haack H. and Scott E.R.D. (1992) Asteroid core crystallization by inwards dendritic growth. Submitted to JGR (Planets). [3] Kracher et al. (1977) Geochem. J. 11, 207-217. [4] Buchwald V.F. (1975) Iron Meteorites. [5] Britt D.T. (1991) Ph.D. Thesis, Brown University; Britt D.T. and Pieters C.M. (1990) LPS XXI, 127-128. [6] Bell J.F. et al., (1990) Preprint. Figure 1, which in the hard copy appears here, shows bidirectional reflectance spectra of powdered samples of troilite from the Mundrabilla iron meteorite. Included are spectra bulk troilite and six particle size separates of the same sample. The particle size of each spectrum is listed in order in the lower right of the figure. Figure 2, which in the hard copy appears here, shows spectra of Mundrabilla troilite and four T-class asteroids. All spectra are scaled to unity at 0.55 micrometers. Asteroid data are from [6].

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