Laboratory reflectance spectra of enstatite and oldhamite mixtures for comparison with Earth-based reflectance spectra of asteroid 2867 teins

2867 teins is a main belt asteroid and was a fly-by target of ESAs Rosetta mission 1. It has been previously studied by ground-based observations (e.g.,2,3,4,5,6,7). It has been classified as an EII-type asteroid. E-type asteroids are characterized by flat or slightly reddish and featureless reflectance spectra in the VIS and NIR and high geometric albedos and are generally associated with aubrites, enstatite achondrites 8. EII-type asteroids additionally show an absorption band at 0.49 m, which has been attributed to oldhamite 9. The depth of the absorption band at 0.49 m in teins spectra has been reported to be 9-13 % 3,5,6,7. Oldhamite usually only occurs as an accessory phase while the abundance required to produce the absorption band is much higher. We present 0.3 to16 m reflectance spectra of synthetic enstatite (Mg2Si2O6), synthetic oldhamite (CaS), and of their mixtures for comparison with spectra of EII-type asteroids such as teins, and investigate the spectral behavior of the mixtures with respect to their oldhamite content. All reflectance spectra were collected using a Bruker Vertex 80v FTIR spectrometer at the Planetary Spectroscopy Laboratory (PSL) of the Institute of Planetary Research at DLR, Berlin 10. The synthesis of the enstatite sample with the composition En99.6Fs0.0Wo0.4 has been described in 11. The oldhamite sample was purchased from abcr GmbH. Mixtures containing 1, 3, 5, 10, 20, 30, 40, 50, 60, 70, 80, and 90 vol% oldhamite were prepared. The spectrum of synthetic oldhamite shows an absorption band at 0.41 m with a relative depth of 11.4 % instead of the absorption band at 0.49 m. This band is visible in the spectra of all mixtures, even in the spectrum of the mixture with only 1 vol% oldhamite. In the MIR, the spectra with 10 vol% oldhamite are very similar to the spectrum of the pure enstatite and are generally dominated by the Christiansen feature and the Reststrahlen bands. The pure oldhamite is significantly brighter than the enstatite spectrum in the MIR. Changes in the band depth and reflectance do not occur as a single trend, but follow two distinct trends. One for mixtures with 10 vol% of oldhamite where changes occur rapidly and another trend for mixtures with 20vol% of oldhamite where changes occur more slowly. The differences in the oldhamite absorption band do not allow for an estimation of the oldhamite content on teins but an overall comparison between the laboratory spectra and Earth-based spectra of teins gives an upper limit for the oldhamite content on the surface of teins of 40vol%. 1Keller et al. (2010) Science, 327, 190-193. 2Barucci et al. (2005) AA, 430, 313-317. 3Nedelcu et al. (2007) AA, 473, L33-L36. 4Dotto et al. (2009) AA, 494, L29-L32. 5Fornasier et al. 2007) AA, 474, L29-L32. 6Fornasier et al. (2008) Icarus 196, 119-134. 7Weissman et al. (2008) Met. Planet. Sci., 43, 905-914. 8Gaffey et al. 1993) Meteoritics 28, 161-187. 9Burbine et al. (2002) Met. Planet. Sci., 37, 1233-1244. 10Maturilli and Helbert. (2019) LPSC, 1846. 11Markus et al. (2018) Planet. Space Sci., 159, 43-55.