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

2867 Šteins is a main belt asteroid and was a fly-by target of ESA’s 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 E[II]-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]. E[II]-type asteroids additionally show an absorption band at 0.49 µm, which has been attributed to oldhamite [9]. The depth of the absorption band at 0.49 µm in Šteins’ 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 to 16 µm reflectance spectra of synthetic enstatite (Mg₂Si₂O₆), synthetic oldhamite (CaS), and of their mixtures for comparison with spectra of E[II]-type asteroids such as Šteins, 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 En_99.6Fs_0.0Wo_0.4 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 µm with a relative depth of 11.4 % instead of the absorption band at 0.49 µm. 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 ≥20 vol% 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 Šteins but an overall comparison between the laboratory spectra and Earth-based spectra of Šteins gives an upper limit for the oldhamite content on the surface of Šteins of 40 vol%. 

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