Reflectance spectra of synthetic Fe-free ortho- and clinoenstatites in the UV/VIS/IR and implications for remote sensing detection of Fe-free pyroxenes on planetary surfaces

For a better spectral characterization of planetary bodies with enstatite-rich surfaces like Mercury or E-type asteroids, we synthesized two different enstatite (Mg2Si2O6) polymorphs: Orthoenstatite and clinoenstatite. Both enstatite polymorphs are known from the meteorite record and are commonly observed in aubrites and enstatite chondrites. The synthesized enstatites are particulate samples suitable for laboratory reflectance measurements and can be used for compositional modelling by preparing mixtures of samples in the laboratory or by using the sample's spectra in mathematical models. We report on the synthesis process, chemical composition, grain size distribution, and reflectance spectra of these synthetic enstatites covering the wavelength range from 0.25 to 17 mu m, compare them to other pyroxenes (meteoritic enstatite and other synthetic enstatites and diopside), and discuss the implications of retrieving surface compositions of planetary bodies like E-type asteroids, comets, or Mercury. Both enstatite spectra are very bright in the VIS and NIR and show almost neutral to slightly bluish spectral slopes with a steep absorption in the UV. Very low iron in the enstatites (below similar to 0.04 wt% FeO) already results in weak albeit noticeable absorptions in the VNIR between 0.4 and 0.9 mu m. Orthoenstatite and clinoenstatite are not distinguishable based only on their spectra in the VIS and NIR. At the Reststrahlen bands in the MIR a systematic difference in the number and exact position of local minima at similar to 10 mu m between clinoenstatite and orthoenstatite is evident. This can be used to discern between the polymorphs in this wavelength range. Additionally, we can distinguish between Fe-free low- and high-Ca pyroxenes in the MIR.