Mid-infrared spectroscopy of alkali feldspar samples for space application
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ORIGINAL PAPER
Mid-infrared spectroscopy of alkali feldspar samples for space application Maximilian P. Reitze1
¨ Helbert3 · Iris Weber1 · Herbert Kroll2 · Andreas Morlok1 · Harald Hiesinger1 · Jorn
Received: 18 November 2019 / Accepted: 18 May 2020 © The Author(s) 2020
Abstract Feldspars are major components of terrestrial planetary surfaces. For future space application and the setup of a comprehensive reference database, Na- and K-rich alkali feldspars, NaAlSi3 O8 – KAlSi3 O8 , have been investigated by infrared reflectance spectroscopy. We related the feldspar spectra to the chemical composition and state of Al,Si order/disorder. The infrared measurements were analyzed with respect to band shifts and peak shapes using the autocorrelation function. Natural samples served as starting materials. Some samples were treated by the alkali exchange method to produce pure end-members, which were then heated to generate various states of Al,Si disorder. X-ray diffraction (XRD) methods served to determine the Al,Si distribution. Our autocorrelation allowed to differentiate between the compositional and the order/disorder influences seen in the spectra in the wavelength range between 7 μm up to 14 μm (1429 cm−1 to 714 cm−1 ). Space missions often analyze the surfaces of planetary bodies using remote sensing. Therefore, our results are essential to characterize and distinguish alkali feldspars on the surfaces of terrestrial planetary bodies like Mercury. Keywords Alkali feldspars · Infrared spectroscopy · X-ray diffraction · Al,Si distribution
Introduction Feldspars are rock-forming minerals, on Earth and on other terrestrial planetary bodies. In the Earth’s crust, they account for more than 50 vol% (Okrusch and Matthes 2014). Lunar highlands, which cover approximately 83% of the lunar surface (Jolliff et al. 2006), are composed mostly of anorthosites and gabbros with a large amount of anorthite (Wood et al. 1970). Furthermore, Mercury’s crust is feldspatic with a labradorite-like composition predicated on the basis of Earth-based infrared emission spectroscopy by Sprague et al. (1997), who measured a Christiansen Editorial handling: A. Beran Maximilian P. Reitze
[email protected] 1
Institut f¨ur Planetologie, Westf¨alische Wilhelms-Universit¨at, Wilhelm-Klemm-Str. 10, 48149, M¨unster, Germany
2
Institut f¨ur Mineralogie, Westf¨alische Wilhelms-Universit¨at, Corrensstr. 24, 48149, M¨unster, Germany
3
Deutsches Zentrum f¨ur Luft- und Raumfahrt, Rutherfordstr. 2, 12489, Berlin, Germany
feature (CF) at 8 μm (1250 cm−1 ) near the equator at approximately 120◦ longitude. After spectral unmixing of Earth-based observations, Mercury’s surface is thought to contain potassium-rich feldspar (Sprague et al. 2009). Direct sampling of Solar System bodies other than Earth requires landing on the surface and returning samples, which is extremely expensive and currently not feasible for most objects. Thus, remote sensing data of planets and moons are necessary to provide the missing information. Between 2011
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