Mid-infrared spectroscopy of alkali feldspar samples for space application

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, NaAlSi3O8 – KAlSi3O8, 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.

Studying the Composition and Mineralogy of the Hermean Surface with the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) for the BepiColombo Mission: An Update

“…Due to their intrinsic spectral complexity, detailed studies of the complete crystallographic and compositional range are necessary. Our investigations of the Na-and K-rich alkali feldspars showed that the autocorrelation method allows differentiating between the compositional and the order/disorder influences seen in the powder reflectance spectra in the wavelength range between 7 µm to 14 µm (Reitze et al 2020). In particular, decreasing the degree of order led to broader RBs and a feature shift.…”

“…Another crucial Mercury surface analog are feldspars (Helbert et al 2007;Sprague et al 2009;Reitze et al 2020). Due to their intrinsic spectral complexity, detailed studies of the complete crystallographic and compositional range are necessary.…”

“…In particular, decreasing the degree of order led to broader RBs and a feature shift. In addition, applying the autocorrelation method we were able to determine the Or content of feldspar at different grain sizes (Reitze et al 2020).…”

Mid‐Infrared Spectroscopy of Anorthosite Samples From Near Manicouagan Crater, Canada, as Analogue for Remote Sensing of Mercury and Other Terrestrial Solar System Objects

“…Investigations of the plagioclases from the Lappajrvi crater, Finland, revealed an almost highest possible degree of disorder, showing a complex interaction of shock and the impact melt (Bischoff & Stffler, 1984). Simulated micrometeorite bombardment with laser irradiation of powder pressed pellets leads to a decrease of spectral contrast of the Reststrahlen bands (RBs) of plagioclase feldspar samples and a formation of smaller grain sizes due to a gardening process, which disturbs the flat surface of the pellets (Moroz et al, 2014;Weber et al, 2020).…”

“…The autocorrelation technique is sensitive to the width of the analyzed peaks in the spectra. Reitze et al (2020) and Reitze et al (2021) used the autocorrelation method to analyze mid-IR reflectance spectra and were able to determine the degree of Al,Si order of alkali feldspars as well as plagioclase feldspars with the technique. The method uses the wavelength of the CF and the width of the spectral autocorrelation function between 7.27 µm and 12.11 µm (Reitze et al, 2021).…”