¹M. Barthez, ¹J. Flahaut, ²M. Guitreau, ¹R. Pik, ¹G. Ito, ³C. Delangle, ³J.-P. Gremilliet, ¹B. Luais, ¹F. Faure
Icarus (in Press) Open Access Link to Article [https://doi.org/10.1016/j.icarus.2026.117027]
¹Université de Lorraine, CNRS, CRPG, Nancy F-54000, France
²Laboratoire Magmas et Volcans, Université Clermont Auvergne, OPGC, CNRS UMR-6524, IRD UMR-163, F-63173 Clermont-Ferrand, France
³Centre Terrae Genesis, F-88120 Le Syndicat, France
Copyright Elsevier

Visible Near-InfraRed (VNIR) spectroscopy is a powerful tool to assess the mineralogical composition of planetary surfaces remotely. Despite decades of investigation with the Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) and the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instruments, new detections are still being made on Mars. Plagioclase signatures were reported on the surface of Mars with CRISM, and it could be indicative of uncommon lithologies, as plagioclase is known to be difficult to detect with VNIR spectroscopy, except for plagioclase-pure rocks (i.e., anorthosite).
The present study investigates the VNIR spectra of 67 plagioclase-bearing terrestrial igneous rocks of various nature (e.g., granite, anorthosite, gabbro, tonalite, syenite and their volcanic equivalents) that were kept uncrushed as Mars candidate analog rocks. Total rock spectra were measured with a point spectrometer, while the spectra of their plagioclase constituents are extracted from hyperspectral camera analyses. In parallel, the petrology of rocks and the chemical compositions of feldspar minerals are evaluated with optical microscopy and electron probe microanalyzer (EPMA) respectively.
Most plagioclase crystals in our sample collection exhibit diagnostic absorption features centered between 1.15 μm and 1.40 μm at the mineral level, unless the mineral is altered and/or weathered. Both physical and spectral evidence for alteration products, such as muscovite and prehnite, are observed. Still, plagioclase signatures are often masked on the total rock spectra (44 out of 67 samples). We found that the main factors that control the spectral signature of plagioclase in the total rock spectra are its chemical composition and grain size, as well as the associated minerals and the rock albedo. Plagioclase spectral signatures are observed in total rock spectra of samples containing as little as 18% plagioclase, challenging previously published estimates and interpretations based on studies of powders and grain mixtures. Diagnostic plagioclase signatures are visible in the spectra of rocks of various natures and textures, including plagioclase-phyric basalt, anorthosite, granite and granodiorite, opening up a range of possibilities for the nature of the lithologies involved in Mars detections.