Abundance and composition of kaolinite on Mars: Information from NIR spectra of rocks from acid-alteration environments, Riotinto, SE Spain

Abstract

Investigation of Earth analogs and their environments is crucial for the full interpretation of geologic outcrops and processes on Mars. Phyllosilicates are important indicators of aqueous processes and their characterization is a significant piece of the geologic puzzle of Mars. They are chiefly investigated with Near-Infrared (NIR) spectroscopy from orbit. While these studies have revolutionized our understanding of aqueous processes on Mars, they are challenged by the chemical and structural complexity of phyllosilicates, non-linear response to mineral abundance, low penetration of infrared radiation in the target rocks, and spectral modifications caused by rock texture. Phyllosilicate-bearing samples from four locations in the Iberian Pyrite Belt (El Villar, Calañas, Quebrantahuesos, and Tharsis) were investigated using NIR, XRD and thermogravimetry in order to document the effects of acidic alteration under multiple environments and inform orbital detections on Mars. The samples are comprised of chlorite, illite, kaolinite, alunite, jarosite, goethite and interstratified chlorite-vermiculite and kaolinite-smectite. Kaolinite dominates the spectral signature relative to other phyllosilicates from abundances as low as 7 wt%. Only alunite and jarosite display spectral intensities similar to kaolinite. NIR spectra of bulk rock and <2 μm size fractions are very similar, indicating that spectra are dominated by the smaller particles. The octahedral Al-Fe-Mg composition of illite and kaolinite determine the positions of their OH combination band (~2.21 μm), commonly used for phyllosilicate characterization. The range of wavelength variation is narrower for kaolinite-dominated spectra, 2.205–2.216 μm, but wider than the spectral resolution of the orbital probe CRISM (~0.007 μm). Thus, in favorable conditions CRISM spectra can identify variations of kaolinite octahedral composition, a valuable tool to investigate kaolinite origin. A survey of kaolinite-bearing spectra from Mars (Leighton Crater, Mawrth Vallis and Nili Fossae regions) showed that most OH combination bands are within 2.205–2.212 μm. One spectrum displayed this band at 2.215–2.219 μm, and is a good candidate for kaolinite with significant Fe/Mg-substitution.