Near-Infrared Signature of Kaolinite: A Proxy for Estimating Its Crystallinity and Its Geological Origin on Earth and Mars

Introduction: Kaolinite, a 1:1 dioctahedral phyllosilicate of chemical formula Al2Si2O5(OH)4, has been detected at the martian surface by orbiters in: (i) Al-clay rich materials associated with opaline silica, overlaid by Fe/Mg-clays in inferred pedogenetic paleoprofiles [1,2,3,4], (ii) alone or associated with a great diversity of alteration phases in contexts suggested to be linked to hydrothermal activity [5,6,7]. On Earth, kaolinite forms through pedogenesis/surface weathering, hydrothermal activity, or diagenetic/sedimentary processes [8]. Thus, the occurrences of kaolinite may be of various origins on Mars as well. Kaolinite crystallinity has been suggested to reflect the physicochemical conditions where it forms, and thus its geological conditions of formation [8]. As spectroscopic sensors on Mars cover the Near-Infrared range, we investigated the NIR signatures of terrestrial kaolinite samples from various origins in order to reveal the degrees of crystalline disorder of this clay mineral.