Experimental transformation of nontronite to kaolinite: implication for ferrallitization of Fe/Mg-rich clays in compositional stratigraphy on Mars

The compositional stratigraphy of Al-rich clays overlying Fe/Mg-rich clays on Mars has been viewed as a window to understanding the atmospheric conditions of early Mars. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) indicates that nontronite is a main component of Fe/Mg-rich clays. However, the role of the ancient climate in the alteration process, which produced and dissolved these phyllosilicates, remains under debate. The present study experimentally modeled the dissolution of nontronite and precipitation of kaolinite in an acidic solution, which enables us to enhance the interpretation of near-infrared remote sensing data from compositional stratigraphy on Mars. The obtained results show that the transformation started from the losses of both tetrahedral Si and interlayer alkali(earth) cations in nontronite, and Fe3+ in the dioctahedral sheet was released and formed hematite with progressive damage of SiO tetrahedral sheets. TG analyses also show that the dissolution of nontronite and precipitation of kaolinite led to an increase in metal-OH (metal = Al, Fe3+ and Mg) content and a simultaneous decrease in H2O content. Accordingly, the gradual increasing trend of visible/near-infrared reflectance (VNIR) spectra ratios of BD1400/BD1900 (BD, band depth) in the kaolinized nontronite can be linked to a gradual upward transition from nontronite to a mixture of nontronite and kaolinite, and finally into kaolinite and hematite, by comparing their spectral features at different evolution stages. Such a trend suggests a weathering process known as ferrallitization, i.e., partial leaching of Si and enrichment of Fe3+ and Al3+. The observed weathering process is consistent with a warm and wet climate capable of sustaining acidic liquid water on its surface over extended geological periods.