ANALYSIS OF NONTRONITE ALTERATION IN COMPLEX ACIDIC SOLUTIONS

Author

Madeline Lewinski, Southern Illinois University CarbondaleFollow

Date of Award

8-1-2025

Degree Name

Master of Science

Department

Geology

First Advisor

Lefticariu, Liliana

Abstract

The presence of hydrated minerals such as clay minerals and sulfates on Mars is indicative of a potentially long planetary history of fluid-rock interactions. Investigations into the alteration of clays in varied acidic conditions are essential to better our understanding of acid-clay interactions in planetary environments. This study was designed to explore the acid-sulfate alteration of nontronite (NAu-1), an Fe-rich Mars analog clay mineral, by exposing it to both chemically complex acid mine drainage (AMD) and a synthetic acid-sulfate fluid (H2SO4) in laboratory settings and then detailing and comparing the alteration products.In the laboratory experiments, 1g of NAu-1 was reacted in 50mL of an initial solution (either AMD or synthetic H2SO4) prepared at an initial pH of 1, 3, 5, or 7 at temperatures of 4, 30, and 80ºC for 3, 7, and 14 days. These experimental conditions were designed to explore their effects on the severity of clay dissolution, the changes in dissolved ion concentrations, and the types of secondary precipitates formed. The initial and reacted solutions were measured for their pH and conductivity, as well as their dissolved ion concentrations, which was quantified by ICP-MS. Dissolution of NAu-1 was evidenced by increases in dissolved ion concentrations above those of the initial solutions, where Si and Al were sourced from the tetrahedral sheets, Fe from the octahedral sheets, and Mg and Ca from the clay interlayers. The initial NAu-1 and reacted solids were investigated by X-Ray Diffraction (XRD) and Raman spectroscopy.The results of this study revealed the importance of solution chemistry and solution pH in controlling the evolution of reacting NAu-1 with acid-sulfate solutions. Specifically, divergent pH evolutions were noted where the solution pH typically decreased in AMD experiments and increased in H2SO4 experiments. There was an increase in the concentration of dissolved ions sourced in NAu-1, with higher gains in lower-pH and higher temperature experiments, suggesting NAu-1 dissolution under all conditions tested. The pH increases in H2SO4 experiments was due to proton uptake by dissolving NAu-1, while the pH decreases in AMD experiments resulted from extensive Fe(III) hydrolysis and precipitation – reactions that release protons in solution. NAu-1 dissolution was also evidenced by Raman analysis, which showed structural changes to the SiO4 tetrahedral structure of altered NAu-1 in pH 1 and 3 experiments but very little change in the pH 5 and 7 experiments.XRD analysis identified the Fe(III) precipitate as goethite as well as gypsum as a secondary phase. Goethite was more common in pH 5 and 7 experiments of both solution types, especially at higher temperatures, while gypsum was more common in AMD experiments under all conditions tested. Higher amounts of Fe(III)-rich phases precipitated from the AMD solutions, which were intrinsically Fe-rich, than from the H2SO4 solutions where fewer precipitates formed even in the more neutral-pH H2SO4 experiments. These Fe(III)-rich precipitates formed coatings on altered NAu-1, thus protecting it against aggressive dissolution even in lower-pH and high-temperature conditions.The results of this study highlight the complexity of reacting systems involving NAu-1 and acid-sulfate solutions under Mars-relevant conditions. These experiments demonstrated that complex natural acidic solutions react with NAu-1 differently than synthetic H2SO4 solutions in terms of pH evolution, the degree of clay dissolution, and the formation of precipitates. The potential for long-term stability of nontronite at the Martian surface under acidic conditions, similar to these AMD experiments, could be enhanced due to the precipitation of protective Fe-rich coatings. The features compiled by this study of NAu-1 alteration by acid-sulfate solutions could also be of use in the identification and ‘reverse-engineering’ of the past surface processes and environmental conditions on Mars.