The effect of clay minerals on Li in martian groundwater simulant

Abstract

The high mobility of Li allows it to be used as a tracer for groundwater processes, recording past aqueous conditions. On Earth, a relationship has been noted in multiple field sites between clay mineral abundances and elevated Li in bedrock. Observations from the Curiosity MSL mission at Gale crater on Mars showed a high-clay mineral and high-Li area near the Vera Rubin ridge (VRR) and Glen Torridon region, suggesting Li was perhaps substituting into clay minerals as was seen in these terrestrial field settings. However, the process of this substitution has not been examined in the laboratory using non-field samples, especially not with Mars-relevant mineralogy. To investigate this open question in the laboratory using Mars-relevant regolith and clay minerals, we conducted continuous flow packed-bed reactor experiments to test whether clay minerals affect the Li concentration of Mars regolith simulant MGS-1 during aqueous alteration. The mechanism for Li sorption was also investigated by conducting experiments with clays mixed with glass beads and investigating changes in other elements alongside Li via laser-induced breakdown spectroscopy (LIBS). We tested four dioctahedral clay minerals (kaolinite, illite, nontronite, mixed layer illite/smectite) and two trioctahedral clay minerals (talc, saponite) and found that both talc and illite are capable of increasing the amount of Li sorbed compared to MGS-1 simulant when exposed to Li-bearing groundwater. For MGS-1, the glass beads, and the clay minerals (talc, illite) the primary mechanism appears to be Li substitution for Mg, Al, and K, respectively. This has implications for ongoing Mars missions as well as astrobiology, specifically relating to understanding habitability of areas on Mars and identifying aqueous environments for future mission concepts.