Iron oxide and hydroxide precipitation from ferrous solutions and its relevance to Martian surface mineralogy

The presence of red magnetic dust in the Martian regolith raises questions as to how surface processes on Mars might differ from those on Earth. Experiments were carried out to determine physico-chemical conditions under which specific iron oxides and hydroxides would precipitate from ferrous salts in aqueous solution. The results indicate that mineral substrates as well as solution chemistry control the precipitation of iron phases. Particular attention was given to identifying conditions favoring the formation of either lepidocrocite or goethite. All iron precipitates were identified and cross checked by using both infrared and magnetic analyses. Specifically, aqueous ferrous sulfate solutions oxidized rapidly at room temperature under neutral conditions yield lepidocrocite; this occurs both with and without fine-grained mineral substrates whose isoelectric points (IEPs) are less than 5 (except quartz) In contrast, mineral substrates with IEP>7 (and quartz, with IEP≤2) yield mixtures of lepidocrocite and goethite. Additional experiments tested the importance of charged interactions in the formation of iron oxyhydroxides by varying the dielectric constant of aqueous solvent. In solutions with a high dielectric constant (>60), lepidocrocite forms during rapid oxidation at neutral pH. In solutions with a low dielectric constant (≤20), goethite is the predominant phase. This implies that electrostatic effects in the aqueous solution and at the substrate surface strongly influence the deposition of iron oxyhydroxides. With reference to Mars, our results suggest that lepidocrocite, in addition to goethite, is more likely to precipitate from low-temperature aqueous solutions in a basalticre golith (assuming sufficient liquid water was present on Mars in its past history). Dehydration of lepidocrocite to maghemite is then an explanation for the ubiquitous, red magnetic phase in the Martian regolith. In contrast, goethite and hematite are the predominant secondary iron phases in the quartzose continental soils of planet Earth. They had a house of crystal pillars on the planet Mars by the edge of an empty sea, and every morning you could see Mrs K eating the golden fruits that grew from the crystal walls, or cleaning the house with handfuls of magnetic dust which, taking all the dirt with it, blew away on the hot wind.