THE NANOPHASE IRON MINERAL(S) IN MARS SOIL

A series of surface-modified clays containing nanophase (np) iron oxid eloxyhydroxides of extremely small particle sizes, with total iron con tents as high as found in Mars soil, were prepared by iron deposition on the clay surface from ferrous chloride solution. Comprehensive stud ies of the iron mineralogy in these ''Mars-soil analogs'' were conduct ed using chemical extractions, solubility analyses, pH and redox, x ra y and electron diffractometry, electron microscopic imaging, specific surface area and particle size determinations, differential thermal an alyses, magnetic properties characterization, spectral reflectance, an d Viking biology simulation experiments. The clay matrix and the proce dure used for synthesis produced nanophase iron oxides containing a ce rtain proportion of divalent iron, which slowly converts to more stabl e, fully oxidized iron minerals. The clay acted as an effective matrix , both chemically and sterically, preventing the major part of the syn thesized iron oxides from ripening, i.e., growing and developing large r crystals. The precipitated iron oxides appear as isodiametric or sli ghtly elongated particles in the size range 1-10 nm, having large spec ific surface area. The noncrystalline nature of the iron compounds pre cipitated on the surface of the clay was verified by their complete ex tractability in oxalate. Lepidocrocite (gamma-FeOOH) was detected by s elected area electron diffraction. It is formed from a double iron Fe( II)/Fe(III) hydroxy mineral such as ''green rust,'' or ferrosic hydrox ide. Magnetic measurements suggested that lepidocrocite converted to t he more stable maghemite (gamma-Fe2O3) by mild heat treatment and then to nanophase hematite alpha-Fe2O3) by extensive heat treatment. After mild heating, the iron-enriched clay became slightly magnetic, to the extent that it adheres to a hand-held magnet, as was observed with Ma rs soil. The chemical reactivity of the iron-enriched clays strongly r esembles, and offers a plausible mechanism for, the somewhat puzzling observations of the Viking biology experiments. Their unique chemical reactivities are attributed to the combined catalytic effects of the i ron oxide/oxyhydroxide and silicate phase surfaces. The reflectance sp ectrum of the clay-iron preparations in the visible range is generally similar to the reflectance curves of bright regions on Mars. This str engthens the evidence for the predominance of nanophase iron oxides/ox yhydroxides in Mars soil. The mode of formation of these nanophase iro n oxides on Mars is still unknown. It is puzzling that despite the lon g period of time since aqueous weathering took place on Mars, they hav e not developed from their transitory stage to well-crystallized end-m embers. The possibility is suggested that these phases represent a con tinuously on-going, extremely slow weathering process.