CHARACTERIZATION OF A NANO-MAGHEMITE BEARING, VOLCANICLASTIC FORMATION FROM IDAHO'S EASTERN SNAKE RIVER PLAIN AS AN ANALOG TO MARTIAN SEDIMENTS

Samples taken from an iron-rich, weathered basalt formation in Box Canyon in Idaho's volcanic eastern Snake River Plain were analyzed for mineral content and crystal structure; pore size and surface area; iron concentration; and proportion of magnetic material, as an analog to Martian sediments. Using magnetic separation, the sample was found to be comprised of at least 89% magnetic materials. Through quantitative analyses, this material was concluded to be Fe-bearing smectite and nano-crystals of maghemite, a cubic mineral (P4₃32) that has a structure very similar to that of magnetite (Fd3m). Analysis of total iron concentration using citrate-bicarbonate-dithionite extraction yielded a mean weight percent Fe(II) of 2.86%. X-ray diffraction (XRD) and X-ray energy dispersive spectroscopy (EDS) data indicate that the sample also contains quartz, phillipsite (a potassium-rich zeolite), and nontronite (an Fe(III)-rich smectite). Transmission electron microscope (TEM) images revealed nano-sized crystals of maghemite with an average size of approximately 5-10 nm, along with larger crystals of phillipsite and nontronite. Quartz crystals examined using an optical microscope were found to be angular particles, indicative of ash fall of primary origin. The surface area of the strongly magnetic portion of the sample was found to be 110 m²/g and the majority of the pores fell within a range of 2 – 15 nm. Thus, the maghemite was found to be comprised of remarkably uniform nano-crystals.

The composition of the sample suggests that the unit experienced chemically stable, low temperature, hydrous conditions as it altered from basaltic volcanic glass to the present mineral assemblage of maghemite, smectite, and phillipsite. The presence of maghemite indicates that a reducing environment, necessary for the formation of precursor materials such as magnetite or green rust, once existed. Low-temperature, reducing environments are common in modern systems which sustain microbial activity. Subsequent oxidizing conditions may have resulted in the transformation of precursor material into maghemite. It has been proposed that nanoporous maghemite is a result of the oxidation of nanoporous, biogenic magnetite (Chen et al., 2005). The presence and history of redox-cycling microbes within the sample is currently being studied.