XAS Study of Fe Mineralogy in a Chronosequence of Soil Clays Formed in Basaltic Cinders

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Abstract:

The characterization of poorly crystalline minerals formed by weathering is difficult using conventional techniques. The objective of this study was to use cutting-edge spectroscopic techniques to characterize secondary Fe mineralogy in young soils formed in basaltic cinders in a cool, arid environment. The mineralogy of a chronosequence of soils formed on 2, 6, and 15 thousand year old basaltic cinders at Craters of the Moon National Monument (COM) was examined using synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy in combination with selective extractions. Fe K-edge XAFS is useful for determining speciation in poorly crystalline materials such as young weathering products. Over 86% of Fe in the soil clay fractions was contained in poorly crystalline materials, mostly in the form of ferrihydrite, with the remainder in a poorly crystalline Fe-bearing smectite. The XAFS spectra suggest that ferrihydrite in the 15 ka soil clay is more resistant to ammonium oxalate (AOD) extraction than is ferrihydrite in the younger materials. Fe in the poorly crystalline smectite is subject to dissolution during citrate-bicarbonate-dithionite (CBD) extraction. The results indicate that relatively few mineralogical changes occur in these soils within the millennial time frame and under the environmental conditions associated with this study. Although the secondary mineral suite remains similar in the soils of different ages, ferrihydrite crystallinity appears to increase with increasing soil age.

Keywords: FE MINERALOGY; FERRIHYDRITE; SMECTITE; WEATHERING; XAFS

Document Type: Research Article

DOI: http://dx.doi.org/10.1346/CCMN.2010.0580605

Publication date: December 1, 2010

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  • The JOURNAL publishes articles of interest to the international community of clay scientists, including but not limited to areas in mineralogy, crystallography, geology, geochemistry, sedimentology, soil science, agronomy, physical chemistry, colloid chemistry, ceramics, petroleum engineering, foundry engineering, and soil mechanics. Clays and Clay Minerals exists to disseminate to its worldwide readership the most recent developments in all of these aspects of clay materials. Manuscripts are welcome from all countries.

    Clays and Clay Minerals is the official publication of The Clay Minerals Society.

    The Editor-in-Chief is Professor Joseph W. Stucki jstucki@illinois.edu

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