XAS Study of Fe Mineralogy of Soil Colloids Formed in Basaltic Tephra Under Cold, Dry Conditions.

Soils (Andisols) on basaltic cinder cones at Craters of the Moon National Park have developed under cool, dry conditions.  X-ray absorption spectroscopy (XAS) of the Fe-edge was done to examine the speciation of Fe in the clay-size fraction of a chronosequence of Vitricryands collected from cinder cones with ages of 2, 6, and 15 ka. Data were analyzed by principal component analysis and linear combination fitting. XAS data showed that only ferric Fe, and no primary basaltic Fe-bearing minerals were present in the soil clay-size fraction.  The predominant Fe mineral detected was ferrihydrite. Soil clays were treated with acid oxalate (AOD), which should remove the poorly crystalline ferrihydrite minerals.  After AOD extraction, the XAS spectra showed that Fe-bearing layer silicates were the predominant Fe mineral present in the soil clay-size fraction.  However, the XAS spectrum of the 15 ka clay-size fraction retained a significant ferrihydrite signature even after AOD treatment, and the XAS clearly shows that the Fe mineral is ferrihydrite rather than hematite or goethite. While XAS is unable to distinguish between 2-line or 6-line ferrihydrite, the decrease in AOD extraction efficiency of ferrihydrite in the 15 ka sample suggests that ferrihydrite crystallinity is increasing with soil age. XAS analyses confirm that most Fe in the clay-size fraction of the samples is in secondary minerals, and that the AOD extraction removes some ferrihydrite, with extraction efficiency dependant on sample age.