Iron-oxide mineralogy of a mollisol from Argentina: A study by selective-dissolution techniques, X-ray diffraction, and Mossbauer spectroscopy

Selective-dissolution techniques by ammonium oxalate (OX), dithionite-citra te-bicarbonate (DCB), and dithionite-ethylenediaminetetraacetic acid (D-EDT A), and X-ray diffraction and Mossbauer spectroscopy were used to identify and characterize iron oxides and oxyhydroxides in the <2-mm, <50-mu m, and <2-mu m size fractions of a Mollisol from Bahia Blanca, Argentina. Iron com pounds are present at low concentrations in mixtures with quartz, Na-rich f eldspar, illite, interstratified illite-montmorillonite, and traces of kaol inite. Total Fe and Al content increases as soil particle size decreases, f rom 4.3 and 13.3 wt. % in the <2-mm size fraction to 8.5 and 22.8 wt. % in the clay fraction (<2 mu m), respectively. No more than 25-30% of the total Fe is associated with the crystalline and the amorphous Fe oxides. Weakly ferromagnetic hematite and goethite were identified in the different fracti ons. These phases have small particle sizes and/or low crystallinity. They may also have Al for Fe substitutions. Crystalline magnetite or maghemite i s rare. These Fe-rich phases are probably coating coarser particles. The efficiency of Fe removal is highest for the D-EDTA treatment and least efficient for the OX method, for all fractions. The opposite is true for Al removal. Poorly crystalline hematite and goethite, which are soluble in ox alate, are only present in the coarser fractions. Poorly crystalline and cr ystalline hematite and goethite, which are soluble in DCB and EDTA, are pre sent in coarser fractions, but do not occur in the clay fraction. DCB treat ment probably dissolves Al in the 2:1 type phyllosilicates occurring in thi s soil, whereas D-EDTA dissolves Fe in the hydroxy interlayers of the smect ite minerals or in the silicate phases.