Selective Dissolution Techniques, X-Ray Diffraction and Mössbauer Spectroscopy Studies of Forms of Fe in Particle-Size Fractions of an Entic Haplustoll

Abstract

Particle-size fractions (∅ = mean diameter, 5–2 μm, 2–1 μm, and <1 μm) from the Ap horizon of an Entic Haplustoll from Argentina were treated with the selective-dissolution techniques ammonium oxalate (OX), dithionite-citrate-bicarbonate (DCB), NaOH, and Na-pyrophosphate (PY), and studied by X-ray powder diffraction (XRD) and 57Fe Mossbauer spectroscopy (MS). Quartz, feldspar, smectite, illite and interstratified illite-smectite are the dominant minerals whereas Fe oxides and oxy-hydroxides are present in low concentration but increase as particle size decreases. Poorly crystallized oxides (highly Al-substituted hematite and goethite) amounts are lower, comparable to or slightly higher than the hematite amounts in the ∅ = 5–2 μm, 2–1 μm and <1 μm fractions respectively. This hematite is well crystallized but presents some degree of Al-substitution. Magnetite/maghemite was identified in the 2–1 μm fraction. Paramagnetic Fe3+ and Fe2+ are associated to the clay minerals and/or hydroxyl-interlayered 2:1 type material present; part of this Fe3+ is located in the hydroxy-interlayers its amount being higher in the smallest fraction. Any possible changes after the PY and NaOH treatments were not detected by MS.