Simulation of Natural Iron Oxide Alteration in Soil: Conversion of Synthetic Ferrihydrite to Hematite Without Artificial Dopants, Observed With Magnetic Methods

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 21; no. 7
• Main Authors: Bilardello, Dario, Banerjee, Subir K., Volk, Michael W. R., Soltis, Jennifer A., Penn, R. Lee
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.07.2020; Wiley
• Subjects: Crystals; Electron microscopy; Environmental conditions; ferrimagnetic ferrihydrite; Goethite; Haematite; hematite; hydromaghemite; Iron oxides; Ligands; loess; magnetic enhancement; Magnetic field; Magnetic fields; Paleoprecipitation; Soil; Stabilizing
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2020GC009037

We present new results on the conversion of pure, undoped synthetic ferrihydrite, wet‐annealed at pH 6.56 and 90°C without stabilizing ligands, to nanophase goethite, hematite, and an intermediate magnetic phase, nanophase maghemite. Our analyses included magnetic field and temperature‐dependent properties and characterization by powder X‐ray diffraction, Mössbauer spectra, and high‐resolution transmission electron microscopy. We sampled alteration products after 0.5 hr, and then in a geometric progression to 32 hr, yielding a detailed examination of the earliest alteration phases. There are many similarities to the latest studies of pure ferrihydrite alteration but with a significant difference: We observe early appearance of oriented nanophase goethite along with a soft magnetic contribution, while rhombohedral hematite crystals form later, as reported in previous studies. Our observations attest to the non‐uniqueness of the magnetic enhancement process and to its strong dependence on environmental conditions, with important implications for use of the hematite/goethite ratio as a paleoprecipitation proxy. Key Points We conducted an alteration study of synthetic ferrihydrite, wet‐annealed up to 32 hr at pH 6.56 and 90°C without stabilizing ligands Very early appearance of goethite is followed by hematite and maghemite, attesting to the non‐uniqueness of the magnetic enhancement process Our observations have important implications for the proposed hematite/goethite ratio as a reliable proxy for paleoprecipitation intensity