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

Dario Bilardello, Subir K. Banerjee, Michael W.R. Volk, Jennifer A. Soltis, R. Lee Penn

Research output: Contribution to journalArticlepeer-review

Abstract

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.

Original languageEnglish (US)
Article numbere2020GC009037
JournalGeochemistry, Geophysics, Geosystems
Volume21
Issue number7
DOIs
StatePublished - Jul 1 2020

Bibliographical note

Funding Information:
We thank Jeanette Voelz for obtaining the XRD patterns and for dialyzing the TEM samples, Jacquelyn Smale for initial magnetic measurements, and Peat Solheid for M?ssbauer analyses. We acknowledge Andrew Roberts and two other anonymous reviewers for their thoughtful reviews that improved the manuscript. The Institute for Rock Magnetism (IRM) is a US National Multi-user Facility supported through the Instrumentation and Facilities program of the National Science Foundation (NSF), Earth Sciences Division (NSF/EAR 1642268), and by funding from the University of Minnesota.

Funding Information:
We thank Jeanette Voelz for obtaining the XRD patterns and for dialyzing the TEM samples, Jacquelyn Smale for initial magnetic measurements, and Peat Solheid for Mössbauer analyses. We acknowledge Andrew Roberts and two other anonymous reviewers for their thoughtful reviews that improved the manuscript. The Institute for Rock Magnetism (IRM) is a US National Multi‐user Facility supported through the Instrumentation and Facilities program of the National Science Foundation (NSF), Earth Sciences Division (NSF/EAR 1642268), and by funding from the University of Minnesota.

Publisher Copyright:
©2020. American Geophysical Union. All Rights Reserved.

Keywords

  • ferrimagnetic ferrihydrite
  • goethite
  • hematite
  • hydromaghemite
  • loess
  • magnetic enhancement

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