Hematite-goethite ratios at pH 2–13 and 25–170 °C: A time-resolved synchrotron X-ray diffraction study

Iron (oxyhydr)oxides are sensitive indicators of pH, Eh, temperature, microbial activity, and climate conditions in the Critical Zone. The most ubiquitous and environmentally significant iron oxides in most soils are two-line ferrihydrite, goethite, and hematite. Here we present a comprehensive study of the transformation of two-line ferrihydrite to hematite and goethite over a wide range of temperature (25–170 °C) and initial pH (2−13) conditions through ex situ batch and in situ synchrotron X-ray diffraction (XRD) experiments. Within the high time resolution of our experiments, goethite and hematite nucleated nearly simultaneously from ferrihydrite in nearly equal concentrations by mass. Hematite increased in abundance relative to goethite until a steady-state ratio was achieved, and both phases ceased growth on the depletion of ferrihydrite. Higher temperatures and lower water activities favored hematite formation at all pH values studied. In both our batch and our time-resolved, angle-dispersive synchrotron X-ray diffraction experiments, hematite was favored relative to goethite at an initial pH of 3 to 5. In contrast, goethite preferentially formed in neutral (initial pH 7–8) and highly alkaline conditions (initial pH ≥ 11). Surprisingly, mildly alkaline conditions (initial pH 9–11) induced the precipitation of a highly Fe-deficient (Feocc = ~0.80–0.90) variety of hematite known as “hydrohematite” in greater concentrations than goethite. Our results are useful for the application of hematite-goethite ratios as paleoclimate proxies for soil and sediment systems with low pH buffering capacities.



Work Title Hematite-goethite ratios at pH 2–13 and 25–170 °C: A time-resolved synchrotron X-ray diffraction study
Open Access
  1. Si Athena Chen
  2. Peter J. Heaney
  3. Jeffrey E. Post
  4. Peter J. Eng
  5. Joanne E. Stubbs
  1. Hematite
  2. Goethite
  3. 2-line Ferrihydrite
  4. Iron Oxides
  5. Time-resolved X-ray diffraction
License In Copyright (Rights Reserved)
Work Type Article
  1. Chemical Geology
Publication Date June 20, 2022
Publisher Identifier (DOI)
  1. https://doi.org/10.1016/j.chemgeo.2022.120995
Deposited February 17, 2023




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Work History

Version 1

  • Created
  • Added Chen_HemGoethRatio_ChemGeol_2022.pdf
  • Added Creator Si Athena Chen
  • Added Creator Peter J. Heaney
  • Added Creator Jeffrey E. Post
  • Added Creator Peter J. Eng
  • Added Creator Joanne E. Stubbs
  • Published
  • Updated Keyword, Publication Date Show Changes
    • Hematite, Goethite, 2-line Ferrihydrite, Iron Oxides, Time-resolved X-ray diffraction
    Publication Date
    • 2022-09-20
    • 2022-06-20
  • Updated