THE FORMATION OF IRON (HYDR)OXIDES IN SURFACE ENVIRONMENTS: A CRYSTALLOGRAPHIC AND KINETIC

Open Access
- Author:
- Chen, Si
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 04, 2021
- Committee Members:
- William Burgos, Outside Unit & Field Member
Peter Heaney, Chair & Dissertation Advisor
Susan Brantley, Major Field Member
Mark Patzkowsky, Program Head/Chair
Jennifer Macalady, Major Field Member - Keywords:
- hematite
goethite
ferrihydrite
hydrohematite
iron oxides
iron hydroxides
transformation
XRD
X-ray diffraction
synchrotron
neutron
neutron scattering
geochemistry
low temperature
kinetics
crystallographic
surface environment
DFT
First principle study
hydrous hematite
hydrous
deficient
vacancy
mineralogy
iron oxyhydroxides - Abstract:
- The transformation of two-line ferrihydrite to (hydro)hematite and goethite over a wide range of pH (2-13) and temperature (25-170 °C) was studied using ex situ batch and in situ experiments. Reaction kinetics and structural mechanisms were studied by combining novel time-resolved synchrotron X-ray diffraction (TRXRD) techniques and Rietveld structural analysis. The rate and extent of hematite and goethite formation depended significantly on temperature and initial pH concentrations. Hematite was favored at high temperatures and acidic conditions, whereas goethite was preferred at lower temperatures and highly alkaline solutions. The transformation rate was minimized at neutral pH and fastest in alkaline conditions. The rate constants for goethite formation were of the same magnitude as those for hematite. An exceptionally Fe-deficient and hydrous variety of hematite, named hydrohematite, formed at pH 9-11. Rietveld structural analyses revealed a nonclassical crystallization pathway for hydrohematite that involved a continuous increase in cation occupancy as defective nanoparticles grew. Incipient hematite particles exhibited a refined Fe site occupancy as low as 0.60, then increased and plateaued at 0.80-0.90, achieving a metastable steady state with a composition halfway between the compositional endmembers FeOOH and Fe2O3 but retaining the R3 ̅c space group symmetry for hematite. Characterization of natural Fe (hydr)oxide samples from around the world revealed that R3 ̅c hydrohematite occurs widely, with Fe occupancies between 80 and 90% and water contents of 3.62 to 7.80 wt% H2O. Inelastic neutron scattering studies of hydrohematite and hematite suggested modes at 600- 900 cm-1, 1630 cm-1, and 3200 cm-1 were characteristic in the natural hydrohematite samples but absent in a stoichiometric hematite standard. Intergrown with natural hydrohematite samples were superhydrous goethite-like phases exhibiting an iron deficiency of 10-20 mol% relative to endmember goethite (α-FeOOH). We propose that hydrohematite is common in low-temperature occurrences of Fe oxide on Earth. By extension it may inventory large quantities of water in apparently arid planetary environments, such as the surface of Mars.