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https://hdl.handle.net/2142/71640
Description
Title
The Role of Iron in Nontronite and Ferrihydrite
Author(s)
Lear, Paul Robert
Issue Date
1987
Doctoral Committee Chair(s)
Stucki, Joseph W.
Department of Study
Agronomy
Discipline
Agronomy
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Agriculture, Agronomy
Abstract
A major constituent of the Earth's crust, Fe is present in many minerals and affects their physicochemical properties. The role of Fe in determining the properties of nontronite and ferrihydrite was studied to better understand these minerals.
The reduction of structural Fe$\sp{3+}$ in the SWa-1 nontronite resulted in an intervalence electron transfer band in the visible absorption spectra. The intensity of the band was correlated to the number of Fe$\sp{3+}$-O-Fe$\sp{2+}$ linkages in the mineral structural. The Fe$\sp{3+}$-O-Fe$\sp{2+}$ linkages also produced ferromagnetic exchange coupling in reduced SWa-1 samples. The magnetic structure of unaltered nontronites, from various sources, was frustrated antiferromagnetic. Depending on the specific nontronite, the frustration was due to: (1) the distribution of Fe$\sp{3+}$ between cis and trans octahedral sites; (2) the presence of Fe$\sp{3+}$ in tetrahedral sites; (3) chemical disorder caused by diamagnetic cation in octahedral sites; and (4) a combination of the above.
The reduction of Fe$\sp{3+}$ decreased the swelling, surface area, and cation exchange capacity of the SWa-1 nontronite. The decreases were related and appeared to be due to the collapse of fully and/or partially expanded layers to unexpanded layers. The decrease in swelling may also have been due to the collapse of fully expanded layers to partially expanded layers. The mechanism of the collapse of expanded layers upon reduction of Fe$\sp{3+}$ is not understood.
The Mossbauer spectra of the unaltered SWa-1 nontronite was best fit to two octahedral Fe$\sp{3+}$ doublets. The reduced Swa-1 spectra for T $<$ 200 K could be used to estimate the ratio of Fe$\sp{2+}$ to total Fe. No significant differences were seen between the unaltered and reoxidized spectra, indicating that any structural changes which occurred during reduction seemed reversible upon reoxidation.
The magnetization curves of the "well ordered" and "poorly ordered" ferrihydrite indicated the spins were frozen near 27 and 15 K, respectively, possibly due to speromagnetic order. The aluminous ferrihydrites also exhibited spin freezing. The Al in the "well ordered" ferrihydrite appeared concentrated in discrete phases; it seemed randomly distributed throughout the structure in the aluminous "poorly ordered" sample.
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