Date of Award
Doctor of Philosophy
A. Duncan Scott
An integrated approach involving several experimental and analytical techniques was used to study the role of structural iron oxidation in the weathering of trioctahedral micas by aqueous solutions. Analytical techniques for the assay of Fe(II) by oxidimetry and for the estimation of octahedral sheet cation occupancies by x-ray diffraction were developed. A weathering apparatus to allow the continuous treatment of micas by fresh aqueous solutions was also developed. This apparatus was used to treat the 10- to 20-[mu]m size-fractions of three trioctahedral micas (a phlogopite, biotite, and siderophyllite) by aqueous solutions that varied in their tendency to promote oxidation, dissolution, and interlayer expansion reactions in the micas. The treatments generally involved 1 M KOAc(pH 4.7)-0.1 M K[subscript]2 EDTA solutions with or without 25% H[subscript]2O[subscript]2 as an oxidant, and were applied at 80°C for periods as long as 36 days. Experiments with deuterated mica samples and with K-depleted samples in 1 M Na solutions were also conducted. The time-dependence of the dissolution of the micas was determined by measuring the amounts of Al, Mg, and Fe in the effluent solutions after different periods and calculating apparent rate constants (ca. 10[superscript]-4 s[superscript]-1) using a heterogeneous kinetic model. Ratios of the rate constants obtained in different solutions were used to estimate the extent of exfoliation of the micas by the weathering treatments. Structural and chemical analyses of the micas before and after treatment were performed by conventional techniques (x-ray diffraction, infrared spectroscopy, Mossbauer spectroscopy) in addition to the two techniques that were developed. These analyses provided strong evidence for the deprotonation of structural hydroxyls and ejection of octahedral cations during the oxidation treatments and for the existence of vacant tetrahedral sites in micas. The main conclusions reached were (1) oxidation of structural Fe(II) in trioctahedral micas does not occur in aqueous solutions without an associated expansion or exfoliation of the interlayer region; (2) the charge created by oxidation is balanced by a combination of the deprotonation of structural hydroxyls, the ejection of octahedral cations (trivalent preferentially to divalent), and the loss of interlayer cations, and (3) the K-selectivity of the mica plays a key role in determining whether oxidation will occur in high-K solutions.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
James Edward Amonette
Amonette, James Edward, "The role of structural iron oxidation in the weathering of trioctahedral micas by aqueous solutions " (1988). Retrospective Theses and Dissertations. 9322.