We have studied the oxidation in air of a well characterized biotite sample. Large single crystal wafers were annealed at various temperatures up to 875$\sp\circ$C and for various times up to 94 hours. The oxidation proceeds primarily via the oxyannite reaction: $\rm (Fe\sp{2+} + OH\sp-)\sb{mica} \to (Fe\sp{3+} + O\sp{2-})\sb{mica} + H$ and was therefore conveniently followed by $\sp{57}$Fe Mossbauer spectroscopy, which resolves the 2+ and 3+ valence states of iron. The main features of the annealing time and temperature dependencies of the $\rm Fe\sp{3+}/Fe\sp{2+}$ amounts are understood in terms of a simple model in which: (i) the overall oxidation reaction proceeds homogeneously via a time-wise bottleneck step that follows a classic thermal activation law and (ii) a certain fraction of the original Fe$\sp{2+}$ is inaccessible to the oxidation reaction. The resulting barrier energy, $\rm E\sb{b} = 2.36\sbsp{-.02}{+.01}$ eV is in the range of measured barrier energies for dehydroxylation of layer silicates. This suggests that the bottleneck step may be local dehydroxylation: $\rm (OH\sp-\ \to O\sp{2-}\ + H\sp+).$ The persistent Fe$\sp{2+}$ can be understood from local crystal-chemical considerations. (Abstract shortened by UMI.)
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/7548 |
| Date | January 1992 |
| Creators | Tume, Pamela. |
| Publisher | University of Ottawa (Canada) |
| Source Sets | Université d’Ottawa |
| Detected Language | English |
| Type | Thesis |
| Format | 131 p. |
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