Ideal annite has structural formula $\rm\{K\sp{+}\}\lbrack Fe\sbsp{3}{2+}\rbrack \langle Al\sp{3+}Si\sbsp{3}{4+}\rangle O\sbsp {2}(OH\sp{-})\sb2$ where $\{\ \},$ () and $\langle\rangle$ represent the interlayer, octahedral, and tetrahedral sites, respectively. Simple crystal chemical considerations suggest that ideal annite should exist as do other end-members having Fe replaced by Mg, Ni, Co, etc. However, due to structural constraints involving long range layer misfit and short range bond matching, ideal annite is never synthesized. Instead, the constraints are satisfied by both non-destructive partial oxydation (via H-loss) of the Fe$\sp{2+}$ and $\rm Fe\sp{3+}-Al\sp{3+}$ exchange. Consequently, real annite must actually contain small fractions of both (Fe$\sp{3+}\rbrack$ and $\rm\langle Fe\sp{3+}\rangle$ with an associated amount of hydrogen deficiency. The observed stoichiometry of real annite (assuming no octahedral vacancies) is: $$\eqalign{\{K\sp{+}\}\lbrack Fe\sbsp{1-x-y}{2+}Fe\sbsp{x}{3+}Al\sbsp{y}{3+}\rbrack\sb3 \langle & Al\sbsp{1-3y}{3+}Fe\sbsp{3y}{3+}Si\sbsp{3}{4+} \rangle\cr & O\sbsp{10+3x+3y}{2-} (OH\sp{-})\sb{2-3x-3y}\cr}$$where $x={{\lbrack Fe\sp{3+}\rbrack}\over{Fe\sb{tot}}}$ and $y={{\langle Fe\sp{3+}\rangle}\over {Fe\sb{tot}}}.$ We measured Fe$\sp{3+}$ (x and y) and Fe$\sp{2+}$ site populations in nine annite samples synthesized using the C-CH$\sb4$ and Ni-NiO buffers, and various equilibration temperatures. We obtained site population measurements with precisions of 0.2-1% of Fe$\sb{tot},$ whereas the generally accepted limit is 1-5% of Fe$\sb{tot}.$ By using temperatures over the entire stability range for annite, we have traced the evolution of $\rm\lbrack Fe\sp{3+}\rbrack$ and $\langle\rm Fe\sp{3+}\rangle$ site populations and have obtained an experimental lower bound on ${{Fe\sp{3+}}\over{Fe\sb{tot}}}.$ Temperature is seen to have a systematic effect that is understood in terms of differential thermal expansion between the octahedral and tetrahedral layers. To explain this effect, we propose a structural misfit model for annite mica that gives definite predictions on the $\rm\lbrack Fe\sp{3+}\rbrack$ and $\langle Fe\sp{3+}\rangle$ site populations and includes a cation size dependant octahedral flattening and the effect of octahedral/tetrahedral sheet differential thermal expansion. (Abstract shortened by UMI.)
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/9835 |
| Date | January 1996 |
| Creators | Mercier, Patrick H. J. |
| Contributors | Rancourt, Denis, |
| Publisher | University of Ottawa (Canada) |
| Source Sets | Université d’Ottawa |
| Detected Language | English |
| Type | Thesis |
| Format | 141 p. |
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