It is generally believed that oxidation of ferruginous biotites proceeds by the oxybiotite substitution:
Fe²⁺ + OH = Fe³⁺ + 0²⁻ + ½H₂
The role of this mechanism as an equilibrium process in hydrothermal systems was studied for two compositions, annite, KFe₃,AISi₃O₁₀(OH)₂, and siderophyllite, KFe₂․₅AI₂,Si₂․₅O₁₀(OH)₂ , an annite with one Tschermak’s substitution. These biotites were synthesized from oxide mixes and then annealed in Shaw-type bombs under controlled hydrogen fugacities. Run conditions were chosen near the maximum thermal stability limit and within the biotite stability field along lines of constant hydrogen fugacity. Yields range from 93-96% for annite and 95-98% for siderophyllite. Sanidine is the major impurity identified along with minor amounts of fayalite or magnetite. The ferrous iron contents of the biotites were determined by the wet chemical technique developed by Whipple (1974). Total iron as FeO, as well as K₂O, AI₂O₃, and SiO₂ were determined with the electron microprobe, and the biotite compositions were found to be close to the stoichiometry of the ideal end members. Ferric iron was determined by the difference between total and ferrous iron. Hydrogen contents were measured by heating the sample in vacuo, reducing all hydrogen species released, and determining the moles of 1-1, volumetrically.
The hydrogen contents of the biotites are low (<3% H₂O) and do not vary with hydrogen fugacity for a given composition. The ratio of ferrous to total iron was found to be directly proportional to the hydrogen fugacity regardless of temperature. For hydrogen fugacities of 1 to 100 bars, this ratio varies from 0.76 to 0.90 for the annites and from 0.86 to 0.96 for the siderophyllites. The amounts of ferrous iron and the nearly constant difference in ferrous iron content between the two compositions is consistent with the proposed instability of a completely reduced annite due to structural misfit between the octahedral and tetrahedral layers (Hazen and Wones, 1972).
The annite stability field as presented by Eugster and Wones (1962) can thus be contoured with lines of constant ferrous iron content. The curves mimic the trends of the solid oxygen buffers which suggests that the mechanism of oxidation is similar for both. Moreover, when mineral formulas are calculated on the basis of 12 anions there is an excess of atoms in the octahedral sites. This excess varies directly with the hydrogen fugacity of the runs. Recalculation of the formulae on the basis of seven octahedral and tetrahedral cations results in a variable number of anions (11.6-12.0) that is positively correlated with the amount of ferrous iron. This, together with the lack of variation of the hydrogen contents in response to changing oxidation state, suggests that the oxybiotite substitution is not the controlling mechanism in hydrothermal biotite oxidation. The data support the following as an alternative reaction:
Fe²⁺ + ¼O₂ = Fe³⁺ + ½O²⁻
In this model the reduced biotites have anion vacancies that are filled as oxidation proceeds without changing the hydrogen content. The data also suggest that other anion vacancy producing reactions may be occurring in addition to the reaction above. / Master of Science
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/53717 |
| Date | January 1986 |
| Creators | Rebbert, Carolyn Rose |
| Contributors | Geological Sciences |
| Publisher | Virginia Polytechnic Institute and State University |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | en_US |
| Detected Language | English |
| Type | Thesis, Text |
| Format | ix, 71 leaves, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | OCLC# 15170488 |
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