Fe-57 Mossbauer effect study of bannisterite: a modulated 2 : 1-type phyllosilicate

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Abstract

Bannisterite is a modulated 2:1-type phyllosilicate having ten octahedral sites with hydroxyl groups in trans, cis and combined trans-cis configurations. Each octahedral site, depending on its proximity to the layer modulation, is exposed to a different degree of distortion. The electric field gradient at the Fe site and the site distortion are used to propose a model for the site assignment and oxidation mechanism in bannisterite. The results show that two models are compatible with the experimental results of fitting the spectrum of bannisterite. However, the model which allows for site distortion accounts better for the observed line overlap of the Fe-[6](2+) sites. Accordingly, the octahedral sites with a greater degree of distortion are assigned to the Fe-[6](2+) with lower electric quadrupole splitting, and the octahedral sites with lower site distortion are assigned to the Fe-[6](2+) with higher electric quadrupole splitting. Moreover, the ease of oxidation decreases in the order trans > trans-cis > cis.

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Subject classification (UKÄ) – MANDATORY

  • Geology

Keywords

  • electric field gradient, distortion, bannisterite, Mossbauer, assignment, site
Original languageEnglish
Pages (from-to)187-199
JournalMineralogical Magazine
Volume70
Issue number2
Publication statusPublished - 2006
Publication categoryResearch
Peer-reviewedYes