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Structural effects of OH Þ F substitution in trioctahedral micas of the system K2O-FeO-Fe2O3-Al2O3-SiO2-H2O-HF.

Abstract : The OH => F Substitution in trioctahedral ferrous micas has been investigated at 720 °C. 1 kbar PH2o- under /02 conditions set by the MW (FeiOj-Fe^O) buffer.The starting compositions belong to the annite-siderophyllite join: K(Fe-, xAlx)(Si3.xAlUx)01(,(OH): with x 0 (annite). 0.5 (Fe-eastonile). and 0.75 (Es). In F-bearing system. the compositions investigated belong to (OH.F)-annite. (OH.F)-Fe-eastonile and (OH.F)-Es joins. A single mica phase was observed lor (OH.F)-annite in 0 < XF < 0.5 compositional ränge, and in 0 < XF < 0.2 ränge for (OH.F)-Fe-eastonite and (OH.F)-Es. Bcyond these values. toward lo F-rich compositions. topaz. quartz. magnetite and glass coexist with mica. The correlation of the reticular distance dÜW] as a function of Xf shows that (OH.F)-annite and (OH.F)-Es behave differently as XF increases. Mössbauer measurements along the annite and Es (OH.F) joins demonstrate that the Fe1' content decreases from XFe3+ (Fe3*/Fe,otai) 9.1% for X,..jnn= 0 to 3.1% for XF..,„„ 0.5 and it increases slighlly from XFc3. 2.4% at XF.Es= 0 to 3% at XF.Es= 0.2. FTIR spectroscopy in the far-infrared region (200-50 cm"1) also shows an opposite trend wavenumber-shift between (OH.F)-annite and (OH.F)-Es with increasing XF. In the single phase ränge, the band observed at 67 cm ' in the (OH)-annite. corresponding to torsional Vibration motion of the tetrahedral layer (mode III).shifts to higher wavenumbers with X,.. In contrast. this band (mode III) decreases from 91 cm ' in the (OH)-Es end-member down to 88 cm-1 at XF 0.2. These shifts are related to changes in the K-site configuration. particularly to the variations of the tetrahedral rotation angle (a).The angle a increases from 2° for XF.ann 0 to 5.5° for„ 0.5 and it decreases from 8.5° at XF.Es= 0 to 5.3° at XF.E, 0.2. The OH => F Substitution induces local cationic changes and consequently a dimensional adaptation of sheets (limited in such micas to a 5.5°). which in turn conlrols the fluorine solubility in these studied micas. The results also show lhat the Fe-'*/Fe,„Ia| ratio in F-bearing micas is not only controlled by /02 but also by structural constraints.The fluorine content of natura] biotiles has to be taken into aecount to cslimate oxygen fugacities prevailing in the rocks.
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Boubker Boukili, Jean-Louis Robert, Jean-Michel Bény, François Holtz. Structural effects of OH Þ F substitution in trioctahedral micas of the system K2O-FeO-Fe2O3-Al2O3-SiO2-H2O-HF.. Schweizerische Mineralogische und Petrographische Mitteilungen, 2001, 81, pp.55-67. ⟨10.5169/seals-61680⟩. ⟨hal-00089707⟩



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