Structural effects of OH double right arrow F substitution in trioctahedral micas of the system: K2O-FeO-Fe2O3-Al2O3-SiO2-H2O-HF

The OH double right arrow F substitution in trioctahedral ferrous micas has been investigated at 720 degreesC, 1 kbar P-H2O, under fO(2) conditions se t by the MW (Fe2O3-Fe1-xO) buffer. The starting compositions belong to the annite-siderophyllite join: K(Fe3-xAlx) (Si3-xAl1-x)O-10(OH)(2) with x = 0 (annite), 0.5 (Fe-eastonite), and 0.75 (Es). In F-bearing system, the compo sitions investigated belong to (OH,F)-annite, (OH,F)-Fe-eastonite and (OH,F )-Es joins. A single mica phase was observed for (OH,F)-annite in 0 less th an or equal to X-F less than or equal to 0.5 compositional range, and in 0 less than or equal to X-F less than or equal to 0.2 range for (OH,F)-Fe-eas tonite and (OH,F)-Es. Beyond these values, toward to F-rich compositions, t opaz, quartz, magnetite and glass coexist with mica. The correlation of the reticular distance d(060) as a function of X-F shows that (OH,F)-annite and (OH,F)-Es behave differently as X-F increases. Moss bauer measurements along the annite and Es (OH,F) joins demonstrate that th e Fe3+ content decreases from XFe3+ (Fe3+/Fe-total) = 9.1% for XF-aan = 0 t o 3.1% for XF-ann = 0.5 and it increases slightly from XFe3+ = 2.4% at XF-E s = 0 to 3% at XF-Es = 0.2. FTIR spectroscopy in the far-infrared region (2 00-50 cm(-1)) also shows an opposite trend wavenumber-shift between (OH,F)- annite and (OH,F)-Es with increasing X-F. In the single phase range, the ba nd observed at 67 cm(-1) in the (OH)-annite, corresponding to torsional vib ration motion of the tetrahedral layer (mode III), shifts to higher wavenum bers with X-F. In contrast, this band (mode III) decreases from 91 cm(-1) i n the (OH)-Es end-member down to 88 cm(-1) at X-F = 0.2. These shifts are r elated to changes in the K-site configuration, particularly to the variatio ns of the tetrahedral rotation angle (alpha). The angle alpha increases fro m approximate to2 degrees for XF-ann = 0 to 5.5 degrees for XF-ann = 0.5 an d it decreases from 8.5 degrees at XF-Es = 0 to 5.3 degrees at XF-Es = 0.2. The OH double right arrow F substitution induces local cationic changes an d consequently a dimensional adaptation of sheets (limited in such micas to alpha = 5.5 degrees), which in turn controls the fluorine solubility in th ese studied micas. The results also show that the Fe3+/Fe-total ratio in F- bearing micas is not only controlled by fO(2) but also by structural constr aints. The fluorine content of natural biotites has to be taken into accoun t to estimate oxygen fugacities prevailing in the rocks.