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

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-FeO) 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 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.