Fe-F and Al-F avoidance rule in ferrous-aluminous (OF, F) biotites.

International audienceThe results of infrared and Raman spectroscopic investigations in the OH-slrelching and lattiee-mode regions in synthetic ferrous-aluminous (OH.F)-biotites are presented. In the OH-stretching region (3800-3200 cm '). all micas studied present a high intensity peak at high frequencies [3669 cm ' for (OH)-annite and 3641 cm-' for (OH)-Es] which can be decomposed into two bands and a low intensity peak at low frequencies [3535 er1 for (OH)-annite and 3589 cm ' lor (OH)-Es] which suggests rather a vacant octahedral site. Along the (OH.F)-annite join. the intense peak at 3669 cm ' shifts to lower frequencies as XF increases from 0 to 0.4. In contrast. this peak shifts to higher frequencies along the (OH.F)-Es join (Es K(Fe:25Al())(Si225Al,75)Ol()(OH.F)2). The low-intensity V-band re¬ mains roughly unchanged. The two bands that compose the 3669 cm ' peak are assigned to a N-band resulting from OH-Fe2+Fe2+Fe2+ (Tri-6p vibrations and a Ib-band due to OH-Fe2*Fe2+AP* (Tri-7-) vibrations. As the amount of fluorine increases in micas of the (OH.F)-annite join. the N-band frequency varies weakly but its intensity decreases significantly, while the Ibband becomes more intense. In contrast, these two bands show an opposite behaviour in (OH.F)-Es micas. TTie Nband intensity increases whereas that of the Ib-band decreases. The opposite evolution of the two main bands in the OH-stretching region shows that F is preferentially linked to Fe rather than to AI in (OH.F)-anmte, whereas F is preferentially linked to AI rather than to Fe in (OH.F)-Es. Conse¬ quently. the bond strengths Al-F or Fe-F are not controlled by the Fe-F or Al-F avoidance rule (which would predict lhat fluorine is preferentially associated to Fe in all micas). but by structural constraints. The Al-F or Fe-F avoidance rule may not play a determining role on the fluorine content of the micas as it is generally agrecd.The maximum fluorine contents in micas mainly depend on the ability of the dimensional adaptation of tetrahedral and octahedral layers