Fluorine in micas: crystal-chemical control of the OH-F distribution between trioctahedral and dioctahedral sites
Robert, Jean-Louis; Beny, Jean-Michel; Ventura, Giancarlο Della; Hardy, Michel
published: Mar 4, 1993
manuscript accepted: Jul 8, 1992
manuscript received: Nov 19, 1991
ArtNo. ESP147050501015, Price: 29.00 €
Abstract The investigation of the synthetic OH-F tetrasilicic magnesium mica (TMM) solid solution K(Mg2.5□0.5)Si4O10(OH,F)2, by FTIR, Raman, XRD and TGA demonstrates that fluorine strongly favours the trioctahedral sites. The partition coefficient DF = (F/OH)Tri/(F/OH)Di of fluorine between trioctahedral and dioctahedral sites is 22 ± 3. This behaviour is interpreted in terms of local charge balance around the hydroxyl group. In the trioctahedral environment, the OH group acts as a point charge; it has no or negligible interaction with the tetrahedral oxygens and can be easily replaced by fluorine. In dioctahedral sites, the hydroxyl proton interacts by hydrogen bonding with the underbonded apical oxygens of adjacent tetrahedra. Because it acts as a dipole rather than a single anion, its replacement by fluorine is difficult. Along the (OH-F)-TMM join, vibrational spectrometries indicate a shortening of Si-0 bonds resulting in a compression of tetrahedra, specially important for XF = F/(F + OH) > 0.5. Structural modifications result from the suppression of OH···O interactions by hydrogen bonding between the hydroxyl proton and the oxygens of adjacent tetrahedra, as F' replaces OH'. The variation of cell dimensions also reflect this partition of fluorine on behalf of trioctahedral sites: b, c and the cell volume V decrease significantly in the range 0 < xf 0.5. These variations are interpreted in terms of perturbations of attractive OH···O and repulsive H+ - K+ interactions, as F' replaces OH'. The partition of fluorine in favour of the trioctahedral phase in naturally occurring two-mica assemblages can be interpreted in the same terms. This work also considers the crystal-chemistry of fluorine in muscovite and lepidolites. In muscovite, it is suggested that hydroxyls pointing towards two apical oxygens of SiO4 tetrahedra can be more easily replaced by fluorine. The common correlation between Li and F in lepidolites may be explained from the same crystal-chemical model, since the OH group acts as a point charge in zinnwaldite, trilithionite and polylithionite. The limit of this correlation is determined by the strength of OH···O bonds between the hydroxyl group and the oxygens of surrounding tetrahedra, and is mainly controlled by the tetrahedral charge. The crystal-chemical model proposed in this work also explains why the Li-rich trioctahedral micas ephesite and bityite do not contain fluorine.