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Role of fluorine in the transfer of Be and the formation of beryl deposits: a thermodynamic model.

Abstract : After a thorough study on the tracing of the emerald origin by oxygen data by Zwaan et al. [12], the paper by Moine et al. [4] entitled ‘The role of fluorine in the formation of the Mananjary emerald deposits (eastern Madagascar)' demonstrates through a thermodynamic model that the metasomatic transformation of amphibolites into fluorine-enriched phlogopites was the cause for beryl precipitation. – This paper is important as it shows that fluorine can be a dominant ligand for some metals, like beryllium. As underlined by the authors, the reason for the BeF affinity in solution is that beryllium, despite the fact it is divalent, still displays a high ionic potential due to its small ionic radius. – Moine et al. [4] calculate that a fluid equilibrated with F-phlogopite (XF=0.7) at 500 °C and 2 kbar contains 275 ppm total fluorine, compared to 55 ppm dissolved in fluids buffered by wollastonite–quartz–fluorite (WQF) in the same conditions [11]. This paper therefore contributes to enlighten that fluorine-rich fluids can circulate through rocks in some natural geological settings. More precisely, it shows that rocks dominantly formed of hydrous silicates constitute a remarkable environment where are contradicted previous conclusions in the literature that “fluoride concentrations in natural hydrothermal solutions are restricted by equilibrium constraints involving fluorite, topaz and other fluoride-bearing minerals with low solubilities” (e.g., [2] and references therein). – The numerous local crystal-chemical factors that control (OH–F) exchanges in hydrous silicates (smectites, micas, amphiboles, tourmalines) are far from being understood and modelled for any given composition. However, this paper is a good illustration of the fact that, at least for a limited number of amphibole and biotite end-members with restricted Fe-free Mg-rich compositions, a thermodynamic database is now available that satisfactorily accounts for fluorination reactions. Numerous systematic structural studies of synthetic end-members have shown that some magnesian minerals like talc, phlogopite or tremolite, host the hydroxyl group in a symmetrical trioctahedral 36[Mg3] environment. Fluorination of such minerals is easy in all proportions and controlled by simple local crystal-chemical parameters. Firstly, there is no steric limitation to the complete substitution of OH− by F− in these minerals because the ionic radii of these two anions are very similar. Also, the proton exchanges one charge with the hydroxyl oxygen and is independent of any lattice atom. Such a hydroxyl group, which behaves as a separate entity, can easily be replaced by fluorine [8].
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Claire Ramboz. Role of fluorine in the transfer of Be and the formation of beryl deposits: a thermodynamic model.. Comptes Rendus Géoscience, Elsevier Masson, 2005, 337, pp.639-640. ⟨10.1016/j.crte.2005.03.007⟩. ⟨hal-00023371⟩



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