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We have particularly focused our attention on the mineralogy of fluorides and on the distribution of fluorine in the alteration products. Although fluorides have been reported in other studies of volcanic incrustations (Naboko, 1957; Stoiber and Rose, 1974; Naughton et al., 1976; Rosenberg Philip, 1988; Keith, 1991a,b; Papike, 1992; Serafimova, 1997), the geochemistry and mineralogy of fluorine in the fumarolic environment is almost unknown. This environment is especially enriched in fluorine because of the condensation of HF gas. HF reacts strongly with silicates, so its role in the alteration of rocks by volcanic gases could be important. Fluorine is also adsorbed on tephra particles inside explosive eruption columns. Analyses of leachates of these tephras indicate SiF2(2/6-) and AlF(+2) as stable species in acidic conditions (Oskarsson, 1980). The high concentration of fluorine (4000 mg l21) in some ash falls results in livestock death, as in the Hekla (1845 and 1947) and Grimsvotn (1934) eruptions (Oskarsson, 1980) in Iceland…

10. Concluding remarks

… The fumarolic environment studied is very rich in fluorine. Whole rock fluorine contents range from 1 to 5 wt%. Aluminum fluorides, which are rare in nature, are commonly observed in this fumarolic environment. In the presence of fluorine and in acidic conditions, the dominant aqueous Al species are fluoride complexes, even in the presence of significant amounts of sulfates in solution. Fluorine enrichment in the altered silicates and in silica incrustations indicates that fluorine plays an important role in the alteration of the primary minerals and in the mobilization of silica into the aqueous phase…

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