Quantitative chemistry and compositional variability of fluorine fibrous amphiboles from Biancavilla (Sicily, Italy)

Compositional variability of the new fluorine fibrous amphiboles (fluoro-edenite) from the volcanic area of Biancavilla (Sicily, Italy) is reported here for the first time. Quantitative chemical analysis of a suite of four samples was performed by a standardized SEM-EDS microanalysis method, previously developed and tested on different typologies of fibrous amphiboles. The results highlighted compositional differences, especially concerning Si, Ca, Fe, and Na contents, both within the same and among the different samples. Compared to the previously investigated fluoro-edenite prismatic variety, the fluorine fibrous amphiboles showed average values of Si and Fe contents always higher, whereas Ca was significantly lower, which we consider a distinctive character for the fluorine fibrous variety. The Fe(3+)/Fe(tot) ratios, evaluated by Mossbauer spectroscopy, reflected different iron oxidation states: Fe(3+) was always more prevalent than Fe(2+), which was very low for two of the four samples analyzed. Employing the Leake classification, all the analyzed fluorine amphibole fibers showed an edenite-winchite trend, with a non negligible content of tremolite component. Both the fluorine amphibole fibers and the prismatic fluoro-edenite from Biancavilla may be correlated with the same genetic process, but the compositional variability reflect different growth conditions. The large variation observed for Fe(3+)/Fe(tot) ratios in the amphibole fibers is probably due to local variations of oxygen fugacity during crystallization. A workable hypothesis is that a hot metasomatizing fluid, enriched in fluorine and other incompatible elements, altered the volcanic rocks and caused the crystallization of either the fibrous fluorine amphiboles, by a very fast cooling, or the prismatic fluoro-edenite, by slow cooling.