Electron- and ion-microprobe analyses, and genetic inferences of tourmalines of the foitite-schorl solid solution, Elba Island (Italy)
Aurisicchio, Carlo; Ottolini, Luisa; Pezzotta, Federicο
European Journal of Mineralogy Volume 11 Number 2 (1999), p. 217 - 226
published: Apr 19, 1999
manuscript accepted: Nov 27, 1998
manuscript received: Aug 5, 1998
ArtNo. ESP147051102012, Price: 29.00 €
Abstract The Li, B and H contents of foitite were investigated for the first time by means of secondary ion mass spectrometry (SIMS). The studied foitite occurs in the pockets of Elba pegmatites, not only as previously described, acicular and fibrous crystals, but also as black-violet, fibrous-to-prismatic overgrowth at the terminations of elbaitic, polychrome tourmalines. A variety of compositions, belonging to the foitite-schorl solid solution, were recognised. Foititic compositions observed as overgrowths of polychrome crystals are associated with the crystallisation of zeolites at the beginning of a zeolite hydrothermal stage. Morphologically similar overgrowths, but richer of schorlitic component, are associated with later hydrothermal circulation along late-stage fractures that crosscut the pegmatites and the host monzogranite. The agreement between the stoichiometric boron concentrations (as B2O3 wt.%), calculated assuming 3 B apfu, and in-situ SIMS data are good, with discrepancies generally within the estimated uncertainty of the ion-probe analytical procedure (3 % rel.). In the case of H, the SIMS values are, mostly, a little lower compared to values calculated assuming 4 H apfu. However, such a discrepancy is in the most cases within 5 %, and for the rest, within 7 % rel., and is comparable to the SIMS analytical uncertainty in this tourmaline compositional range. In the case of Li, the ion microprobe values closely match those derived by calculation of the Li required to fill the Y site. The results testify the high potential of SIMS for light-element, accurate analysis of compositionally complex samples.