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Accurate determination of F in minerals is a diffi cult task even when high F concentrations are present. Fluorine usually is determined by means of electron micro-probe analysis (EMPA) standardized on non-silicate-matrix compounds (e.g., fl uorite), and some previous work has revealed the diffi culties in determining F at high concentrations such as found in the humite-group minerals. Moreover, when both single-crystal structure refi nement (SREF) and EMPA are available for the same crystal, the two estimates do not always agree. On the other hand, the secondary ion mass spectrometry (SIMS) technique is not easily applied at high F concentrations due to the existence of matrix effects related to the chemical composition and structure of the sample as well as to the concentration of the element itself. We tested the agreement among these analytical techniques in the estimation of high F contents and propose an analytical procedure for the analysis of fl uorine. Our results indicate that careful selection of working conditions for EMPA of F together with appropriate correction, can yield accurate fl uorine concentrations in minerals. Fluorine data extracted from refi ned site occupancies are systematically overestimated. New accurate working curves have been worked out for SIMS analysis of F taking Si and Mg, in turn, as the reference element for the matrix. Humite-group minerals show SIMS matrix effects on the order of ~10%. In analyzing fl uoborite in the most unfavorable cases, the difference in Ion Yield (F/Mg) between "disoriented" humite-group minerals and "oriented" fl uoborite samples can reach ~27%. Finally, a lower than expected IY(F/Si) from the F/Si working curve (made with humite minerals) is shown by topaz, which can be ascribed to chemical matrix effects, as well as to the covalent-type bonding between F and the major element in the matrix (Al). they possess lower resolving power than TAP (Fialin et al. 1993). With these new refl ectors, multiple-order interferences greater than the second-order are absent (e.g., the third-order PKα peak in apatite). Nevertheless, interferences due to second-order peaks from Mg and Al and fi rst-order L-lines from Fe still survive and constitute a limitation in the determination of fl uorine in minerals rich in Mg, Al, and transition elements (Raudsepp 1995).

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Biotites from mafic rocks occurring at different stratigraphic levels of the Ivrea-Verbano Mafic Complex are studied. The rocks are gabbros and diorites. All the biotites are intermediate between phlogopite and annite [0.28