AN ELECTRON MICROPROBE ANALYSIS, SECONDARY ION MASS SPECTROMETRY, AND SINGLE-CRYSTAL X-RAY DIFFRACTION STUDY OF PHLOGOPITES FROM MT. VULTURE, POTENZA, ITALY: CONSIDERATION OF CATION PARTITIONING

Abstract

Mt. Vulture trioctahedral micas- 1M mainly consist of phlogopite-annite solid solutions with a minor component of brittle micas. However, both Li-free and Li- and F-rich compositions may coexist in the same volcano-stratigraphic level. We report the results of electron microprobe analysis (EMPA), secondary ion mass spectrometry (SIMS), and single-crystal X-ray diffraction (SCXRD) for three crystals (LC7-27Go, LC7-3Go, and LC7-1R) that are representative of both compositions. EMPA and SIMS showed that sample LC7-1R is richer in TiO2, Li2O, and F and poorer in H2O than the other samples. Structure refinements using anisotropic displacement parameters for the three samples, performed in space group C2/m, converged at 3.46≤R≤4.34, 3.89≤Rw≤4.39. When considering bond distances, the three samples can be described as homo-octahedral whereas mean atomic numbers suggest that only LC7-1R is meso-octahedral. Significant differences occur among the samples for some distortion parameters commonly used for micas. In particular, LC7-1R displays higher values of BLDM2, shiftM2 and lower values of ΔK-O4 and tK-O4 than those of samples LC7-3Go and LC7-27Go. These differences are ascribed to F- and Ti-substitutions. Cation distributions were obtained by combining EMPA, SIMS, and SCXRD data after analysis of the effect of normalization schemes commonly used in mica formula recalculation. In LC7-3Go and LC7-27Go, Ti is incorporated according to the Ti-Tschermak mechanism, whereas more than one Ti-substitution mechanism occurs in LC7- 1R. For the latter sample, octahedral vacancies are present leading to a complex substitution pattern and complex structural distortions related to the special octahedral compositions that were determined.