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Volcanic activity at Mt. Vulture lasted about 750 ka and produced Si[O.sub.2]-undersaturated volcanic rocks that can be classified as old (~700 ka), intermediate (~600-550 ka), and young (~130 ka). The intermediate deposits consist of pyroclastic falls and flows and lavas with compositions ranging from phonolite to foidite. A recent revision of the stratigraphic setting allowed these deposits to be classified into one synthem (the Barile Synthem) and further subdivided into four subsynthems (Toppo S. Paolo, Rionero, Vulture-S. Michele, and Ventaruolo). In the present investigation, trioctahedral micas from sample VUT 191 in the Vulture-S. Michele Subsynthem are considered. The host rock has modal diopside (20.2%), analcime (22.8%), plagioclase (27.8%), hauyne (5%), phlogopite (8.9%), and magnetite (6.3%). The micas were studied using chemical (EPMA, C-H-N, SIMS), structural (SCXRD), and spectroscopic (Mossbauer) methods. EPMA of 36 crystals from thin sections and 6 discrete crystals selected for the structural analysis showed remarkable compositional variability, as follows (in wt%): Si[O.sub.2] = 33.14-38.01, [Al.sub.2][O.sub.3] = 15.56-20.45, MgO = 13.[O.sup.2-]20.81, Fe[O.sub.tot] = 6.34-14.08, Ti[O.sub.2] = 2.34-6.02, [K.sub.2]O = 6.03-9.48, [Na.sub.2]O = 0.50-0.78, and BaO = 0.89-4.06; all crystals proved to be phlogopite. Elemental C-H-N analyses yielded [H.sub.2]O = 2.86 [+ or -] 0.36 wt%. The water content was also determined by SIMS on two single crystals, labeled VUT 191_2 and VUT 191_19, which yielded values of 3.81 [+ or -] 0.12 and 1.72 [+ or -] 0.08 wt% [H.sub.2]O, respectively. Mossbauer investigation showed that all the iron in VUT191 mica is octahedral with [Fe.sub.2+] 25.5% and [Fe.sup.3+] = 74.5%, confirming that Vulture micas are particularly [Fe.sup.3+]-rich, as also found from previous investigations. Structure refinements using anisotropic displacement parameters were performed in space group C2/m and converged at 1.89 < R [less than or equal to] 3.17, 2.09 [less than or equal to] [R.sub.w] [less than or equal to] 3.43%. All of the analyzed micas belong to the 1M polytype but exhibit remarkable variations in the c parameter from 10.1569(4) to 10.2458(4) [Angstrom]. The chemical and structural parameters indicate that the studied micas can be divided into two groups: the first encompassing strongly dehydrogenated micas affected mainly by Ti-oxy [[sup.IV][M.sup.2+] + 2(OH) [left and right arrow] WTi4+ + 2[0.sup.2-] + [H.sub.2]] and [M.sup.3+]-oxy [[sup.IV] [M.sup.2+] + (OH)-[left and right arrow] W[M.sup.3+] + [O.sup.2-] + 1/2 [H.sub.2], with [M.sup.3+] = [Fe.sup.3+], [Al.sup.3+] substitutions. The second group consist of samples in which vacancy-bearing mechanisms, 2 [sup.VI][M.sup.2+] [left and right arrow] [sup.VI][Ti.sup.4+]+ [sup.VI] [cube root]and 3[sup.IV][M.sup.2+] [left and right arrow] 2'a[M.sup.3+] +[sup.VI][cube root] occur. Keywords: Volcanic phlogopite, Mossbauer spectroscopy, structure refinement, CHN, SIMS, crystal chemistry, substitution mechanisms

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 LCT-1R) that are representative of both compositions. EMPA and SIMS showed that sample LC7-1R is richer in Ti[O.sub.2], [Li.sub.2]O, and F and poorer in [H.sub.2]O than the other samples. Structure refinements using anisotropic displacement parameters for the three samples, performed in space group C2/m, converged at 3.46 [less than equal to] R [less than or equal to] 4.34, 3.89 [less than or equal to] [R.sub.w] [less than or equal to] 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, LCT-1R displays higher values of [BLD.sub.M2], [shift.sub.M2] and lower values of [DELTA][K.sub.04] and [t.sub.K-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 LCT-3Go and LCT-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. Keywords: Analysis, chemical (mineral), EPMA and SIMS investigation, crystal structure, trioctahedral micas--1M, major and minor elements, Ti-F-bearing phlogopites, trace elements and REE, Li-bearing micas, XRD data, single-crystal structure refinement

The iron sulfate FeOHS[O.sub.4] studied was obtained as a dehydration product of metahohmannite [Fe.sub.2][([H.sub.2]O).sub.4][O[(S[O.sub.4]).sub.2]] during a synchrotron real-time powder diffraction experiment. As quoted in the literature, FeOHS[O.sub.4] has iron atoms octahedrally coordinated with two hydroxyl groups and four sulfate O atoms, while each hydroxyl group is bonded to two iron atoms. This compound is commonly described in the orthorhombic system with space group Pnma, lattice parameters [a.sub.j] = 7.33, [b.sub.j] = 6.42, and [c.sub.j] = 7.14 A ([a.sub.j], [b.sub.j], and [c.sub.j] are the Johansson lattice parameters), and Z = 4. However a preliminary Rietveld refinement of the pattern at about 220 [degrees]C using the structural model from the literature yielded a poor fit of the observed data and a final [R.sub.p] value of about 23%. A careful analysis of the calculated powder diffraction pattern showed unexpected peaks, not observed in the experimental trace, for h = 2n + 1, while sharp reflections for h = 2n seemed to point to different lattice constants and space group. The recognition of the order-disorder character of the FeOHSO4 compound was the key to successfully interpreting the unexpected features of the experimental powder pattern and the misfit with respect to the calculated pattern. In fact, FeOHS[O.sub.4] belongs to a family of OD structures formed by equivalent layers of symmetry Pbmm. Only two MDO (Maximum Degree of Order) polytypes are possible. MDO1 results from a regular alternation of stacking operators [2.sub.1/2] and [2.sub.-1/2], and yields an orthorhombic structure with space group Pnma and lattice parameters [a.sub.j] = 7.33, [b.sub.j] = 6.42, and [c.sub.j] = 7.14 A. MDO2 results from the [2.sub.1/2][12.sub.1/2] [12.sub.1/2] ... sequence of symmetry operators and yields a monoclinic structure with space group P[2.sub.1]/c,[a.sub.M] = 7.33, [b.sub.M] = 7.14, [c.sub.M] = 7.39 A, and [beta] = 119.7[degrees]. The analysis of one-dimensional stacking disorder was performed by fitting the observed XRPD pattern with a calculated intensity curve generated by DIFFaX. The disorder model was investigated by taking into account a probability matrix for the occurrence of OD layer sequences. The best fit ([R.sub.p] = 0.009) to the observed powder pattern was obtained with a 61:39 ratio of monoclinic and orthorhombic polytypes for a fully disordered OD layers sequence.