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The crystal chemistry of Ti-rich phlogopite from deposits of the oldest activity (~740 ka) of Mt. Vulture, Potenza, Italy, was investigated to identify the substitution mechanisms in Ti-rich phlogopite and to determine its potential as a geohygrometer. Substitution mechanisms were determined by electron probe microanalysis (EPMA), single-crystal X-ray diffraction (SCXRD), Mössbauer spectroscopy, and micro-Fourier transform infrared (FTIR) spectroscopy. Magnetite and sanidine from the mica-rich host rocks were also analyzed. Use of the biotite-magnetite-sanidine geohygrometer in these volcanics (essentially trachytic-phonolitic ignimbrites) is exploited. All investigated phlogopite samples consist of the 1M polytype, with 5.3275(2) ≤ a ≤ 5.3635(4) Å, 9.2211(4) ≤ b ≤ 9.2958(8) Å, 10.1211(5) ≤ c ≤ 10.281(1) Å, and 99.980(3) ≤ β ≤ 100.097(2)°. Structure refinements in space group C2/m converged to 2.42 ≤ R1 ≤ 4.00% and 2.04 ≤ wR2 ≤ 4.50%. VIFe3+ /Fetot from Mössbauer analyses ranged from 34(1)-89(1)%. The main bands in the OH-stretching region are 3709, 3682, and 3658 cm-1, and were assigned to 3Mg-OH--K-OH , 3Mg-OH--K-O2-, and 2MgFe3+ - OH--K-O2- local configurations, respectively. The overall crystal chemical features are compatible with the M3+-Tschermak substitutions (VIM2+ + IVSi4 ↔ VIM3+ + IVAl, with M3+ = Al, Fe3+), M3+-oxy [VIM2+ + (OH)- ↔ M3+ + (O) 2- + 1/2H2↑ and Ti-oxy substitutions [VIM2+ + 2(OH)-1 → VITi4+ + 2(O)2- + H2↑]. The magnetite composition varies between Mt82Uv18 and Mt93Uv07, whereas the proportion of orthoclase with respect to albite plus anorthite is 0.77 ≤ XOr =≤ 0.82. A partly ionic model was used to estimate annite activity, starting from experimentally determined substitutions in phlogopite. The resulting water fugacity was an order of magnitude lower than that calculated using stoichiometry to determine the phlogopite formula. The results are consistent with the occurrence of a well-developed hydrothermal system that altered the magmatic oxygen fugacity conditions at Mt. Vulture. [ABSTRACT FROM AUTHOR]

Trioctahedral micas from kamafugitic lavas of Presidente Olegario (Brazil) are Ti-Fe-bearing fluorophlogopites. They were investigated using a combination of electron probe microanalysis (EPMA), single-crystal X-ray diffraction (SCXRD), and Mössbauer spectroscopy. EPMA data yielded the following ranges: Al2O3 (8.7-10.1 wt%), MgO (19.6-20.6 wt%), FeO (5.7-6.5 wt%), TiO2 (5.9-7.5 wt%), K2O (8.9-9.7 wt%), Na2O (0.4 -0.6 wt%), and fluorine (3.4-4.1 wt%). Mössbauer investigation indicated: VIFe2+ ~ 60%, VIFe3+ ~ 10%, IVFe3+ ~ 30%. X-ray analysis indicated the 1M polytype, with cell parameters in the range a = 5.3208-5.3376, b = 9.2210-9.2464, c= 10.1227-10.155 Å, β= 100.157-100.194°. Structure refinements using anisotropic displacement parameters were performed in space group C2/m and converged to 2.01 ≤ R1 ≤ 3.51,2.00 ≤ wR2 ≤ 3.51%. The micas have no VIAl3+ and Na is probably partitioned over both interlayer and octahedral sites. Major substitutions are OH- ↔ F-, and the Ti-oxy substitution: VIM2+ + 2(OH)- ↔ VITi4+ + 2O2- + H2↑. Accordingly, the structural features of the studied micas encompass those typical of both fluorophlogopites and micas affected by Ti-oxy substitution. [ABSTRACT FROM AUTHOR]

Selected phlogopite flakes from Mt. Vulture in southern Italy were studied using a combination of single-crystal techniques: electron microprobe analysis (EMPA), secondary ion mass spectrometry (SIMS), single-crystal X-ray diffraction (SCXRD), and micro-X-ray absorption near-edge spectroscopy (XANES). The latter technique was employed to analyze the structure of the Fe-K absorption edge over the region from 7080-8100 eV and to determine Fe3+/ΣFe at a micrometer scale, albeit with large error bars due to known effects of orientation on pre-edge energy. The annite component, Fe/(Mg+Fe), of the samples studied ranged from 0.16 to 0.31, the Ti content from 0.11 to 0.27 atoms per formula unit (apfu) and the Ba content from 0.03 to 0.09 apfu. SIMS analysis showed H2O (wt%) = 1.81-3.30, F (wt%) = 0.44-1.29, and Li2O (wt%) = 0.001-0.027. The intra single-crystal chemical variability for major/minor elements (Mg, Fe, Al, Ba, Ti, and K) was found particularly significant for samples VUT191_11 and PG5_1, less significant for the other samples of the set. SIMS data relative to crystals VUT187_24, VUT191_10, VUT191_11, and VUT187_28 showed a noteworthy variation in the concentrations of some light elements (H, Li, and F) with coefficient of variation CV (as 1σ%) up to ~18% for H2O. The analyzed micas belong to the 1M polytype. Structure refinements using anisotropic displacement parameters were performed in space group C2/m and converged at 3.08 ≤ R ≤ 3.63, 3.32 ≤ Rw ≤ 3.98%. Micro-XANES results yielded Fe3+/ΣFe from 51-93%. Previous Mössbauer data from powdered samples suggested Fe3+/ΣFe values ranging from 49-87%. However, the Fe3+ content determined by both techniques is sometimes remarkably different, in part because of the large errors (±10-15%) presently associated with the micro-XANES technique and in part because the Fe3+ content of a single crystal may significantly depart from the average value obtained from routine Mössbauer analysis.The combination of EMPA, SIMS, and micro-XANES resulted in the characterization of the samples at a comparable spatial scale. By means of in-situ data and the results of crystallographic investigations, the occurrence of different relative amounts of M3+-oxy [VIM2+ + (OH)- ↔ VIM3+ + O2- + 1/2H2↑], Ti-oxy substitutions [VIM2+ + 2(OH)- ↔ VITi4+ + 2O2- + H2↑], and Ti-vacancy (⇔) substitution (2VIM2+ ↔ VITi4+ + VI⇔) was ascertained for the studied samples. [ABSTRACT FROM AUTHOR]