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Ti-rich phlogopite from Mt. Vulture (Potenza, Italy) investigated by a multianalytical approach

¹ Dipartimento Geomineralogico, Università di Bari, via E. Orabona 4, 1-70125 Bari, Italy
² Dipartimento di Scienze Geologiche, Università di Roma Tre, Largo S. Leonardo Murialdo 1, I-00146 Roma, Italy
³ Dipartimento di Sanità Pubblica, Sezione di Fisica, Plesso Biotecnologico Integrato, via Volturno 39, I-43100 Parma, Italy

^* Corresponding author, e-mail: f.scordari{at}geomin.uniba.it

Trioctahedral mica samples, collected at Cava St. Antonio (Mt. Vulture, Italy) were studied by combining electron-microprobe and C-H-N elemental analyses, single-crystal X-ray diffraction refinement, Mossbauer and Fourier transform infrared spectroscopies.

Electron-microprobe analyses show the crystals to be quite homogeneous with TiO₂3 wt% and F ranging from 0.42 to 0.59 wt%. Quantitative analyses of H combined with ferric/ferrous ratios from Mossbauer data allowed reliable crystal-chemical formulae to be derived. The results suggest that the entry of both Ti⁴⁺ and Fe³⁺ in the structure occurs through R-oxy substitution mechanisms involving deprotonation at O(4). This inference is supported by X-ray structure-refinement results (notably the c cell-parameter, the off-centering of the M2 cation towards O(4), the bond-length distortions of the cis-M2 octahedron) obtained using anisotropic thermal parameters in space group C2/m. The amount of oxy-substitutions from both electron-microprobe and X-ray data is in agreement with carbon-hydrogen-nitrogen analyses which give an average anion composition (OH_1.25O_0.65F_0.10).

Polarized-light infrared spectroscopy shows a complex OH-stretching spectrum which is composed of several overlapping (at least five) components. These can be assigned to the main octahedral local configurations that are compatible with the chemical composition. Pleochroic Fourier transform infrared spectroscopy measurements done along the principal optical directions show that the O-H bond axis is tilted from [001] and provide the average orientation of the O-H dipole in the structure: O-H ^ 23° and O-H ^ 56°.

Key-words: trioctahedral 1M-micas bearing titanium, crystal chemistry, EMPA, C-H-N elemental analysis, SREF, Mossbauer, polarized-light FTIR.

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