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Phlogopite from the Ventaruolo subsynthem volcanics (Mt Vulture, Italy): a multi-method study

¹ Dipartimento Geomineralogico, Università degli Studi di Bari, Via E. Orabona 4, I-70125 Bari, Italy
² Dipartimento di Scienze della Terra, Università G. d’Annunzio, Via dei Vestini 3, I-66013 Chieti Scalo, Italy
³ Dipartimento di Sanita’ Pubblica, Sezione di Fisica e CNISM, Plesso Biotecnologico Integrato, via Volturno 39, I-43100 Parma, Italy

^* E-mail: f.scordari{at}geomin.uniba.it

Volcanic activity at Mt Vulture lasted throughout the Middle Pleistocene and produced SiO₂-undersaturated volcanics. Deposits from the Monte Vulture stratovolcano have been classified into four subsynthems and clustered into the Barile Synthem. In the present investigation, trioctahedral micas from the uppermost units (the Ventaruolo Subsynthem) of the Barile Synthem are considered. The samples are labelled VUT187. The phlogopitic micas were separated from the host rock (an olivine-foidite) and underwent chemical (electron microprobe analysis - EMPA and C-H-N), structural (single-crystal X-ray diffraction) and spectroscopic (Mössbauer) investigations.

The EMPA yielded: MgO (17.62–21.89 wt.%), FeO_tot (5.98–9.78 wt.%), TiO₂ (1.81–3.92 wt.%) and Al₂O₃ (14.47–17.98 wt.%), with H₂O contents = 2.86 (±0.42) wt.% determined by C-H-N analyses. Mössbauer investigation provided ^[VI]Fe²⁺ = 12.6%, ^[VI]Fe³⁺ = 87.4%. The chemical and structural data are consistent with the occurrence of Ti-oxy, ^[VI]M²⁺ + 2(OH)^{– [VI]}Ti⁴⁺ + 2O^2– + H₂, and M³⁺-oxy substitutions, ^[VI]M²⁺ + (OH)^{– [VI]}M³⁺ + O^2– + H₂, with M³⁺ = Fe³⁺, Al³⁺. In particular, Fe³⁺-oxy substitution has affected the Fe²⁺/Fe³⁺ ratio in the studied sample. This is probably due to the fact that interaction with underground water or a hydrothermal system may have altered the oxygen fugacity and raised the Fe³⁺ content of VUT187 phlogopite with respect to magmatic values.

KEYWORDS: volcanic phlogopite, Mössbauer spectroscopy, structure refinement, C-H-N elemental analysis, crystal chemistry, substitution mechanisms