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

Abstract

Volcanic activity at Mt Vulture lasted throughout the Middle Pleistocene and produced SiO 2 -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 olivinefoidite) and underwent chemical (electron microprobe analysis -EMPA and C-H-N), structural (singlecrystal X-ray diffraction) and spectroscopic (Mössbauer) investigations. The EMPA yielded: MgO (17.62À21.89 wt.%), FeO tot (5.98À9.78 wt.%), TiO 2 (1.81À3.92 wt.%) and Al 2 O 3 (14.47À17.98 wt.%), with H 2 O contents = 2.86 (Ô0.42) wt.% determined by C-H-N analyses. Mössbauer investigation provided [VI] Fe 2+ = 12.6%, [VI] Fe 3+ = 87.4%. The chemical and structural data are consistent with the occurrence of Ti-oxy, [VI] M 2+ + 2(OH) À > [VI] Ti 4+ + 2O 2À + H 2 , and M 3+ -oxy substitutions, [VI] M 2+ + (OH) À > [VI] M 3+ + O 2À + ÝH 2 , with M 3+ = Fe 3+ , Al 3+ . In particular, Fe 3+ -oxy substitution has affected the Fe 2+ /Fe 3+ 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 Fe 3+ content of VUT187 phlogopite with respect to magmatic values.

The role of SIMS in the investigation of the complex crystal chemistry of mica minerals

“…This applies particularly to micas from Vulture deposits of intermediate stratigraphic position (and age), to which also SA phlogopite belongs, that experienced to a different extent the effects of late-magmatic or post magmatic events (freatomagmatic activity, polybaric/polythermal magmatic evolutions, etc. [10][11]23]).…”

“…All these involve petrologically significant cations, such as Ti 4+ , Fe 3+ and Al 3+ [11,[21][22][23]. Before the advent of SIMS, formula recalculation of mica was performed using different normalisation schemes.…”

Fluorophlogopite from Piano delle Concazze (Mt. Etna, Italy): Crystal chemistry and implications for the crystallization conditions

“…On the other hand, the tetrahedral thickness remains unchanged (2.24 Å) and the octahedral thickness increases slightly from 2.11 Å in the Biancavilla fluorophlogopite (Gianfagna et al 2007) to 2.12 Å in the Olegario fluorophlogopite ) and in the Piano delle Concazze fluorophlogopite of this study. The latter (Gianfagna et al 2007); triangle pointing upward = SA phlogopite characterized by Ti and Fe 3+ -oxy substitutions (Ventruti et al 2009); triangle pointing downward = synthetic near end-member phlogopite (Redhammer and Roth 2002); triangle with plus = HO20 F-free, Ti-rich oxyphlogopite (Cesare et al 2003); triangle with cross = MA1_14 Ti-rich fluorophlogopites from Olegario ; diamond = VUT 191_19 phlogopite (Matarrese et al 2008); pentagon = VUT 187_1 phlogopite (Schingaro et al 2007); hexagon with plus = VUT 187_18 phlogopite (Schingaro et al 2007); solid hexagon = PG5_1 phlogopite (Scordari et al 2010); star = IGNA_24 phlogopite ; solid circle = VUT215_1 phlogopite ; square with plus = fluorophlogopite (Joswig 1972); hexagon with cross = VUT215_8 phlogopite ; square with vertical line = fluorophlogopite (Hazen and Burnham 1973); square with horizontal line = synthetic fluorophlogopite (Takeda and Morosin 1975); hexagon = VUT001_2 phlogopite (Scordari et al 2010). Solid lines: join end-member phlogopite (Redhammer and Roth 2002)-end-member fluorophlogopite (Takeda and Morosin 1975); join end-member phlogopite (Redhammer et al 2005)-oxyphlogopite (Cesare et al 2003).…”

“…This results in the alteration of several structural parameters that, in turn, are employed to identify the occurrence of M 3+,4+ -oxy substitution mechanisms (Schingaro et al 2005(Schingaro et al , 2007Scordari et al 2006Scordari et al , 2008Scordari et al , 2010Cesare et al 2008;Matarrese et al 2008;Ventruti et al 2008).…”

“…Schingaro et al (2011) studying various F-rich phlogopite from kamafugitic lavas of Presidente Olegario (Brazil) with the highest Ti content in nature and with a minor tetraferriphlogopite component, discuss the mechanisms of Ti and Fe incorporation in the phlogopite structure. On the other hand several phlogopites of volcanic origin with variable oxy component have been investigated recently (Schingaro et al 2007;Matarrese et al 2008;Scordari et al 2010;Lacalamita et al 2011) to understand how oxy substitutions affect the mica structure.…”