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

Abstract

Fluorine is an important proxy for magmatic differentiation processes in the shallow parts of volcanic plumbing systems. Fluorphlogopite is one of the more important fluorine carriers in magmatic rocks. In the present study, a full crystal chemical investigation of fluorophlogopite 1M from Piano delle Concazze, Mt. Etna volcano, Italy, is carried out. The fluorophlogopite occurs in a benmoreitic lava from prehistoric volcanic activity at Mt. Etna (post-caldera forming phase of the "Ellittico" eruptive center; ~15 ka BP). It is primarily associated with fluorapatite covered with amorphous SiO 2 and crystallized during syn/post-eruption pneumatolytic stages. The mica sample studied here is among the most Fe-and Ti-rich fluorophlogopite found in nature. EPMA data yielded the following mean chemical formula for this mineral (K 0.83 Na 0.13 )(Fe 2+ 0.44 Structure refinements on four fluorophlogopite crystals, performed in space group C2/m, converged at R = 0.03-0.04, with cell parameters in the ranges a = 5.323-5.324, b = 9.219-9.222, c = 10.116-10.119 Å, b = 100.1-100.3°. Major substitutions are OH -↔ F -,M 3+ -oxy ( VI M 2+ +OH -↔ VI M 3+ +O 2-) and Ti-oxy substitution: VI M 2+ +2(OH) -↔ VI Ti 4+ +2O 2-. The fluorophlogopite from Piano delle Concazze exhibits the shortest c-parameter with respect to other fluorophlogopites found in nature. The short c parameter is essentially due to the absence of the hydroxyl group in favor of F -and especially of O 2-and to the thus increased attractive interaction between the interlayer cation and the anion content (F -, O 2-) located at the O4 site. A comparison with other natural fluorophlogopites (namely from Biancavilla, Etna and Presidente Olegario, Brazil) show intermediate crystal-chemical features for the Piano delle Concazze fluorophlogopite. Particularly at Etna, differences in the chemical composition of the crystallized fluorophlogopites could be related to the various extent of enrichments by transfer of a gas phase achieved in specific parts of the volcanic plumbing system.

Surface Wetting Controls Calcium Carbonate Crystallization Kinetics

“…OH phlogopite is characterized by a doublet peak at ∼3711 and ∼3670 cm –1 , attributed to the structural (octahedral) hydroxyl group, , and a small feature due to adsorbed water between 3600 and 3000 cm –1 (Figure A inset). F phlogopite, on the other hand, lacks such features, which is in accordance with previous studies. , The region below 1000 cm –1 is dominated by the strong phlogopite signal, and consequently peak assignments for CaCO 3 polymorphs were performed from 1800 to 1200 cm –1 .…”

Crystal-chemistry of micas belonging to the yangzhumingite-fluorophlogopite and phlogopite-fluorophlogopite series from the Apuan Alps (northern Tuscany, Italy)

“…The Russian yangzhumingite (Shchipalkina et al 2020) and the fluorophlogopite in Hazen et al (1981) fall out of the trend because the c cell parameter (~ 10.20 Å) is affected by the occurrence of Fe 3+ in the tetrahedral site. On the contrary, fluorophlogopite samples studied by Scordari et al (2013) and Schingaro et al (2011) exhibit the shortest c cell parameters (~ 10.12 Å) due to the additional significant oxy-substitutions (OH − → O 2− replacement) associated to the Ti-incorporation in the octahedral site.…”

“…The two studied MA crystals exhibit almost the same unitcell parameters which are, on average, a ~ 5.30, b ~ 9.18, c ~ 10.14 Å, β ~ 100.12°, V ~ 486.22 Å 3 (see Table 2). The value of the c cell parameter in mica strictly depends on the anionic content (Cruciani and Zanazzi 1994;Ventruti et al 2008;Ottolini et al 2010;Zema et al 2010;Scordari et al 2013). Figure 2 shows the correlation between the c parameter and the F − + O 2− content for micas with composition close to yangzhumingite and fluorophlogopite.…”