Crystal chemistry of Ti-rich fluorophlogopite from Presidente Olegario, Alto Paranaiba igneous province, Brazil

Abstract

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 substitutio

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

“…Regarding individual distances, the M1-O4 and M2-O4 ones are the shortest due to the F for OH substitution at the O4 site. Similar structural features were previously found for the Kvaløya yangzhumingite by Schingaro et al (2011). The synthetic tetrasilicic fluormica in Toraya et al (1976Toraya et al ( , 1978, having only Mg in the octahedral sites, showed short values (~ 2.063 Å) for both the < M1-O > and < M2-O > distances.…”

“…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 crystal-chemical features of fluorophlogopite were described in several studies also in relation to the (OH)-end member phlogopite (e.g. Gianfagna et al 2007 and references therein;Schingaro et al 2011;Balassone et al 2013;Lacalamita et al 2017). In particular, Gianfagna et al (2007) showed that fluorophlogopite from Biancavilla (Sicily) crystallizes in the 1M polytype (C2/m s.g.) and exhibits a more compact structure than that of phlogopite.…”

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

“…This mica is characterized by an F content (F = 1.35 apfu) intermediate between those from Biancavilla (F = 1.90 apfu; Gianfagna et al 2007) and the Ti-rich fluorophlogopite from Presidente Olegario (F = 0.90 apfu; Schingaro et al 2011). The main goal is to gain a deeper understanding of the structural effects caused in trioctahedral micas by F --OH -substitution as well as by O 2--OH -via deprotonation associated with the intake of M 3+ and Ti 4+ on the octahedral layer.…”

“…However, recently, Gianfagna et al (2007) performed a crystal-structure analysis of * E-mail: fernando.scordari@uniba.it the end-member F-bearing phlogopite from Biancavilla (Etna) that had been approved by the IMA-CNMMN as a new mineral with the name fluorophlogopite, whereas Brigatti et al (2007) carried out a structural study of a fluorannite emphasizing the structural changes due to F content with respect to annite. 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.…”