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 substitution.