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

Abstract The present work reports the crystal-chemical characterization of micas from the Monte Arsiccio and Buca della Vena mines (Apuan Alps, Italy) through electron microprobe analysis, single-crystal X-ray diffraction, and Raman spectroscopy. The sample from the Monte Arsiccio mine can be classified as an intermediate member of the yangzhumingite-fluorophlogopite series, with average crystal-chemical formula (K 0.85 Na 0.01 Ba 0.09 ) Σ=0.95 (Mg 2.11 Fe 2+ 0.23 Fe 3+ 0.11 Cr 3+ 0.01 Al 0.20 Ti 0.04 ☐ 0.30 ) Σ=3.00 (Si 3.20 Al 0.80 ) Σ=4.00 O 10.00 F 1.90 Cl 0.02 (OH) 0.08 . Unit-cell parameters are a ~ 5.30, b ~ 9.18, c ~ 10.14 Å, β ~ 100.12°, V ~ 486.22 Å 3 , corresponding to the 1 M polytype. Structure refinements, performed in C 2/ m space group, converged to R 1 = 3.54 and 4.46% and provided Mg plus Fe occupancy in the range 86–94% for the octahedral M 1 and M 2 sites. Raman spectroscopy shows very weak bands in the OH stretching region at ~ 3690 and 3580 cm −1 . The sample from the Buca della Vena mine has been identified as an (OH)-rich fluorophlogopite, with average crystal-chemical formula (K 0.84 Na 0.02 Ca 0.01 ) Σ=0.87 (Mg 2.12 Fe 2+ 0.55 Fe 3+ 0.10 Al 0.18 ☐ 0.05 ) Σ=3.00 (Si 2.99 Al 1.01 ) Σ=4.00 O 10.00 F 1.02 Cl 0.09 (OH) 0.89 . Its unit-cell parameters are a ~ 5.33, b ~ 9.22, c ~ 10.23 Å, β ~ 100.09°, V ~ 494.39 Å 3 . Structure refinements gave good R 1 values (3.27 and 4.37%) and revealed octahedral occupancy of 82–84% Mg and 16–18% Fe. Strong Raman signals at ~ 3702 cm −1 and 3595 cm −1 were observed in the OH stretching region. The findings allow to better understand not only the mineralogy of the Apuan Alps but, more generally, the crystal chemical details of intermediate dioctahedral-trioctahedral mica belonging to the yanzhumingite-fluorophlogopite series.