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

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 classifiedas an intermediate member of the yangzhumingite-fluorophlogopite series, with average crystal-chemical formula (K _0.85Na _0.01Ba _0.09) _Σ=0.95(Mg _2.11Fe ²⁺ _0.23Fe ³⁺ _0.11Cr ³⁺ _0.01Al _0.20Ti _0.04☐ _0.30) _Σ=3.00(Si _3.20Al _0.80) _Σ=4.00O _10.00F _1.90Cl _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 1M polytype. Structure refinements, performed in C2/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 M1 and M2 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 _0.84Na _0.02Ca _0.01) _Σ=0.87(Mg _2.12Fe ²⁺ _0.55Fe ³⁺ _0.10Al _0.18☐ _0.05) _Σ=3.00(Si _2.99Al _1.01) _Σ=4.00O _10.00F _1.02Cl _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.