Crystal-chemistry of an intermediate member of the yangzhumingite-fluorophlogopite and phlogopite-fluorophlogopite series from the Apuan Alps (Tuscany, Italy)

The present work reports for the first time 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 Monte Arsiccio mica is an intermediate yangzhumingite-fluorophlogopite with average crystal-chemical formula (K0.85Na0.01Ba0.09)=0.95(Mg2.11Fe2+0.23Fe3+0.11Cr3+0.01Al0.19Ti0.04 Li0.01☐0.31)=3.01(Si3.20Al0.81)=4.01O10.00F1.90Cl0.02OH0.08. The crystals belong to the 1M polytype and exhibit unit-cell parameters a ~ 5.30, b ~ 9.18, c ~ 10.14 Å,  ~ 100.12° and V ~ 486 Å3, and M1 and M2 sites occupancy in the range 86-94% . The Raman spectrum shows very weak signals in the OH stretching region at ~ 3690 and 3580 cm-1. Buca della Vena mica is an OH-rich fluorophlogopite with average crystal-chemical formula (K0.84Na0.02Ca0.01)=0.87(Mg2.12Fe2+0.55Fe3+0.10Al0.17☐0.07)=3.01(Si2.99 Al1.02)=4.01O10.00F1.02Cl0.09OH0.89 and unit-cell parameters a ~ 5.33, b ~ 9.22, c ~ 10.23 Å,  ~ 100.09° and V ~ 494.4 Å3. X-ray site scattering refinements provided occupancies of about 82-84% Mg and 16-18% Fe for the M sites. Strong Raman signals at ~ 3702 cm-1 and 3595 cm-1 were observed in the OH stretching region. The high F content of both Monte Arsiccio and Buca della Vena micas may explain the origin of fluo-sulfates from Apuan Alps as the product of interaction between sulphur acid and (F-rich mica)-bearing rocks.