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 (K0.85Na0.01Ba0.09)Σ=0.95(Mg2.11Fe2+0.23Fe3+0.11Cr3+0.01Al0.20Ti0.04☐0.30)Σ=3.00(Si3.20Al0.80)Σ=4.00O10.00F1.90Cl0.02(OH)0.08. Unit-cell parameters are a ~ 5.30, b ~ 9.18, c ~ 10.14 Å, β ~ 100.12°, V ~ 486.22 Å3, corresponding to the 1M polytype. Structure refinements, performed in C2/m space group, converged to R1 = 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 (K0.84Na0.02Ca0.01)Σ=0.87(Mg2.12Fe2+0.55Fe3+0.10Al0.18☐0.05)Σ=3.00(Si2.99Al1.01)Σ=4.00O10.00F1.02Cl0.09(OH)0.89. Its unit-cell parameters are a ~ 5.33, b ~ 9.22, c ~ 10.23 Å, β ~ 100.09°, V ~ 494.39 Å3. Structure refinements gave good R1 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.

Non-destructive determination of the biotite crystal chemistry using Raman spectroscopy: how far we can go?

“…7a). In fact, this is in accordance with the peak assignment by Tlili et al (1989) and Lacalamita et al (2020), attributing the peaks near 650 and 680 cm −1 to Si-O b -Al and Si-O b -Si bond vibrations, respectively. Our analysis revealed that both ω 680 and FWHM 680 are sensitive to M Ti content (see Fig.…”

“…Therefore, the dataset of sample B3 was not taken into consideration while constructing calibration curves. On the other hand, high total oxides equal to 100 wt % can be explained by high F content such as in fluorophlogopite (e.g., Gianfagna et al, 2007;Scordari et al, 2013;Lacalamita et al, 2020). The fraction of trivalent iron and its distribution over the M and T sites for samples B4, B13, B14, B17, B19, and B21 was derived from the Mössbauer spectra (see Table S2 and Fig.…”

“…two crystallographic sites, M1 and M2, and each M1M2M2 triplet is bonded to two X-site anions. Based on single-crystal XRD analysis, it has been established that M2 is the preferred site for Ti 4+ and Fe 3+ , whilst divalent M cations as well as Al 3+ are randomly partitioned on both M1 and M2 sites (e.g., Redhammer et al, 2000;Scordari et al, 2006;Lacalamita et al, 2011). Finally, A-site cations are coupled with six O b atoms of the tetrahedral sheets.…”

“…So far, the substitutional mechanisms of biotites with various chemical compositions have been thoroughly studied by single-crystal XRD, EMPA, and Mössbauer spectroscopy (e.g., Brigatti et al, 2000;Redhammer et al, 2002;Scordari et al, 2006;Lacalamita et al, 2011;Schingaro et al, 2013). In addition, Fourier-transform infrared (FTIR) and Raman spectral features of biotites together with peak assignment of the framework vibrational modes (15-1215 cm −1 ) and OH-stretching modes (3550-3750 cm −1 ) have been reported over the last 3 decades (e.g., McKeown et al, 1999;Tutti and Lazor, 2008;Wang et al, 2015;Singha and Singh, 2016).…”

Sulfates from the Pyrite Ore Deposits of the Apuan Alps (Tuscany, Italy): A Review

“…The latter is a fluo-sulfate. Recently, Lacalamita et al [78] detected high F content in micas from the Monte Arsiccio mine; these phases may be the source not only of K and Al, but also for F. Krausite was observed as overgrowths on the other sulfates (i.e., römerite, coquimbite-Figure 17) and it has been found also on alunogen and halotrichite. Krausite seems to postdate the crystallization of alunogen, whose individuals are superficially corroded, whereas krausite crystals are shiny, without any hints of alteration.…”