Fluor-tsilaisite, NaMn3Al6(Si6O18)(BO3)3(OH)3F, a new tourmaline from San Piero in Campo (Elba, Italy) and new data on tsilaisitic tourmaline from the holotype specimen locality

Fluor-tsilaisite, NaMn3Al6(Si6O18)(BO3)3(OH)3F, is a new mineral of the tourmaline supergroup. It occurs in an aplitic dyke of a LCT-type pegmatite body from Grotta d’Oggi, San Piero in Campo, Elba Island, Italy, in association with quartz, K-feldspar, plagioclase, elbaite, schorl, fluor-elbaite and tsilaisite. Crystals are greenish yellow with a vitreous lustre, sub-conchoidal fracture and white streak. Fluor-tsilaisite has a Mohs hardness of ~7 and a calculated density of 3.134 g/cm3 . In plane-polarized light, fluor-tsilaisite is pleochroic (O = pale greenish yellow and E = very pale greenish yellow), uniaxial negative. Fluor-tsilaisite is rhombohedral, space group R3m, a = 15.9398(6), c = 7.1363(3) A˚ , V = 1570.25(11) A˚ 3 , Z = 3. The crystal structure of fluor-tsilaisite was refined to R1 = 3.36% using 3496 unique reflections collected with MoKa X-ray intensity data. Crystal-chemical analysis resulted in the empirical formula: X(Na0.69&0.29Ca0.02)S1.00Y(Mn2+ 1.29Al1.21Li0.56Ti0.03)S6.00ZAl6 T (Si5.98Al0.03)S6.00B2.92O27V(OH)3 W[F0.39(OH)0.25O0.36]S1.00. Comparisons were performed between fluor-tsilaisite and a tsilaisitic tourmaline from the same locality as the holotype specimen. This latter tourmaline sample was selected for this study due to its remarkable composition (MnO = 11.63 wt.%), the largest Mn content found in tourmaline so far. Fluor-tsilaisite is related to tsilaisite through the substitution WF $ W(OH) and with fluor-elbaite through the substitution Y(Al + Li) $ 2YMn2+, and appears to be a stepwise intermediate during tourmaline evolution from tsilaisite to fluor-elbaite.