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, NaMn 3 Al 6 (Si 6 O 18 )(BO 3 ) 3 (OH) 3 F, 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/cm 3 . 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 R 3 m, a = 15.9398(6), c = 7.1363(3) Å, V = 1570.25(11) Å 3 , Z = 3. The crystal structure of fluor-tsilaisite was refined to R 1 = 3.36% using 3496 unique reflections collected with Mo Kα X-ray intensity data. Crystal-chemical analysis resulted in the empirical formula: X (Na 0.69 〈 0.29 Ca 0.02 ) Σ1.00 Y (Mn 2+ 1.29 Al 1.21 Li 0.56 Ti 0.03 ) Σ6.00 Z Al 6 T (Si 5.98 Al 0.03 ) Σ6.00 B 2.92 O 27 V (OH) 3 W [F 0.39 (OH) 0.25 O 0.36 ] Σ1.00 . 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 W F ↔ W (OH) and with fluor-elbaite through the substitution Y (Al + Li) ↔ 2 Y Mn 2+ , and appears to be a stepwise intermediate during tourmaline evolution from tsilaisite to fluor-elbaite.