Tsilaisite, NaMn3Al6(Si6O18)(BO3)3(OH)3OH, a new mineral species of the tourmaline supergroup from Grotta d'Oggi, San Pietro in Campo, island of Elba, Italy

Tsilaisite, NaMn 3 Al 6 (Si 6 O 18 )(BO 3 ) 3 (OH) 3 OH, is a long-expected new mineral of the tourmaline supergroup. It occurs in an aplitic dike of a LCT-type pegmatite body from Grotta d’Oggi, San Pietro in Campo, island of Elba, Italy, in association with quartz, K-feldspar, plagioclase, elbaite, and schorl. Crystals are greenish yellow with a vitreous luster, a white streak, and show no fluorescence. Tsilaisite has a Mohs hardness of approximately 7; it is brittle with a sub-conchoidal fracture, and has a calculated density of 3.133 g/cm 3 . In plane-polarized light, tsilaisite is pleochroic, O = pale greenish yellow, E =very ale greenish yellow; it is uniaxial negative, ω = 1.645(5), Ɛ = 1.625(5). Tsilaisite is rhombohedral, space group R3m, a = 15.9461(5), c = 7.1380(3) Å, V = 1571.9(1) Å 3 , Z = 3. The strongest eight ‑raydiffraction lines in the powder pattern [d in Å(I)(hkl)] are: 3.974(100)(220), 2.942(94)(122), 2.570(79) (051), 2.034(49)(152), 4.205(41)(211), 6.329(22)(101), 2.377(21)(003), and 1.592(21)(550). Analysis by a combination of electron microprobe, secondary ion mass spectrometry, and optical absorption spectroscopy gives SiO 2 = 36.10(3), TiO 2 = 0.32(4), Al 2 O 3 = 37.10(5), MnO = 9.60(10), CaO = 0.09(4), Na 2 O = 2.11(7), K 2 O = 0.03(1), F = 0.79(3), B 2 O 3 = 10.2(6), Li 2 O = 0.8(1), H 2 O = 3.1(2), sum 99.95 wt%. The unit formula is X(Na 0.67 ⃞ 0.30 Ca 0.02 K 0.01 ) Y (Mn 2+ 1.34 Al 1.14 Li 0.54 Ti 0.04 ) Z Al 6 T (Si 5.94 Al 0.06 )B 2.91 O 27 V (OH) 3 W (OH 0.39 F 0.41 O 0.20 ). The structure, refined also taking into account the positional disorder of the O1 and O2 anions, converged to statistical indices R1 for all reflections of about 2%. The resulting site populations indicate that the Z site is occupied by Al and that the Y site is dominated by Mn 2+ . Aluminum is incorporated at Y through two types of substitutions: Y Al+ W O 2- → Y Mn 2+ + W OH, which has the result of replacing OH at the W site by O 2- , and Y (Al+Li)+ W F → 2 Y Mn 2+ + W OH, which relates fluor-elbaite to the tsilaisite component. Infrared absorption spectra measured in the principal OH-stretching region were interpreted on the basis of local arrangements consistent with the short-range bond-valence requirements. A compositional trend from fluor-elbaite to tsilaisite is observed in samples from Elba Island. The occurrence of tsilaisite is very rare in nature, as a consequence of both the requirement of extraordinary petrogenetic conditions and limited structural stability.