Fluor-elbaite, Na(Li1.5Al1.5)Al6(Si6O18)(BO3)3(OH)3F, a new mineral species of the tourmaline supergroup

Fluor-elbaite, Na(Li_1.5Al_1.5)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃F, is a new mineral of the tourmaline supergroup. It is found in miarolitic cavities in association with quartz, pink muscovite, lepidolite, spodumene, spessartine, and pink beryl in the Cruzeiro and Urubu mines (Minas Gerais, Brazil), and apparently formed from late-stage hydrothermal solutions related to the granitic pegmatite. Crystals are blue-green with a vitreous luster, sub-conchoidal fracture and white streak. Fluor-elbaite has a Mohs hardness of approximately 7.5, and has a calculated density of about 3.1 g/cm³. In plane-polarized light, fluor-elbaite is pleochroic (O = green/bluish green, E = pale green), uniaxial negative. Fluor-elbaite is rhombohedral, space group R3m, a = 15.8933(2), c = 7.1222(1) Å, V = 1558.02(4) ų, Z = 3 (for the Cruzeiro material). The strongest eight X-ray-diffraction lines in the powder pattern [d in Å(I)(hkl)] are: 2.568(100)(051), 2.939(92)(122), 3.447(67)(012), 3.974(58)(220), 2.031(57)(152), 4.200(49)(211), 1.444(32)(642), and 1.650(31)(063). Analysis by a combination of electron microprobe, secondary ion mass spectrometry, and Mössbauer spectroscopy gives SiO₂ = 37.48, Al₂O₃ = 37.81, FeO = 3.39, MnO = 2.09, ZnO = 0.27, CaO = 0.34, Na₂O = 2.51, K₂O = 0.06, F = 1.49, B₂O₃ = 10.83, Li₂O = 1.58, H₂O = 3.03, sum 100.25 wt%. The unit formula is: ^X(Na_0.78□_0.15Ca_0.06K_0.01)^Y(Al_1.15Li_1.02Fe²⁺_0.46Mn²⁺_0.28Zn_0.03) ^ZAl₆^T(Si_6.02O₁₈)^B(BO₃)₃^V(OH)₃^W(F_0.76OH₀₂₄).

The crystal structure of fluor-elbaite was refined to statistical indices R1 for all reflections less then 2% using MoKα X-ray intensity data. Fluor-elbaite shows relations with elbaite and tsilaisite through the substitutions ^WF ↔ ^WOH and ^Y(Al + Li) + ^WF ↔ 2^YMn²⁺ + ^WOH, respectively.