Bosiite, NaFe3+ 3(Al4Mg2)(Si6O18)(BO3)3(OH)3O, a new ferric member of the tourmaline supergroup from the Darasun gold deposit, Transbaikalia, Russia

Abstract

Bosiite, NaFe3+3(Al4Mg2)(Si6O18)(BO3)3(OH)3O, is a new mineral species of the tourmaline supergroup from the Darasun gold deposit (Darasun mine), Vershino-Darasunskiy, Transbaikal Krai, Eastern-Siberian Region, Russia (52°20′24″N, 115°29′23″E). Bosiite formed as a hydrothermal phase in a gold-bearing quartz-vein spatially related to the Amudzhikan–Sretensky subvolcanic K-rich granodiorite-porphyry intrusion. Ores of this deposit are enriched in sulfides (up to 60%). Bosiite is intimately associated with other tourmalines. The first tourmaline generation is bosiite, which is followed by a second generation of oxy-dravite and a third generation of dravite. Bosiite also coexists with quartz and pyrite; further associated minerals in the vein are gangue minerals (quartz, calcite, and dolomite), sulfides (pyrite, arsenopyrite, chalcopyrite, pyrrhotite, tetrahedrite, sphalerite, and galena) and native gold. Crystals of bosiite are dark brown to black with a pale-brown streak. Bosiite is brittle and has a Mohs hardness of 7; it is non-fluorescent, has no observable parting and cleavage. It has a measured density of 3.23(3) g/cm3 (by pycnometry) and a calculated density of 3.26(1) g/cm3. In plane-polarized light, it is pleochroic, O = yellow-brown, E = red-brown. Bosiite is uniaxial negative, ω = 1.760(5), ϵ = 1.687(5). The mineral is trigonal, space group R 3 m , a = 16.101(3), c = 7.327(2) A, V = 1645.0(6) A3. The eight strongest X-ray diffraction lines in the (calculated) powder pattern [ d in A( I ) hkl ] are: 2.606(100)(50-1), 8.051(58)(100), 3.008(58)(3-1-2), 4.025(57)(4-20), 3.543(50)(10-2), 4.279(46) (3-11), 2.068(45)(6-1-2), 4.648(28)(300). Analysis by a combination of electron microprobe (EMPA), inductively coupled plasma mass spectrometry (ICP-MS), Mossbauer spectroscopic data and crystal-structure refinement results in the empirical structural formula: ![Formula][1] According to the IMA-CNMNC guidelines, the dominant valence at the Y site is 3+ and the dominant cation is Fe3+. To accommodate the disorder and allocating cations to the Z and Y sites, the recommended procedure leads to the optimized empirical formula (based on 31 O): X (Na0.73Ca0.23□0.04) Y (Fe3+2.40Fe2+0.59Ti4+0.01) Z (Al3.36Mg1.69Fe3+0.95) T (Si5.92Al0.08O18) (BO3)3 V (OH)3 W [O0.85(OH)0.15]. Bosiite, ideally NaFe3+3(Al4Mg2)(Si6O18)(BO3)3(OH)3O, is related to end-member povondraite, ideally NaFe3+3(Fe3+4Mg2)(Si6O18)(BO3)3(OH)3O, by the substitution Z Al4 → Z Fe3+4. Further, bosiite is related to oxy-dravite, ideally Na(Al2Mg)(Al5Mg)(Si6O18)(BO3)3(OH)3O, by the substitutions [6]Fe3+3 → [6]Al3. Bosiite is named after Dr. Ferdinando Bosi, researcher at the University of Rome La Sapienza, Italy, and an expert on the crystallography and mineralogy of the tourmaline-supergroup minerals and the spinels. [1]: /embed/mml-math-1.gif