Chlorbartonite, K6Fe24S26(Cl,S), a new mineral species from a hydrothermal vein in the Khibina massif, Kola Peninsula, Russia:: Description and crystal structure

Chlorbartonite, ideally K6Fe14S16(Cl,S), is a new potassium iron sulfide chloride found as an accessory mineral in a microcline - pectolite - sodalite - aegirine vein within feldspathic urtite at Mount Koashva, Khibina massif, Kola Peninsula, Russia. The mineral occurs as roundish grains up to 2 cm across included in socialite and natrolite crystals; it is associated with djerfisherite and rasvumite. Chlorbartonite is weakly magnetic, brittle, and has a conchoidal fracture. Cleavage and parting were not observed. Macroscopically, the mineral is opaque, brown-black, with a black streak and submetallic luster. In reflected light, it appears optically isotropic. yellowish brown, and without internal reflections; interpolated values of reflectance (R, %) at the standard wavelengths (nm) are: 10.8 (470), 13.5 (546), 15.1 (589), and 17.2 (650). The hardness VHN40 is 203-212, average 207 kg/mm(2); the Mobs hardness is 4. The density is 3.70 g/cm(3) (measured) and 3.65 g/cm(3) (calculated). The electron-microprobe analysis of six grains gave. as an average composition: S 33.84, Cl 0.99, K 9.62, Fe 54.09, Co 0.04, Cu 0.57, total 99.15 wt.%. The empirical formula calculated on the basis of a total of 57 atoms is K-6.08(Fe23.93Cu0.22Co0.01)Sigma(24.16)S(26.00)(Cl0.69S0.07)Sigma(0.76), which yields an ideal formula of K6Fe24S26(CI,S). The mineral is tetragonal, I4/mmm, a 10.3810(8), c 20.614(2) Angstrom, V 2221.5(3) Angstrom(3), Z = 2. The strongest eight lines in the X-ray powder-diffraction pattern [d in Angstrom (1)(hkl)] are: 9.25(33)(10 1), 5.97(65)(112), 3.121(45)(3 12), 2.986(100)(224) 2.380(38)(332), 2.374(57)(3 16), 1.834(51)(440). and 1.830(82)(408). The crystal structure has been refined to an Rl of 0.020 (447 unique observed reflections with F-o greater than or equal to 4sigmaF(o)). The structure consists of a framework of the Fe8S14 clusters of eight edge-sharing FeS4 tetrahedra. The framework has large cavities occupied by CIK6 octahedra. As the name indicates, chlorbartonite is the Cl-dominant analogue of bartonite.