Abstract Niobobaotite (IMA 2022-0127a), ideally Ba4(Ti2.5Fe2+1.5)4Nb4(Si4O12)O16Cl, is a new mineral discovered in the Bayan Obo REE-Nb-Fe deposit, Inner Mongolia, China. It occurs as columnar or platy, subhedral to anhedral grains ranging from approximately 20 to 80 μm, associated with columbite-(Mn), pyrite and monazite-(Ce), as well as hematite, magnetite, dolomite, calcite, rutile, ilmenite, aeschynite-(Ce), bastnäsite-(Ce), parisite-(Ce), magnesio-arfvedsonite and quartz. The new mineral exhibits a brown to black color with a yellow streak and a vitreous lustre. It is brittle with uneven fractures, and has a calculated density of 4.684 g/cm3. The average Vickers microhardness is 647 kg/mm2 under a 50 g load, corresponding to the Mohs hardness of approximately 6. Optically, niobobaotite is uniaxial (+) with strong pleochroism: ω = yellow and ɛ = brown. Electron probe micro-analysis, combined with X-ray photoelectron spectroscopy, yields the following mean values (wt%) from eight analysis points: Nb2O5 26.04, SiO2 14.23, TiO2 14.93, Al2O3 0.29, SrO 0.15, BaO 35.53, CaO 0.02, FeO 2.75, Fe2O3 4.43, MnO 0.07, Na2O 0.06, Cl 2.10, and Cl≡O -0.48, total 100.12. The empirical formula, calculated on the basis of 16 cation atoms per formula unit (apfu), is A(Ba3.88Sr0.02Na0.03Ca0.01)Σ3.94B1(Ti2.68Fe2+0.64Fe3+0.31Nb0.35Mn0.02)Σ4.00B2(Nb2.93Fe3+0.62Ti0.45) Σ4.00T(Si3.96Al0.10)Σ4.06XO28ΦCl0.99. Niobobaotite crystallizes in the tetragonal system, space group I41/a (# 88), with unit cell parameters a = 20.0264(6) Å, c = 5.9592(2) Å, V = 2389.98(16) Å3, and Z = 4. The crystal structure was determined using single crystal X-ray diffraction on the basis of 1503 independent reflections with I2σ(I), yielding a final R1 = 2.78%. Niobobaotite is isostructural with baotite, but has Nb dominance at the B2 site. The eight strongest lines in the X-ray powder diffraction pattern d in Å (I/I0) (hkl) are: 4.070 (47) (231), 3.532 (100) (440), 3.319 (58) (051), 3.159 (97) (26¯0), 2.893 (49) (16¯1), 2.767 (88) (460), 2.692 (50) (13¯2), and 2.251 (57) (352). The crystal structure of niobobaotite consists of square columns of Ti octahedra and Nb octahedra aligned parallel to the c-axis which are connected with Si4O12 square rings parallel to (0 0 1) to form a framework. The Ba cations are located in the framework channels and are square-arranged between Si4O12 square rings with Cl atoms located at the center of the square of Ba cations. Cation substitutions in niobobaotite involve 3Ti4+ → 2Nb5+ + Fe2+ and 2Ti4+ → Nb5+ + Fe3+, collectively expressed as 5Ti4+ → 3Nb5+ + Fe3+ + Fe2+. The formation environment of niobobaotite requires high activities of Si, Nb, Ti, and Ba coupled with low activity of Ca in the fluid. Niobobaotite has a unique capacity of incorporating Cl from an F-rich hydrous fluid.
Ge et al. (Wed,) studied this question.