Xianhuaite-(Ce), K 2 CeNb 5 O 15 , a new niobium mineral from the Bayan Obo deposit, China

Abstract. Xianhuaite-(Ce), K2CeNb5O15, is a new mineral species discovered in the Bayan Obo deposit, Inner Mongolia, China. The mineral is named in honor of Professor Xianhua Li in recognition of his outstanding contributions to research on the Bayan Obo deposit. It occurs as brownish-red to pale yellow-brown tetragonal prisms or irregular granules and exhibits a Mohs hardness of 5–6, an adamantine luster, and a calculated density of 5.23 g cm−3. Electron microprobe analysis yields the empirical formula A1(K1.30Ba0.69Sr0.02)Σ2.01A2(Ce0.39La0.31Nd0.04Pr0.02Ca0.15Na0.09)Σ1.00B(Nb4.75Fe0.133+Ti0.09Mg0.04)Σ5.01O15. Xianhuaite-(Ce) crystallizes in the tetragonal system, space group P4/mbm, with unit-cell parameters a=12.5355(4) Å, c=3.9213(2) Å, V=616.19(5) Å3, and Z=2. Its crystal structure adopts a tetragonal tungsten bronze (TTB)-type framework, characterized by corner-sharing NbO6 octahedra forming channels hosting K, Ba, and rare earth element (REE) cations. Xianhuaite-(Ce) is associated with minerals typical of skarnization assemblages (e.g. dolomite, forsterite, phlogopite, and chlorite), as well as other Nb phases, including fersmite, columbite-(Fe), fergusonite-(Ce), and aeschynite. As the first naturally occurring mineral with a TTB-type structure, xianhuaite-(Ce) expands the known structural diversity of Nb minerals and provides new mineralogical constraints on Nb mineralization at Bayan Obo.