The first barium-HREE fluorocarbonate new mineral bayanoboite-(Y), Ba2Y(CO3)2F3, from Bayan Obo deposit, Inner Mongolia, China

Bayanoboite-(Y) (IMA2023-084), Ba2Y(CO3)2F3, is a new mineral discovered at the Bayan Obo deposit in Baotou City, Inner Mongolia, China. The mineral is named after its type locality (Bayan Obo deposit) with mineral symbol “Byb-Y”. The holotype of bayanoboite-(Y) is found within a crack between sulfide grains dispersed in dolomite. The host sulfide grains are predominantly pyrite, with some regions partially transformed into pyrrhotite. Bayanoboite-(Y) is brittle with conchoidal fracture, a Mohs hardness of 4–5, and perfect cleavage along {001}. The calculated density is 4.69 g/cm3. The empirical chemical formula for the holotype is A(Ba1.94Sr0.06)Σ2.00B(Y0.77Dy0.06Gd0.04Fe0.03Ca0.02Nd0.02Er0.02Yb0.02Sm0.01)Σ1.00(CO3)2(F2.79O0.08)Σ2.87 based on (A + B)apfu = 3. Bayanoboite-(Y) is orthorhombic, space group Pbcn (#60), with unit-cell parameters a = 9.4528(4) Å, b = 6.9499(2) Å, c = 11.7638(5) Å, V = 772.83(5) Å3, and Z = 4. The crystal structure of bayanoboite-(Y) represents a novel structure type for minerals, and it is characterized by two 9-coordinated large cation sites: a Y site and a Ba site, both forming tetrakaidecahedra with surrounding O and F atoms. Bayanoboite-(Y) has a layered-framework structure composed of two distinct blocks: (1) thick slabs formed by two adjacent, oppositely oriented layers where [YO6F3] polyhedra are connected by [CO3] triangles; and (2) a framework built from [BaO5F4] polyhedra arranged as edge-shared wavy-line chains, which are further connected by F atoms and [CO3] triangles. These two blocks are well bonded through the shared edges and faces of [YO6F3] and [BaO5F4] polyhedra. In addition to Y, it also contains Gd (0.60–1.32 wt%), Dy (1.51–2.13 wt%), Er (0.51–0.85 wt%), and Yb (0.36–0.79 wt%). Therefore, the discovery of bayanoboite-(Y) holds significant implications for comprehending the occurrence of heavy rare earth elements (HREEs) in the Bayan Obo deposit. It also provides mineralogical evidence for the evolution of the Bayan Obo deposit, and this new mineral has the potential to serve as an indicator for the exploration of HREEs in this region.