Oboniobite (Mg4Nb2O9), a new magnesic-niobic oxide mineral from the Bayan Obo deposit, China

Abstract A new magnesic-niobic oxide mineral, oboniobite (IMA 2023-118a), ideally Mg4Nb2O9, has been discovered in the Bayan Obo REE-Nb-Fe deposit, China. The mineral occurs in a Meso-Proterozoic dolomite-calcite carbonatite dike and is associated with fersmite, columbite, pyrochlore, apatite, magnetite, dolomite, and calcite. Individual crystals range in size from 15 to 70 μm, and no twinning is observed. The mineral is yellowish-brown to brown in color and transparent, with a vitreous luster. The calculated density is 4.585 g/cm3. Seventeen electron microprobe analyses yielded the main compositions of oboniobite (total 97.61 wt%): Nb2O5 (57.65 wt%), MgO (27.24 wt%), FeO (6.15 wt%), MnO (5.98 wt%), and TiO2 (0.59 wt%). The empirical formula calculated on the basis of O = 9 apfu is Mg1(Mg1.42Fe0.30Mn0.24)Σ1.96Mg2(Mg1.71Fe0.10Mn0.15)Σ1.96(Nb2.01Ti0.03)Σ2.04O9, and the simplified formula is (Mg,Fe,Mn)4(Nb,Ti)2O9. Oboniobite is trigonal, with corundum-related structure, space group P3¯c1 (#165), Z = 2, and unit-cell parameters a = b = 5.18043(19) Å, c = 14.0768(6) Å, α = β = 90°, γ = 120°, and V = 327.16(3) Å3. The crystal structure of oboniobite has infinite layers of metal-centered MO6 octahedra along the c-axis, and Mg and Nb atoms are all hexacoordinated with oxygen. Isolated Nb2O9 spindles formed from face-sharing NbO6 octahedra are interspersed in the open hexagonal channels, which are formed by the Mg2O9 spindles sharing edges of O2-O2 along the c-axis. Oboniobite shares a similar chemical composition with columbite-(Mg) and ternovite, yet it is notably different in its crystallographic, physical, and optical properties compared to the other two minerals. Columbite-(Mg) forms needle-like opaque crystals, has a higher density of 5.04 g/cm3, contains more Nb2O5 at 68.36 wt% than oboniobite, and belongs to the orthorhombic system with the Pcan space group. Ternovite typically appears as radial translucent aggregates, has a low density of 2.95 g/cm3, has the highest Nb2O5 content at 70.24 wt% among the three minerals, and possesses a monoclinic symmetry with the P2/m or P2 or Pm space group. Therefore, oboniobite is not a variant of columbite-(Mg) and ternovite; instead, it is a unique new mineral species that contains niobium and magnesium.