Fuyuanite [Mg7Nb6O18(OH)8], a new hydrous magnesic-niobic oxide mineral from the Bayan Obo deposit, China

A new hydrous magnesic-niobic oxide mineral, fuyuanite (IMA 2024-059), ideally Mg7Nb6O18(OH)8, has been discovered in the Bayan Obo REE-Nb-Fe deposit, China. The mineral occurs in a Mesoproterozoic dolomite-calcite carbonatite dike, and it is associated with oboniobite, fersmite, pyrochlore, magnetite, dolomite, apatite, and calcite. Individual crystals range in size from 40 to 60 μm, and no twinning is observed. The mineral is reddish-brown to brown and translucent, with a vitreous luster. The calculated density is 4.156 g/cm3. Electron microprobe analyses of 18 spots yielded the main components of fuyuanite (total 99.41 wt%): Nb2O5 (average of 67.34 wt%), MgO (21.66 wt%), FeO (2.89 wt%), MnO (1.18 wt%), TiO2 (0.21 wt%), and H2O (6.12 wt%, calculated assuming H = 8 apfu). The empirical formula calculated on the basis of O = 26 apfu is (Mg6.31Fe0.53Mn0.20)Σ7.03(Nb5.96Ti0.03)Σ5.99O18(OH)8, and the simplified formula is (Mg,Fe,Mn)7(Nb,Ti)6O18(OH)8. Fuyuanite is trigonal, with space group P3¯ (#147), Z = 1, and unit-cell parameters a = 10.7174(5) Å, c = 4.7110(2) Å, and V = 468.62(4) Å3. The crystal structure of fuyuanite consists of metal-octahedra (MO6) layers along the c-axis, where Mg and Nb are octahedrally coordinated with oxygen. In the fuyuanite structure, the layer (L1) of edge-sharing MgO5(OH) octahedra (Mg2) is stacked with the layer (L2) of discrete rings of six edge-sharing NbO4(OH)2 octahedra enclosing a Mg(OH)6 octahedron (Mg1), and the cation stacking follows the sequence of –Mg2–(Mg1,Nb)–Mg2–. Fuyuanite shares a similar chemical composition with oboniobite (Mg4Nb2O9), columbite-(Mg) (MgNb2O6) and ternovite (MgNb4O11·8–12H2O), yet it is notably different in its crystallographic, physical, and optical properties compared to the other three minerals. Therefore, fuyuanite is not a variant of oboniobite, columbite-(Mg), or ternovite. Instead, it is a unique new mineral species.