Magnesiohongruiite-(Fe3+) (Mg2Fe3+)NbFe3+O7(OH), a new hydrous magnesic-ferric-niobic oxide mineral from the Bayan Obo deposit (China)

Abstract A new hydroxyl-bearing magnesic-ferric-niobic oxide mineral, magnesiohongruiite-(Fe3+) (IMA 2025-049), with the ideal formula (Mg2Fe3+)NbFe3+O7(OH), has been discovered in the Bayan Obo REE-Nb-Fe deposit, China. The mineral occurs in a Mesoproterozoic carbonatite dyke and is typically associated with oboniobite, magnetite, hydroxycalciopyrochlore, and dolomite. Magnesiohongruiite-(Fe3+) occurs as well-formed platy to subrounded single crystals (5-30 μm across). The crystals are translucent, reddish-brown to brown in color, with a vitreous luster, and have a calculated density of 4.373 g·cm−3. Electron microprobe analyses of 19 spots revealed a composition of 100.17 wt.% in total, comprising Nb2O5 (31.69-33.38 wt.%), MgO (13.97-18.39 wt.%), Fe2O3 (42.25-47.28 wt.%), and a calculated H2O content of ∼2.34 wt.% (assuming H = 1 a.p.f.u.). The empirical formula, calculated on the basis of 8 O atoms per formula unit, is M1(Mg1.69Fe1.14Mn0.14)Σ2.97M2(Nb0.96Ti0.04)Σ1.00T(Fe0.92Mg0.07)Σ0.99O7(OH), which can be simplified to (Mg2Fe3+)NbFe3+O7(OH). Magnesiohongruiite-(Fe3+) crystallizes in the hexagonal system (space group P63mc, #186) with Z = 2 and unit-cell parameters a = 5.97109(16) Å, c = 9.4375(3) Å, and V = 291.404(18) Å3. Magnesiohongruiite-(Fe3+), ideally formulated as (M122+M13+)M25+T3+O7(OH), has a structure consisting of edge-sharing M1O5(OH) octahedra forming an octahedral layer (L1), which is linked to a connector layer (L2) composed of corner-sharing TO4 tetrahedra and M2O6 octahedra. The crystal structure of magnesiohongruiite-(Fe3+) confirms its affiliation with the nolanite supergroup, but its dominant cation occupancy differs markedly from that of other minerals in the supergroup, suggesting that it may represent a potential new mineral group. The chemical composition of magnesiohongruiite-(Fe3+) closely resembles that of columbite-(Fe) (FeNb2O6), columbite-(Mg) (MgNb2O6), oboniobite (Mg4Nb2O9), and fuyuanite [Mg7Nb6O18(OH)8] yet it exhibits distinct crystallographic, physical, and optical properties. In addition, magnesiohongruiite-(Fe3+) is characterized by the presence of constitution water (OH-) and trivalent iron (Fe3+), features that are absent in the other minerals. Therefore, its distinctive composition and crystal structure demonstrate that it represents a previously unrecognized mineral species.