Yttrotungstite-(Nd), a new mineral species from the Nyakabingo tungsten mine, Kigali, Rwanda

Abstract. Yttrotungstite-(Nd), NdW2O7(OH)(H2O), is a new mineral species discovered in the Nyakabingo tungsten mine, Kigali, Rwanda. The mineral occurs in the oxidized zones of the deposit, where it forms tiny yellow-orange tabular crystals associated with anthoinite and hydrokenoelsmoreite. The crystals are transparent, reach a length of a maximum of 100 µm, and frequently form radiated groups. The dominant form is {010}, and a twinning has been observed parallel to (001). The streak of yttrotungstite-(Nd) is white, the lustre is vitreous, and the mineral is non-fluorescent. The Mohs hardness is 1; the tenacity is brittle; and observed cleavage planes are (010) perfect and (100) and (001) excellent. The calculated density is 6.25 g cm−3, and refraction indices are α= 1.90(2) and γ= 2.10(2). The mineral is optically biaxial, its optical plane is parallel to (010), and the optical orientation is X ∧ c = 6° and Z ∧ a = 9°. Pleochroism is from light yellow (X) to yellow (Z). The empirical formula of yttrotungstite-(Nd), calculated on the basis of 15 positive charges assuming 1 (OH) group and 1 H2O molecule per formula unit, corresponds to the following: (Nd0.36Sm0.13Ce0.12Pr0.08La0.06Gd0.06Y0.05Dy0.03Yb0.02Ca0.01)Σ0.92(W2.02P0.02)Σ2.04O7(OH)(H2O). The crystal structure was refined in space group P21/m to the final R1 value of 0.0362, and the measured unit-cell parameters are a= 5.8534(3), b= 8.6881(3), c= 7.0488(4) Å, β= 105.336(5)°, and V= 345.70(3) Å3. The structure of yttrotungstite-(Nd) is characterized by edge-sharing WO6 octahedra, forming kinked chains parallel to the b axis. The NdO8 complex polyhedra connect three chains together by sharing edges or corners with adjacent WO6 octahedra. A view perpendicular to the b axis shows that the NdO8 and WO6 polyhedra occur in two different types of planes, stacked parallel to the [−101] direction. Neodymium occurs in a very large eight-coordinated site, which can be described as two tetragonal pyramids sharing one common basal edge and sharing the opposite basal edge with a square. Calculated bond-valence sums are discussed in detail, as well as the Raman spectral results. The species and its name were approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association under number IMA 2023-064; its abbreviation is “Ytgs-Nd”.