Characterization of W- and Nb-bearing columbite-supergroup crystals from Tahara, Gifu Prefecture, Japan

We report prismatic crystals of a W-and Nb-bearing columbite-supergroup mineral from a granitic pegmatite in Japan.Electron microprobe analyses indicate that the crystals consist of a relatively W-rich core and a W-poor mantle.High-resolution scanning transmission electron microscopy observations show that the core has a uniform wolframite (ferberite-hübnerite series) structure containing a significant amount of Nb, whereas the mantle is not a simple compositional growth zone but consists of wolframite and columbite structural units that are irregularly interstratified along the a-axis.Regular 1:1 stacking of one wolframite unit cell and half a columbite unit cell along the a-axis, previously reported as qitianlingite, is common.Another superstructure with one wolframite unit cell and one columbite unit cell also occurs frequently.The mantle structure is interpreted to have formed through post-growth re-equilibration during cooling.Charge compensation associated with the substitution of W 6+ by (Nb,Ta) 5+ in wolframite is also discussed.