Fergusonite n. : A mineral of a brownish black color, essentially a tantalo-niobate of yttrium, erbium, and cerium; -- so called after Robert Ferguson.

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Taxonomy
Fergusonite, mineral, free encyclopedia, species, radiation damage, mineral collector, metamict, Fractures, search, chemical formula, Rare earth, Dipyramidal, Conchoidal, brittle materials, surfaces, hardness, isotropic, characteristics.

Defination from the web:
Fergusonite is a mineral comprising a complex oxide of various rare earth elements. The chemical formula of fergusonite species is (Y,RE)NbO4, where RE = rare-earth elements in solid solution with Y. Yttrium is usually ...