| Noun | 1. | fergusonite - a dark mineral consisting of oxides of yttrium and erbium and tantalum and other minerals atomic number 68, Er, erbium - a trivalent metallic element of the rare earth group; occurs with yttrium atomic number 73, Ta, tantalum - a hard grey lustrous metallic element that is highly resistant to corrosion; occurs in niobite and fergusonite and tantalite atomic number 39, Y, yttrium - a silvery metallic element that is common in rare-earth minerals; used in magnesium and aluminum alloys mineral - solid homogeneous inorganic substances occurring in nature having a definite chemical composition |