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Category Oxide minerals
(repeating unit)
REENbO4 (REE: Rare earth element)
Strunz classification 7.GA.05
Crystal system Tetragonal
Crystal class Dipyramidal (4/m)
H-M symbol: (4/m)
Space group I41/a
Formula mass 245.81 g/mol (Y)
Color Black, Black, Brown, Gray, Yellow
Crystal habit Metamict - Mineral originally crystalline, now amorphous due to radiation damage
Cleavage Indistinct
Fracture Sub Conchoidal - Fractures developed in brittle materials characterized by semi-curving surfaces
Mohs scale hardness 5.5-6
Luster Sub Metallic
Streak Brown
Density 4.3 - 5.8, Average = 5.05
Refractive index 2.05-2.19, isotropic
Other characteristics Nonmagnetic, Non-fluorescent, Not Radioactive
References [1]

Fergusonite is a mineral comprising a complex oxide of various rare earth elements. The chemical formula of fergusonite species is (Y,REE)NbO4, where REE = rare-earth elements in solid solution with Y. Yttrium is usually dominant (the species fergusonite-(Y)), but sometimes Ce or Nd may predominate in molar proportion (species fergusonite-(Ce) and fergusonite-(Nd)). All the other rare earth elements are present in subordinate amount, and tantalum substitutes for some of the niobium. There are Fergusonite-beta-(Nd), Fergusonite-beta-(Y), Fergusonite-beta-(Ce) too, but they are classified as 4.DG.10 in the Nickel-Strunz system. The mineral has tetragonal crystal symmetry and the same structure as scheelite (calcium tungstate, CaWO4), but can be metamict (amorphous) due to radiation damage from its small content of thorium. It is found as needle-like or prismatic crystals in pegmatite. It was named after British Politician and mineral collector Robert Ferguson of Raith (1767–1840).[2]

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