Fergusonite

Sulfate mineral series

(repeating unit)REENbO₄ (REE: rare-earth element)IMA symbolFgs^[1]Strunz classification7.GA.05Crystal systemTetragonalCrystal classDipyramidal (4/m)
H-M symbol: (4/m)Space groupI4₁/aIdentificationFormula mass245.81 g/mol (Y)ColorBlack, black, brown, gray, yellowCrystal habitMetamict – mineral originally crystalline, now amorphous due to radiation damageCleavageIndistinctFractureSubconchoidal – fractures developed in brittle materials characterized by semi-curving surfacesMohs scale hardness5.5–6LusterSubmetallicStreakBrownDensity4.3–5.8, average = 5.05Refractive index2.05–2.19, isotropicOther characteristicsNonmagnetic, non-fluorescent, non-radioactiveReferences^[2]

Fergusonite is a mineral comprising a complex oxide of various rare-earth elements. The general chemical formula of fergusonite is (Y,REE)NbO₄, where REE = rare-earth elements in solid solution with Y. Yttrium is usually dominant (the mineral in this case being referred to as fergusonite-(Y)), but sometimes Ce or Nd may be the major rare-earth component (in fergusonite-(Ce) and fergusonite-(Nd), respectively). The other rare-earth elements are present in smaller amounts, and tantalum sometimes substitutes for some of the niobium. There are fergusonite-beta-(Nd), fergusonite-beta-(Y), fergusonite-beta-(Ce) forms 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).^[3]

References

^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
^ Fergusonite-(Y) Mineral Data. Webminerals.
^ Robert Ferguson of Raith. University of Edinburgh.