Fergusonite

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]

Fergusonite
General
Category	Oxide minerals
Formula (repeating unit)	REENbO4 (REE: rare-earth element)
IMA symbol	Fgs[1]
Strunz classification	7.GA.05
Crystal system	Tetragonal
Crystal class	Dipyramidal (4/m) H-M symbol: (4/m)
Space group	I41/a
Identification
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	Subconchoidal – fractures developed in brittle materials characterized by semi-curving surfaces
Mohs scale hardness	5.5–6
Luster	Submetallic
Streak	Brown
Density	4.3–5.8, average = 5.05
Refractive index	2.05–2.19, isotropic
Other characteristics	Nonmagnetic, non-fluorescent, non-radioactive
References	[2]