Fergusonite: Properties and Occurrences

Fergusonite is a mineral comprising a complex oxide of various rare-earth elements. It is a dark mineral consisting of oxides of yttrium and erbium and tantalum and other minerals. 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.

Fergusonite-beta-(Y) was named after the allusion to its dimorphous relation to fergusonite-(Y).

Fergusonite-beta-(Nd) was named after the allusion to its dimorphous relationship to fergusonite-(Nd).

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).

General Information

  • Category: Oxide minerals
  • Formula: REENbO4 (REE: rare-earth element)
  • Crystal system: Tetragonal
  • Crystal class: Dipyramidal (4/m)

Fig: Fergusonite-beta-(Y)

Properties

All the other rare-earth elements are present in a subordinate amount, and tantalum substitutes for some of the niobium. There are Fergusonite-beta-(Nd), Fergusonite-beta-(Y), Fergusonite-beta-(Ce) too. 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.

  • 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

Occurrence of Fergusonite-Beta-(Nd) and Fergusonite-Beta-(Y)

Fergusonite-beta-(Nd) occurs in sodium-metasomatized carbonatite-derived dolomitic marble. It is often associated with minerals such as riebeckite, ferroan dolomite, biotite, magnetite, pyrite, monazite, bastnasite, aeschynite-(Nd), ilmenite, fergusonite-(Ce), and fergusonite-(Nd).

Fergusonite-beta-(Y) occurs in microcline-bearing granite stocks. This mineral is often associated with minerals such microcline and quartz.

 

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