Fergusonite

oxide

Bayan Obo

Formulae:
fergusonite-(Ce) CeNbO4.0.3H2O
fergusonite-(Ce)-β CeNbO4
fergusonite-(Nd)-β NdNbO4
fergusonite-(Y) YNbO4
fergusonite-(Y)-β YNbO4
Multiple oxides containing the rare earth elements niobium Nb, cerium Ce, neodymium Nd and yttrium Y, usually metamict.

Specific gravity: Ce 5.48; Ce-β 5.34; Nd-β 6.45; Y 4.2 to 5.7; Y-β 5.65
Hardness: 5½ to 6½ (All)
Streak:
Ce reddish brown
Ce-β pale rose
Nd-β pale brown
Y brown, yellow-brown, greenish grey
Colour
Ce dark red to black
Ce-β red, reddish brown
Nd-β red, brownish red, yellowish brown
Y black to brownish grey, grey, yellow, brown
Y-β yellow
Solubility: Fergusonite-(Ce)-β is insoluble in acids (AM 62.397).
Environments

Pegmatites
Carbonatites
Placer deposits
Metamorphic environments

Fergusonite-(Ce)

At Bayan Obo, Inner Mongolia, China, fergusonite-(Ce) occurs in magnesium-rich skarn of the outer granite contact zone of a carbonatite-derived dolomitic marble (HOM). At the type locality, the Chernigovskiy Massif, Zaporizhia Oblast, Ukraine, fergusonite-(Ce) occurs in a carbonatite associated with magnetite, phlogopite, graphite, chlorite, quartz, pyrite, columbite, allanite, bastnäsite, parisite, apatite, aeschynite, fersmite, monazite, calcite and dolomite (HOM, Dana, AM 74.946).

Fergusonite-(Ce)-β

At the type locality, the Bayan Obo deposit, Baotou League, Inner Mongolia, China, fergusonite-(Ce)-β occurs in magnesian skarn around carbonatite-derived dolomitic marble, associated with diopside, phlogopite and apatite (Dana, AM 60.485).

Fergusonite-(Nd)-β

At the type locality, the West Mine, Bayan Obo deposit, Baotou League, Inner Mongolia, China, fergusonite-(Nd)-β occurs in a niobium-rare earth deposit in riebeckitised dolomite associated with riebeckite, ferroan dolomite, biotite, magnetite, monazite, bastnäsite, aeschynite-(Nd), ilmenite, pyrite and fergusonite-(Ce) (Dana, AM 69.406-407).

Fergusonite-(Y)

Fergusonite-(Y) is a typical mineral of rare-earth-bearing granite pegmatites, and it also occurs in placers. Associated minerals include monazite, gadolinite, thalénite, euxenite, allanite, zircon, biotite and magnetite (HOM).

At Murchison, Ontario, Canada, fergusonite-(Y) occurs with euxenite (Dana).

The type locality for fergusonite-(Y) is Kangeq, Sermersoq Island, Kujalleq, Greenland, Denmark.

At Hundholman, Norway fergusonite-(Y) occurs with allanite, fluorite variety yttrofluorite, xenotime and gadolinite (Dana).

At Iveland, Norway, fergusonite-(Y) occurs in pegmatites with gadolinite and thalénite (Dana).

At the Blum mine, Ilmenskie mountains, Russia, fergusonite-(Y) occurs with samarskite and rutile variety ilmenorutile (Dana).

At Ytterby, Sweden, fergusonite-(Y)occurs in a pegmatite with xenotime, zircon variety cyrtolite and biotite (Dana).

At spruce Pine, Mitchell county, North Carolina, USA, fergusonite-(Y)occurs in pegmatites with allanite (Dana).

At Barringer Hill, Llano county, Texas, USA, fergusonite-(Y) occurs with gadolinite and other rare earth minerals (Dana).

Fergusonite-(Y)-β

At Mount Cervandone in the Western Alps of Italy, an arsenic-rich variety of fergusonite-(Y)-β occurs in cavities within pegmatite dikes, associated with quartz, biotite, potassium feldspar, and synchysite-(Ce). The fergusonite crystallised under amphibolite facies conditions (AM 95.487-494).

At the type locality, Asht-Sai, Shaidan Massif, Kuraminskii Range, Sughd, Tajikistan, fergusonite-(Y)-β occurs in microcline granite rocks (Dana, Webmin).

Common impurities: U,Th

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