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Formula: Ce(CO3)F
Bastnäsite-(Ce) is the most common member of the
bastnäsite group
Anhydrous carbonate containing halogen, cerium-bearing mineral, may be found
as pseudomorphs after
fluocerite or
chevkinite, and
forms oriented overgrowths on fluocerite, with crystal axes of both
species oriented in parallel position
(Mindat)
Crystal System: Hexagonal
Specific gravity: 4.9 to 5.2 measured, 5.12 calculated
Hardness: 4 to 5
Streak: White
Colour: Yellow, reddish-brown; colourless to light yellow in transmitted light
Solubility: Soluble in strong, hot acids
Weakly RADIOACTIVE
Environments:
Pegmatites
Carbonatites
Metamorphic environments
Hydrothermal environments
Bastnäsite is most commonly found in metamorphic rocks and
pegmatites, associated with
allanite, cerite-(CeCa),
fluocerite, fluorite and
tornebohmite
(Mindat).
Bastnäsite-(Ce) is typically hydrothermal, although primary
igneous occurrences are known, in granite and alkali
syenite and
pegmatites
in carbonatites, in
contact metamorphic deposits and rarely as a detrital
mineral in placers. Associated minerals include
allanite-(Ce), cerianite-(Ce),
synchysite-(Ce), parisite-(Ce),
cerite-(CeCa), fluocerite-(Ce) and
fluorite
(HOM).
Localities
At Tvedalen, Larvik, Vestfold, Norway, crystals of bastnäsite-(Ce) 10 to 20 mm in size are found in
syenite
pegmatites in both Tuften and Bjørndalen
larvikite quarries. Aggregates and rosettes of tiny platy crystals
are also observed in Heia larvikite quarry.
In a syenite
pegmatite dyke in Treschow quarry well developed
bastnäsite-(Ce) crystals have been found lining small vugs in
analcime. These vugs are actually negative crystals of a
primary, probably
gadolinite group, mineral. This
gadolinite mineral is the precursor of bastnäsite-(Ce)
which is sometimes intergrown with parisite-(Ce)
(JWW p156).
The type locality for bastnäsite-(Ce) is the Bastnäs Mines, Riddarhyttan, Skinnskatteberg, Västmanland County,
Sweden. Here bastnäsite-(Ce) occurs in narrow bands in a
contact metamorphic
amphibole skarn, associated with
fluocerite, fluorite,
cerite-(CeCa), allanite-(Ce) and
törnebohmite. The skarn is
composed largely of
amphiboles, that run parallel and adjacent to the
hematite
ore belt, in the granulite formation, consisting of
granulite, mica
schist and limestone -
dolostone layers
(Mindat, AM 30.609-612).
At Jamestown, Jamestown Mining District, Boulder county, Colorado, USA, centimetre to decimetre sized mineralised
pods and veins consist of zoned mineral assemblages dominated by
fluorbritholite-(Ce)
in a core 10 cm thick, with monazite-(Ce),
fluorite and minor quartz,
uraninite and sulphides. The core is surrounded by a
typically millimetre thick rim of allanite-(Ce), with
minor monazite-(Ce) in the inner part of the rim.
Bastnäsite-(Ce),
törnebohmite-(Ce) and
cerite-(CeCa) appear in an intermediate zone between core and
rim, often just a few hundreds of microns wide
(R&M 96.3.252-253).
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