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Formula:
Bastnäsite-(Ce): Ce(CO3)F is the most common member of the bastnäsite group
Bastnäsite-(La): La(CO3)F
Bastnäsite-(Nd): Nd(CO3)F
Bastnäsite-(Y): Y(CO3)F
All of these are anhydrous carbonates containing halogen
Bastnäsite-(Ce) may be found as pseudomorphs after
fluocerite or chevkinite, and it forms oriented
overgrowths fluocerite, with crystal axes of both species oriented in parallel position
(Mindat)
Specific gravity: 4.9 to 5.2
Hardness: 4 to 5
Streak: White
Colour: Yellow, reddish-brown; colourless to light yellow in transmitted light; bastnäsite-(Nd) may be pale purplish
pink to colourless
Solubility: Bastnäsite-(Ce) is 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,
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-(Ce), fluocerite-(Ce) and
fluorite
(HOM).
Bastnäsite-(Nd) occurs in the outer rim zone in crystals of bastnäsite-(Ce), found in cavities in yttrium-bearing
fluorite in quartz -
microcline
niobium–yttrium–fluorine
pegmatite in granitic
gneiss, and in granite pegmatites. Associated
minerals include bastnäsite-(Ce), stetindite,
atelisite-(Y), calcioancylite-(Nd),
kozoite-(Nd) and vyuntspakhkite-(Y)
(HOM).
Localities
In specimens from Ruanda-Urundi, Democratic Republic of the Congo, bastnäsite occurs as tabular masses, 5 to 8 millimeters long and
about 2 milimeters wide, scattered at random through the rock. Other minerals sparsely distributed through the rock include
quartz, pyrite,
microcline, biotite and
limonite. The entire rock appears to be secondary
(AM 30.608-609).
The type locality for bastnäsite-(Y) is the Verkhnee Espe Massif, Akzhaylyautas Mountains, Tarbagatai Range, East Kazakhstan, Kazakhstan.
Here bastnäsite-(Y) occurs as a rare secondary mineral in a
microcline – quartz
pegmatite vein, associated with
fluorite,
microcline, hematite and
quartz
(HOM).
At Itorendrika-Ifasina, Imorona valley, Itremo, Ambatofinandrahana, Amoron'i Mania, Madagascar, bastnäsite occurs in a
contact metamorphic zone with the
pegmatite facies of
alkali-granite. The
bastnäsite is most closely associated with chevkinite. Other minerals present are
magnesio-riebeckite, aegirine,
biotite, hematite,
magnetite and rutile
(AM 30.612-613).
At Odegi, Nasarawa, Nigeria, bastnäsite-(La) is associated with fluocerite and
cerianite-(Ce)
(HOM).
The type locality for bastnäsite-(Nd) is the Stetind pegmatite,
Narvik, Nordland, Norway.
The type locality for bastnäsite-(La) is the Belaya Zima REE-Nb deposit, Tuva, Russia. Here bastnäsite-(La) occurs
in late
ankerite carbonatite
(HOM).
At Kyshtym, Chelyabinsk Oblast, Russia, bastnäsite is found with cerite,
britholite-(Y), törnebohmite and more abundant
allanite, in pebbles in auriferous gravels
(AM 30-613).
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-(Ce), 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 Pike's Peak, El Paso county, Colorado, USA, a fist-sized crystalline mass has been found. About half of which is laminated bastnäsite,
and the rest of the sample is fluocerite. The fluocerite
appears to occur in parallel growth with the bastnäsite in feldspar in
a granite pegmatite
(AM 30.609=612).
At Jamestown, Jamestown Mining District, Boulder county, Colorado, USA, bastnäsite occurs with
cerite
and other cerium-bearing minerals along the contact between
pegmatite-aplite
bodies in granite and inclusions of
biotite
schist in the granite
(AM 30.613).
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-(Ce) appear in an intermediate zone between core and
rim, often just a few hundreds of microns wide
(R&M 96.3.252-253).
At the Gallinas Mountains, Red Cloud District, Lincoln county, New Mexico, USA, bastnäsite has been found in
fluorite deposits. The Gallinas Mountains are characterised by igneous intrusions which have penetrated
granite and overlying sediments. The principal intrusive rocks are
prophyritic quartz -
monzonite, rhyolite and
syenite. The sedimentary rocks are conglomerate,
arkose and fine-grained sandstone interbedded
with some limestone and siltstone overlying
the granite. These underwent slight metamorphism previous to the igneous intrusions.
Bastnäsite has been identified as a minor constituent of the fluorite ore in the Conqueror
prospects, the Eagles Nest claims, the Eureka mine and the Buckhorn mine. The rockmass in which the bastnäsite occurs is composed largely
of an aggregate of interlocking crystals of baryte, bastnäsite,
fluorite, quartz,
goethite and hematite
pseudomorphs after pyrite.
Pyrite appears to have been deposited at the same time as the
baryte and older fluorite, and at a later period the
entire zone was invaded by magmatic emanations rich in fluorine, carbon dioxide and rare earths, at which time a new generation of
fluorite and the bastnäsite were deposited. bastnäsite appears to have been one of the
last minerals to be deposited. Associated minerals include baryte,
barytocelestite, calcite,
fluorite, goethite,
hematite, limonite,
orthoclase, pyrite and
quartz
(AM 30.601-608).
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