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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
Crystal System: They are all hexagonal
Specific gravity: 4.9 to 5.2
Hardness: 4 to 5
Streak: White
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).
Localities for bastnäsite group, species unspecified.
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).
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 Kyshtym, Chelyabinsk Oblast, Russia, bastnäsite is found with
cerite,
britholite-(Y), törnebohmite
and more abundant
allanite, in pebbles in auriferous gravels
(AM 30-613).
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).
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,
barium-rich celestine,
calcite,
fluorite, goethite,
hematite, limonite,
orthoclase, pyrite and
quartz
(AM 30.601-608).
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