Bastnäsite

minerals

fluocerite

allanite

neodymium

Images
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 microclinequartz 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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