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Monazite-(Ce) is overwhelmingly the most common member of the
monazite group
Formula: Ce(PO4)
Phosphate
Crystal System: Monoclinic
Specific gravity: 5 to 5.5 measured, 5.26 calculated
Hardness: 5 to 5½
Streak: White
Colour: Commonly reddish brown to brown; shades of green to brown, yellow brown, rarely
nearly white; yellow, colourless in transmitted light.
Solubility: Slightly soluble in hydrochloric, sulphuric and nitric acid
Environments:
Plutonic igneous environments
Pegmatites
Carbonatites
Sedimentary environments
Metamorphic environments
Hydrothermal environments (infrequent)
(SC)
Monazite is a comparatively rare mineral occurring as an accessory in some plutonic igneous rocks, in pegmatites and
as rolled grains in sands because of its resistance to chemical attack and its high specific gravity.
It may be found in
granite including
aplite,
syenite,
schist,
gneiss and
granulite.
In clastic sedimentary deposits it is associated with other resistant and heavy minerals such as
magnetite,
ilmenite,
rutile and
zircon.
Localities
At Llallagua, Bolvia, monazite occurs both as an igneous mineral, with a high thorium content, and also as a hydrothermal mineral,
with a characteristically low thorium content. It is associated with fluorapatite,
other hydrous phosphates
and cassiterite. As the temperature drops, monazite begins to crystallise out at
about 550oC
and continues to grow on down to about 300oC
(Mineralogy and Petrology 111:547-568).
At the type locality, the Ilmen Nature Reserve, Chelyabinsk Oblast, Russia, monazite is associated with
zircon and ilmenite
(Mindat).
At the Longs Peak - St Vrain batholith near Jamestown, Jamestown 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-(Ce) 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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