Monazite-(Ce) is overwhelmingly the most common member of the monazite group
Specific gravity: 5 to 5.5
Hardness: 5 to 5½
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
Plutonic igneous environments
Hydrothermal environments (infrequent)
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.
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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