Formula: U4+O2
Simple oxide of uranium, uraninite group, forms a series with thorianite
Pitchblende is a massive, often globular, possibly impure, variety of uraninite.
Crystal System: Isometric
Specific gravity: 10.63 to 10.95 measured, 10.88 calculated
Hardness: 6
Streak: Black
Colour: Black, grey, brownish
Solubility: Slightly soluble in hydrochloric acid; moderately soluble in sulphuric and nitric acid
Common impurities: Th,Zr,Pb,Ra,Ac,Po,Ce,Y,Er,La

Sedimentary environments
Hydrothermal environments

Uraninite occurs as a primary mineral in several distinct geological settings:
1. In pegmatites, typically as well-formed crystals or dendritic growths associated with monazite, columbite-tantalite and zircon.
2. In high temperature hydrothermal veins, typically as massive pitchblende associated with cassiterite, arsenopyrite and cobalt - nickel - bismuth - arsenic minerals.
3. In moderate temperarure hydrothermal veins with galena and other sulphides.
4. In sandstone deposits with vanadium, copper and other metal ores.
In pegmatites uraninite crystals contain up to ~10% thorium and ~15% rare earth elements. This is because the size and valence of these ions prevent their inclusion in the main rock-forming silicates as they crystallise. Pitchblende, on the other hand, is usually fairly pure because it has crystallised from a hydrothermal fluid that does not contain these elements. Both varieties of uraninite contain up to ~20% of radiogenic lead, depending on the age of the deposit (Lauf2).

At the Eldorado mine, Great Bear Lake, Northwest Territories, Canada, uraninite occurs in veins with native silver (Dana).

At the Shinkolobwe Mine, Shinkolobwe, Kambove District, Haut-Katanga, DR Congo, good specimens of sheet and wire gold associated with uraninite have been (R&M 96.3.220).

The type locality is Jáchymov, Karlovy Vary District, Karlovy Vary Region, Czech Republic.

At Placeres de Guadalupe, Chihuahua, Mexico, uraninite occurs with gold (Dana).

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).

At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, uraninite has been found rarely in massive microcline, commonly replaced by schoepite-group minerals and other secondary uranium minerals. In the proximity of uraninite, autunite and meta-autunite are seen. The Emmons pegmatite is situated in a belt of metasedimentary rocks which originated as marine sediments which were subsequently deformed and metamorphosed. The Emmons pegmatite is a highly evolved boron-lithium-cesium-tantalum enriched pegmatite (R&M 94.6.516).

At the pegmatites on Noyes Mountain, Greenwood, Oxford county, Maine, USA, uraninite is found in a garnet layer in masses to 1.5 cm, associated with coffinite and less commonly with autinite and meta-autinite. Accessory zircon, monazite and xenotime group species are intimately associated with the uranium assemblage (R&M 95.3.271).

At the the Chickering Mine, Walpole, Cheshire County, New Hampshire, USA, a single sample of uraninite has been found with zircon in a pegmatite (R&M 90.5.421).

At the Ruggles pegmatite, Grafton county, New Hampshire, USA, uraninite occurs intergrown with albite-rich feldspar or with muscovite, and associated with smoky quartz, curite and uranophane (Dana, AM 23.334-341).

At Baringer hill, Llano county, Texas, USA, uraninite occurs in a pegmatite with gadolinite (Dana).

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