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Formula: U(SiO4).nH2O
Nesosilicate (insular SiO4 groups), zircon group
Specific gravity: 5.1
Hardness: 5 to 6
Streak: Greyish black
Colour: Black
Common impurities: Al,Fe,As,V,Pb
RADIOACTIVE
Environments
Sedimentary environments
Hydrothermal environments
Coffinite occurs in Colorado Plateau-type black unoxidised
uranium-vanadium deposits, replacing organic material in
sandstone, and in other sedimentary and hydrothermal vein
uranium deposits. Associated minerals include uraninite,
thorite, pyrite,
marcasite, roscoelite,
clay minerals and amorphous organic matter
(HOM). Coffinite is usually metamict, and formed from
uraninite in a
supergene, reducing, alkaline environment by absorption
of silica and hydration
(Dana).
Localities
At the Olympic Dam Mine, Roxby Downs, Stuart Shelf, South Australia, the
iron-oxide/copper/gold/silver/uranium
deposit contains three dominant uranium minerals, uraninite,
coffinite and brannerite. Coffinite is typically
globular and may enclose quartz. It is often found with
uraninite, and may be enveloped by it, or form aureoles of
coffinite on the margins of uraninite crystals. It is also
associated with brannerite.
Brannerite and coffinite have probably precipitated as part
of a late-stage hydrothermal event
(MM 81.6.1323–1366).
At Bangombé, Léboumbi-Leyou Department, Haut-Ogooué Province, Gabon, spontaneous fission reactions occurred
in several uranium deposits approximately two billion years ago. The reactor zones, from 10 to 5O cm thick, are
found in sandstone and consist of high-grade uranium ore
mantled by illite and/or
chlorite. several grains of a coffinite with unusually high
concentrations of phosphorous and enriched in light rare earth elements have been found
(MM 60.665-669).
At the Witwatersrand goldfield, South Africa, silicates of uranium and thorium are extremely rare in most
ore-bearing conglomerates. Authigenic coffinite, however, is fairly common in the Dominion Reef and on
the West Rand Goldfield. The coffinite was formed in situ from detrital
uraninite though the uptake of silica.
Thorite and thorogummite
of detrital origin also are present in the Dominion Reef
(CM 27.643).
Copper King mine, Prairie Divide District, Larimer county, Colorado, USA. Although coffinite was first
found in the uranium deposits in the sedimentary rocks of the Colorado Plateau, it has been found more recently
with uraninite and pitchblende in the vein deposits of the Copper
King mine. It is suggested that, in the sediments at least, reducing conditions were necessary for the deposition
of coffinite
(AM 41.675-688).
At the type locality, La Sal No. 2 mine, Beaver Mesa, Mesa county, Colorado, USA, coffinite has been found
with uraninite, carbonaceous material and
pyrite, impregnating sandstone
and replacing wood
(AM 30.1037). It is closely associated with fine-grained black low-valence
vanadium minerals, with uraninite,
and with finely disseminated black organic material
(AM 41.675-688).
In the Uravan District, Mesa county, Colorado, USA, coffinite and
uraninite seem to be the most abundant uranium minerals in the black ores
(AM 41.675-688).
At the Woodrow Mine, Laguna subdistrict, Cibola county, New Mexico, USA, well-crystallised coffinite has been
found as a vug filling of botryoidal material with a radially fibrous internal structure. The coffinite
is associated with pyrite, baryte,
small amounts of galena, wurtzite
and cobaltite, and trace amounts of
chalcopyrite
(AM 47.26-33).
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