Formula: (K)2(UO2)2(Si5O13).4H2O
Nesosilicate (insular SiO4 groups), uranium-bearing mineral
Specific gravity: 4.1
Hardness: 1 to 2
Streak: Yellow
Colour: Yellow
Common impurities: Al,Fe,Ca,Ba,Na,C

Igneous environments
Sedimentary environments

Weeksite occurs in opal veinlets in rhyolite and agglomerates, as well as in sandstones and limestones. Associated minerals include opal, chalcedony, calcite, gypsum, fluorite, uraninite, thorogummite, uranophane, boltwoodite, carnotite and margaritasite (HOM, Dana).


At Chihuahua, Mexico, weeksite has been found associated with carnotite on the dumps of an abandoned mercury mine, 4½ miles southwest of Lajitas, Texas, USA (AM 45.39-52).

At the Red Knob Mine, Muggins Mountains uranium prospects, Muggins Mining District, Muggins Mountains, Yuma county, Arizona, USA, weeksite is associated with opal, carnotite, vanadinite, gypsum, calcite and azurite (AM 45.39-52).

At an unnamed U occurrence at Inyo county, California, USA, weeksite has been found as a coating along fractures in granite, associated with uranophane (AM 45.39-52).

At the Red Rock Mine, Plumas county, California, USA, weeksite is found in fractures in rhyolite (AM 45.39-52).

At the Jackpile Mine, Laguna subdistrict, Cibola County, New Mexico, USA, weeksite occurs in a sandstone-type deposit (AM 45.39-52).

At the Sherrer Quarry, C.K. Williams Quarry complex, Chestnut Hill, Easton, Northampton county, Pennsylvania, USA, weeksite is found in fractures in limestone associated with thorian uraninite, thorogummite, uranophane, carnotite and boltwoodite (AM 45.39-52).

At the Mammoth Mine, Presidio county, Texas, USA, weeksite lines cavities in a welded tuff (AM 45.39-52).

The type locality is the Autunite No. 8 claim, Topaz Mountain, Thomas Range, Juab County, Utah, USA. The oldest rock in the vicinity is a porphyritic rhyolite containing numerous phenocrysts of sanidine and quartz, some phenocrysts of plagioclase and a few of biotite. This rhyolite is overlain by a black glass welded tuff consisting of pieces of pumice and crystals of plagioclase and biotite. This is overlain in turn by a white vitric tuff containing volcanic rock fragments, mainly pumice, and some tiny crystal fragments of sanidine and quartz.
The weeksite occurs in numerous veinlets, up to 1/4 inch thick, which fill fractures in the rhyolite. The veins consist chiefly of opal, which fluoresces bright yellowish green. The only uranium mineral observed is weeksite, which occurs in finely fibrous yellow rosettes, up to 1 mm across, both encrusted and intergrown with the opal, and also on fracture surfaces free of opal. Other vein minerals are calcite and fluorite (AM 45.39-52).

At the Good Will property, Thomas Range, Juab Coounty, Utah, USA, uranium minerals are found in two types of occurrences: disseminated in tuffaceous sandstone and as replacement in limestone cobbles in the conglomerate which overlies the sandstone. Uranophane-β, the chief ore mineral, fills numerous pore spaces in the sandstone. The only other uranium mineral noted in this rock was schröckingerite, which occurredin a few veinlets in one pit.
Weeksite occurs in fine grains in the conglomerate, where it replaces parts of limestone pebbles or cobbles. Weeksite occurs in extremely fine grains and is the only mineral replacing the limestone pebbles. No weeksite has been found in the uranium deposits in the underlying tuffaceous sandstone, and neither uranophane-β nor schröckingerite has been found in the conglomerate (AM 45.39-52).

At the Silver Cliff Mine, Lusk, Niobrara county, Wyoming, USA, weeksite occurs in calcareous sandstone with uranophane (AM 45.39-52).

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