Formula: (UO2)(MoO4).2H2O
Hydrated molybdate, uranyl mineral
Crystal System: Triclinic
Specific gravity: 4.53 to 4.66 measured, 4.49 calculated
Hardness: 2
Colour: Black, blue-black, dark green
Solubility: In 1:1 nitric acid umohoite effervesces, then turns yellow and yellow-brown; it is insoluble in hydrochloric acid

Sedimentary environments
Hydrothermal environments

Umohoite is a rare secondary mineral, typically in sediment-hosted uranium-bearing deposits, formed in the oxidation zone above the water table. Associated minerals include uraninite, ilsemannite, jordisite, iriginite, schoepite, uranophane, rutherfordine, calcurmolite, fluorite, pyrite, gypsum and quartz (HOM).


At the type locality, the Freedom No. 2 mine, Marysvale Mining District, Piute County, Utah, USA, umohoite occurs as small flat black flakes with triangular markings, in veinlets, and in disseminated grains. Associated minerals include uraninite, quartz, pyrite, jordisite, ilsemannite and fluorite (Mindat).

At the Lucky MC mine, Fremont County, Wyoming, USA, the uranium deposits are localised in coarse sandstones and conglomerates, and in mudstones and siltstones to a lesser extent. The abundant molybdenum in these deposits has given rise to uranium molybdates in the early stages of oxidation. The unoxidised ore contains uraninite, coffinite, iron sulphides, and jordisite as an interstitial cement in the sediments. Oxidation is rapid by moist air above the water table. Umohoite is present in limited amounts, and crystallises in the early stages of oxidation. It is commonly intergrown with gypsum in juxtaposition with iron sulphides and uranium oxides.
The umohoite forms delicate rosettes to 2 mm in size of tabular plates terminated by rather sharply angled points. The unaltered crystals are splendent blue black or dark green (AM 42.657-660).

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