Images
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
Strongly RADIOACTIVE
Environments
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).
Localities
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).
Back to Minerals