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Formula: Cu(UO2)2(SO4)2(OH)2.8H2O
Hydrated sulphate, uranyl mineral
Crystal System: Triclinic
Specific gravity: 3.32 measured, 3.44 calculated
Hardness: 2 to 2½
Streak: Pale green
Colour: Emerald-green, apple-green
Solubility: Decomposed by water, soluble in acids
RADIOACTIVE
Environments
Johannite is a rare secondary mineral in the oxidised
portions of sulphide-bearing uraninite deposits, commonly of post-mine
formation. Associated minerals include gypsum,
zippeite, uranopilite,
brochantite and chalcanthite
(HOM).
Localities
The type locality is the Elias mine, Jáchymov, Karlovy Vary District, Karlovy Vary Region, Czech Republic.
Johannite from the Elias mine - Image
At Johanngeorgenstadt, Erzgebirgskreis, Saxony, Germany, johannite, an alteration product of
pitchblende, was found rarely in the oxidation zones of
uranium ore-bearing veins. It appeared as tiny crystals and reniform aggregates
of a grass-green colour
(MinRec 55.5.599).
At La Creusaz Uranium prospect, La Creusaz, Les Marécottes, Salvan, Saint-Maurice, Valais, Switzerland,
johannite
is associated with marécottite
(MinRec 35.4.358).
Johannite from La Creusaz - Image
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