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Formula: Cu5Zn5(AsO4)2(OH)14
Anhydrous arsenate
Crystal System: Orthorhombic
Specific gravity: 4.3 measured, 4.45 calculated
Hardness: 1½
Streak: Off-white
Colour: Greenish blue, turquoise-blue, pale turquoise-green, pale emerald-green
Environments
Sedimentary environments
Hydrothermal environments
Theisite is a rare secondary mineral in
tennantite-tetrahedrite
deposits
(Mindat).
Localities
The type locality, Tuckerville prospects, Hinsdale County, Colorado, USA, is a small
uranium prospect; the wallrocks are
marly sediments cut by near-vertical
breccia zones that are cemented by sulphides and
gangue minerals such as quartz,
calcite and baryte.
Primary ore minerals may survive as relicts, and they include
uraninite, tetrahedrite,
galena and sphalerite. In most
places, however, oxidation has been severe. Sulphides alter in situ to
chalcocite,
covellite, and cuprite surrounded
by malachite, azurite and
kolwezite. Other species noted and only locally present include
parnauite, anglesite,
cerussite, tenorite,
adamite, hemimorphite,
chrysocolla, zeunerite and
duftite.
Theisite is sparingly present and is one of the very last minerals to form. It occurs in thin seams cutting
the oxide assemblage. These structures usually carry no minerals other than theisite
(MM 46.49-50).
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