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Formula: Ag3(AsO4)
Arsenate of silver
Crystal System: Isometric
Specific gravity: 6.622 calculated
Hardness: 2
Streak: Grey to violet-grey streak; when readily fresh, its streak is brownish-red
Colour: Dark violet, changing to reddish and black when exposed to the air and light
Luminescence: Non-fluorescent in short wave and long wave UV
Environments
Theuerdankite is a new mineral, approved in 2023 and to date (December 2024) reported only from the type
locality.
Localities
At the type locality, the Alter Theuerdank Mine, Beerberg, St Andreasberg, Braunlage, Goslar District, Lower Saxony,
Germany, theuerdankite was found on a single specimen. Associated minerals within the specimen are
chlorargyrite, native silver and
calcite. Theuerdankite is most probably of
supergene origin but could also be late hydrothermal if the fluids
were oxidising enough to convert As in the primary minerals to
arsenates.
Theuerdankite occurs as irregular aggregates up to 3 mm in size, growing in cavities of strongly
supergene-weathered material consisting of
native silver and chlorargyrite.
Theuerdankite is dark grey with pinkish tints; there are dark red internal reflections, visible around fractures,
and inclusions of native silver
(MM 88.5.557-564).
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