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Formula: Ag2S
Sulphide of silver,
acanthite group
Crystal System: Monoclinic
Specific gravity: 7.2 to 7.4 measured, 7.24 calculated
Hardness: 2
Streak: Black
Colour: Lead grey
Solubility: Acanthite is slightly soluble in hydrochloric acid and sulphuric acid
Common impurities: Se
Environments:
Acanthite is a primary silver mineral
that occurs in epithermal (low temperature) hydrothermal silver ore
veins. It may also be of secondary origin.
At atmospheric
pressure, acanthite is stable below 173°C. Above 173°C the
structure changes to produce crystals of twinned acanthite known as argentite.
Argentite is unstable
below 173°C,
and if the temperature drops below this level it will change back to acanthite.
Common associates include aguilarite,
calcite, chalcopyrite,
galena, polybasite,
proustite, pyrargyrite,
quartz, silver,
sphalerite and stephanite
(Mindat).
Localities
At the Mount Lyell Mines, Queenstown district, West Coast municipality, Tasmania, Australia, acanthite
pseudomorphs after argentite have been found in a vein
of chalcocite, bornite and
tetrahedrite between massive pyrite and
hematite bodies
(AJM 21.2.21).
At the Old Hope of God pit, Freiberg, Saxony, Germany, acanthite pseudomorphs
after galena have been found with proustite
and baryte
(KL p126).
At the San Juan de Rayas mine, Guanajuato, Mexico, acanthite forms paramorphs
after argentite. At the same locality
acanthite-chalcopyrite forms
pseudomorphs
after polybasite
(KL p123-124)>.
At the Santa Catarina mine, Guanajuato, Mexico, acanthite forms pseudomorphs
after pyrargyrite
(KL p125)>.
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