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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 Johanngeorgenstadt, Erzgebirgskreis, Saxony, Germany, acanthite was found in rich concentrations in the
upper zones of the ore veins during the heyday of mining in the 17th and 18th centuries. Cubic and octahedral
crystals and penetration twins of acanthite (actually
pseudomorphs after the crystal habit of higher-temperature
argentite) to more than 2 cm were found. Acanthite occurred in
many forms including hairlike to wire-like habits, cubes with modified corners, octahedrons, dendritic groups,
four-sided and six-sided prisms, thick, platy crystals and solid masses weighing more than 1.5 kg. Leafy to
granular acanthite occurred in chert and
jasper, forming the so-called “silver agate”.
Proustite-pyrargyrite was
commonly found intermixed in thick bodies of massive acanthite.
Species commonly associated with acanthite include native silver,
proustite-pyrargyrite,
galena, pyrite, and
cobalt-nickel ores, as well as
quartz and dolomite
(MinRec 55.5.584-587).
Acanthite from Johanngeorgenstadt -
Image
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).
Acanthite from the San Juan de Rayas Mine -
Image
At the Santa Catarina mine, Guanajuato, Mexico, acanthite forms pseudomorphs
after pyrargyrite
(KL p125)>.
Acanthite from the Santa Catarina Mine -
Image
At Charcas, Charcas Municipality, San Luis Potosí, Mexico,
the primary minerals are
sphalerite, galena,
chalcopyrite, bornite,
tetrahedrite,
arsenopyrite, pyrite and
silver minerals such as jalpaite,
diaphorite and acanthite. In
the host rock, as metamorphic or alteration minerals, danburite,
datolite, hedenbergite,
epidote, chlorite,
andradite, actinolite
and wollastonite have been reported.
Quartz, calcite and
danburite crystallised during the entire life of the systems, throughout
the intrusive emplacement, metamorphism, and mineralising events. With depth, both
sphalerite and galena decrease
while chalcopyrite increases.
Secondary sulphides formed include
bornite, covellite,
digenite and chalcocite.
Native silver, native gold,
hematite and goethite were
deposited after the sulphides
(Minrec 55.6.727-728).
Acanthite paramorphic after
argentite is the most common
silver mineral in the mines of Charcas, occurring as inclusions and
exsolutions in galena. Specimens with visible crystals are rare and of
modest quality
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