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Formula: Ag5SbS4
Sulphosalt, silver- and antimony- bearing mineral
Crystal System: Orthorhombic
Specific gravity: 6.26 measured, 6.28 calculated
Hardness: 2½
Streak: Shiny black
Colour: Lead grey, iron black
Solubility: Insoluble in water, hydrochloric and sulphuric acid; soluble in nitric acid if heated
Stephanite is stable below 197oC (Mindat)
Common impurities: As,Cu,Fe
Environments:
Stephanite is a late stage primary mineral in epithermal
(low temperature)
hydrothermal silver veins (Webmin, HOM). Associated minerals include
proustite,
acanthite, silver,
tetrahedrite, galena,
sphalerite and pyrite
(HOM).
Localities
The type locality is Freiberg, Mittelsachsen, Saxony, Germany.
The Pallancata deposit, Parinacochas Province, Ayacucho, Peru, is a world-class intermediate-sulphidation
deposit, hosted by volcanics.
Ore deposition is related to protracted boiling of very diluted, mainly meteoric fluids, starting at 250oC
to 260oC, under ~300 m hydrostatic head. There has been a complex sequence of mineralisation and mineral
reactions consistent with Ag2S enrichment or Sb2S3 depletion, or both, during cooling
over the temperature range 250oC to 200oC (pyrite,
sphalerite, galena,
miargyrite,
pyrargyrite-proustite,
chalcopyrite,
polybasite-pearceite,
argentite and electrum).
Data document the thermal evolution of the system below 200oC (stephanite,
uytenbogaardtite, jalpaite,
stromeyerite and
mckinstryite, among others). The end of the most productive stages is
marked by the precipitation of stephanite at temperatures below ~197oC, but precipitation of residual
silver continues through the waning stages of the hydrothermal system down to
less than 93.3oC (stromeyerite) or in a
supergene redistribution
(acanthite II)
(CM 51.67-91).
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