Stephanite

stephanite

proustite

acanthite

tetrahedrite

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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:

Hydrothermal 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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