The Ag-Au-S system

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Determination of phase relations in the Ag-Au-S system between 100° and 850°C by silica tube, differential thermal analysis, and high-temperature X-ray experiments reveals that the condensed system consists of three basic parts: the Ag-Au complete solid solution, the Ag 2S-Au 2S partial solid solution, and sulfur. At low temperatures, two ordered silver-gold sulfides exist, Ag 3AuS 2 and AgAuS; they invert at 185° and 310°C, respectively, to a simple, cubic solid solution. Above 310°C a complete solid solution exists from Ag 2S to approximately Ag 0.9Au 1.1S. This solid solution has a body- centered cubic structure for silver-rich compositions, which changes via a second-order transition to a simple, cubic structure between Ag 2S and Ag 1.5Au 0.5S. The silver-rich end of the solid solution inverts to a face-centered modification at high temperatures. The sulfides melt in the interval 680° to 838°C with a minimum in the liquidus at Ag 1.3Au 0.7S. Enthalpies of transition for the Ag 2S III -II, Ag 3AuS 2 low-high, and AgAuS low-high transitions were measured by differential scanning calorimetry and found to be, respectively: 950; 2,270(min)-2,760(max); and l,570(min)-l,920(max) calories/mole. Within experimental error, the sulfide solid can be modeled as a regular solution with components AgS 0.5 and AuS 0.5 and with Wg = -1,000 calories. Estimated temperature- dependent standard free energies of formation are (T = K and units of calories): -43,880 + 20.8T (acanthite, Ag 2S III), - 40,410 + 20.8T (uytenbogaardtite, low Ag 3AuS 2), -37,910+ 15.3T (high Ag 3AuS 2), -36,740 + 23.4T (low AgAuS), -33,340 + 17.6T (high AgAuS), and -22,700 + 29.6T (Au 2S).

Original languageEnglish (US)
Pages (from-to)303-316
Number of pages14
JournalEconomic Geology
Issue number2
StatePublished - Jan 4 1980

ASJC Scopus subject areas

  • Geophysics
  • Geology
  • Geochemistry and Petrology
  • Economic Geology


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