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The Ag-Au-S system

M. D. Barton
The Ag-Au-S system
Economic Geology and the Bulletin of the Society of Economic Geologists (April 1980) 75 (2): 303-316


Determination of phase relations in the Ag-Au-S system between 100 degrees and 850 degrees 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 (sub 2) S-Au (sub 2) S partial solid solution, and sulfur. At low temperatures, two ordered silver-gold sulfides exist, Ag (sub 3) AuS (sub 2) and AgAuS; they invert at 185 degrees and 310 degrees C, respectively, to a simple, cubic solid solution. Above 310 degrees C a complete solid solution exists from Ag (sub 2) S to approximately Ag (sub 0.9) Au (sub 1.1) S. 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 (sub 2) S and Ag (sub 1.5) Au (sub 0.5) S. The silver-rich end of the solid solution inverts to a face-centered modification at high temperatures. The sulfides melt in the interval 680 degrees to 838 degrees C with a minimum in the liquidus at Ag (sub 1.3) Au (sub 0.7) S.Enthalpies of transition for the Ag (sub 2) S III-II, Ag (sub 3) AuS (sub 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 1,570(min)-1,920(max) calories/mole. Within experimental error, the sulfide solid can be modeled as a regular solution with components AgS (sub 0.5) and AuS (sub 0.5) and with W (sub G) [asymp] - 1,000 calories. Estimated temperature-dependent standard free energies of formation are (T = K and units of calories): -43,880 + 20.8T (acanthite, Ag (sub 2) S III), -40,410 + 20.8T (uytenbogaardtite, low Ag (sub 3) AuS (sub 2) ), -37,910 + 15.3T (high Ag (sub 3) AuS (sub 2) ), -36,740 + 23.4T (low AgAuS), -33,340 + 17.6T (high AgAuS), and -22,700+ 29.6T (Au (sub 2) S).

ISSN: 0361-0128
EISSN: 1554-0774
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 75
Serial Issue: 2
Title: The Ag-Au-S system
Author(s): Barton, M. D.
Affiliation: Univ. Chic., Dep. Geophys. Sci., Chicago, Ill., United States
Pages: 303-316
Published: 198004
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
References: 43
Accession Number: 1980-033810
Categories: Economic geology of ore deposits
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. tables
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Society of Economic Geologists
Update Code: 1980
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