Ag---Cu exchange equilibria between pyrargyrite, high-skinnerite, and polybasite solutions

Details Ag---Cu exchange equilibria between pyrargyrite, high-skinnerite, and polybasite solutions Ag---Cu exchange equilibria between pyrargyrite, high-skinnerite, and polybasite solutions

Mar 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59..867h&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 59, Issue 5, pp.867-874

Physics

1

Scientific paper

A model is developed for the thermodynamic properties of pyrargyrite (Ag,Cu ) 3 SbS 3 and high-skinnerite (Cu, Ag ) 3 SbS 3 solid solutions over the temperature range 150-350°C. It makes explicit provision for Ag---Cu exchange equilibria between pyrargyrite, high-skinnerite, and polybasite and for the presence of a miscibility gap between pyrargyrite and high-skinnerite. These features are constrained by 150-350°C Ag---Cu exchange experiments (evacuated silica tubes; variable mass ratio) between pyrargyrite and polybasite and high-skinnerite and polybasite. Ag---Cu exchange experiments between polybasite and equimolar mixtures of pyrargyrite and skinnerite are used to define the limbs of a miscibility gap between pyrargyrite and high-skinnerite. Explicit provision for site distortion resulting from the Cu + for Ag + substitution (or Ag + for Cu + substitution) is made by assuming that the standard state properties of the Ag-pyrargyrite and Cu-pyrargyrite endmembers are linearly dependent on the Cu/(Cu + Ag) ratio. Copper-substitution in pyrargyrite is shown to be asymmetric and substantially nonideal ( W CuAg Pyr = 21.0; W AgCu Pyr = 11.0 ± 0.5 kJ/gfw). In contrast, substitution of Ag in high-skinnerite is strongly ordered ( ) on two sites (2:1) ( W CuAg A Skn = 12.0; W CuAg B Skn A = 9.0 ± 0.5 kJ/gfw). Finally, these equilibria are consistent with estimates of Gibbs energies of formation (from the simple sulfides) of a hypothetical Cu 16 Sb 2 S 11 polybasite endmember of 15.00 kJ/gfw (400°C).

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