Arsenic-Silver Incompatibility in Fahlore

Denton S. Ebel and Richard O. Sack
Department of Earth and Atmospheric Science, Purdue University, West Lafayette, Indiana, U.S.A. 47907

Abstract: Silver-bearing zinc-iron tetrahedrite-tennantite and freibergit fahlores approximating the simplified formula (Ag,Cu)10(Fe,Zn)2(As,Sb)4S13 have been equilibrated with excess electrum (AuxAg1−x) and chalcopyrite + pyrite + iron-bearing sphalerite (CuFeS2 + FeS2 + Fe0.05Zn0.95S) in evacuated silica tubes at 300 °C, in reversed silver-copper exchange experiments with less than 0.1 mg NH4Cl added as a transport medium. A thermodynamic formulation and parameters describing As-Ag incompatibility at 400 °C (Ebel and Sack, 1989), which incorporate large temperature dependencies of standard-state properties and composition-ordering systematics, are shown to apply equally well to these 300 °C results. A generalised graphical model for this mineral assemblage is presented, describing fahlore composition as a function of temperature and the compositions of coexisting electrum and (Fe,Zn)S, which define the Ag(Cu)−1 and Fe(Zn)−1 exchange properties controlling fahlore compositions.

Keywords: tetrahedrite • thermodynamics • fahlore • silver • sulphosalt • geothermometer • electrum

Mineralogical Magazine; December 1991 v. 55; no. 381; p. 521-528; DOI: 10.1180/minmag.1991.055.381.04
© 1991, The Mineralogical Society
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