The state of Ag in pyrrhotite: insights from X-ray absorption spectroscopy

Ag-bearing pyrrhotites Fe1-xS were synthesized using the salt flux technique. The concentration of Ag
in pyrrhotite reached 0.08 wt.% at 540 °C and 1.3 wt.% at 750-760 °C. SEM and EPMA analyses
revealed that at low sulfur fugacity (CFe > 48.8 at.%), Ag is disseminated in pyrrhotites in an
“invisible” form and concentrates on the grain boundaries of crystals as metallic rims (group (i)). At
high sulfur fugacity (CFe < 48.8 at.%), Ag occurs as a minor form of Ag-bearing oriented submicron
inclusions but mostly enriches pyrrhotite as the “invisible” form disseminated in the pyrrhotite matrix
(group (ii)). Analysis of Ag K-edge XANES spectra recorded at ambient temperature revealed that
pyrrhotites of group (i) mostly contain Ag°, group (ii) accommodate Ag as Ag+. EXAFS spectra fitting
demonstrated that samples of group (i) contain a small fraction of the “invisible” form while the
majority of Ag is disseminated in pyrrhotite crystals as Ag°; samples of group (ii) mostly contain the
“invisible” form presented by Ag+2S-like clusters. Heating of the samples containing the “invisible”
form of Ag up to 750 °C performed via capillary technique demonstrated the decrease of Ag
coordination number in the second coordination sphere and increase of Debye-Waller parameters for
Ag and Fe in the distant coordination shells. This observation demonstrates that the size of Ag2S-like
form is decreasing with temperature. However, the “invisible” Ag presented by Ag2S-like clusters is
stable up to the temperature of synthesis of 750 °C, hence, the formation of clusters cannot be
accounted for upon cooling. The predicted concentrations of Ag in pyrrhotite coincide with those from
natural samples of various origins.