State of Ag in Pyrrhotite: Insights from X-ray Absorption Spectroscopy

Ag-bearing pyrrhotites Fe1-x S were synthesized by using the salt flux technique. The concentrationof Ag in pyrrhotite reached 0.08 wt % at 540 & DEG;C and 1.3 wt %at 750-760 & DEG;C. The synthesized samples are divided intotwo groups according to the values of sulfur fugacity in the experimentalsystem and the content of Fe (C (Fe)) inpyrrhotite. In group (i) at low sulfur fugacity (log f(S-2) & AP; -6.5, C (Fe) > 48.8 at. %), Ag is disseminated in pyrrhotites in an "invisible"form but mostly concentrates on the grain boundaries of crystals asmetallic rims. In group (ii) at high sulfur fugacity (log f(S-2) & AP; -1.0, C (Fe) < 48.8 at. %), Ag occurs as an invisible form disseminatedin the pyrrhotite matrix together with the minor form of Ag-bearingsubmicron inclusions. Analysis of Ag K-edge XAS spectra recorded atambient temperature revealed that the samples of group (i) containa small fraction of the invisible Ag+ form, while the majorityof Ag presents in pyrrhotite crystals as Ag & DEG;. The samples ofgroup (ii) at a temperature of 750 & DEG;C contain the invisible formof Ag-Ag-2 S+-like clusters. The decreasein temperature leads to the partial decomposition of the invisibleform and the appearance of the second (minor) form, Ag-bearing submicroninclusions. The predicted concentrations of Ag in pyrrhotite coincidewith those from natural samples of various origins.