Referenza:
▪ Sack, R.O., Lyubimtseva, N.G., Bortnikov, N.S., Anikina, E.Yu., Borisovsky, S.E. (2022): Sulfur vacancies in fahlores from the Ag-Pb–Zn Mangazeyskoye ore deposit (Sakha, Russia). Contributions to Mineralogy and Petrology, 177, 82.
Abstract:
Fahlores from the epithermal Mangazeyskoye Ag-Pb–Zn ore deposit (Sakha, Russia) typically occur in rhythmically zoned crystals. In contrast to typical (Ag,Cu)10(Fe,Zn)2Sb4S13 fahlores, up to about 2.6 of the 13 sulfur atoms in the formula unit may be vacant and many of them have Ag/(Ag + Cu) ratios substantially above those established for (Ag,Cu)10(Fe,Zn)2Sb4S13 fahlores coexisting with miargyrite, pyrargyrite, and sphalerite by experimental, field, and theoretical investigations. The charge balance inequalities consequent on the removal of sulfur are compensated by metallization of the nominally Ag+ and Cu+ ions in the octahedral metal clusters that are formed by the removal of S2− from the octahedral site at the center and corners of the unit cell of fahlore (space group I-43m, Z = 2). The development of these vacancies leads to an effective ionic charge of [(Ag,Cu)(6−2∗vac)+6] for these octahedral clusters, resulting in an intrinsic limit on sulfur vacancies of 3 per formula unit (pfu). The increases in the Ag/(Ag + Cu) ratio of some of the Mangazeyskoye fahlores relative to those defined for (Ag,Cu)10(Fe,Zn)2Sb4S13 fahlores are readily attributed to the lowering of the activities of Ag10Zn2Sb4S13 and Ag10Fe2Sb4S13 components in fahlore by sulfur vacancies. This results in expansion of the field of fahlore stability relative to that given by the fahlore breakdown reaction Ag10Zn2Sb4S13 (fahlore) = AgSbS2 (miargyrite) + 3 Ag3SbS3 (pyrargyrite) + 2 ZnS (sphalerite) for (Ag,Cu)10(Fe,Zn)2Sb4S13 fahlores. A preliminary analysis demonstrates that the increased Ag/(Ag + Cu) ratios observed in the Mangazeyskoye fahlores relative to those defined for (Ag,Cu)10(Fe,Zn)2Sb4S13 fahlores are due to sulfur vacancies being almost exclusively in the octahedral sulfur site where the vacancy for sulfur substitution appears to be approximately ideal. However, it appears that a maximum in Ag/(Ag + Cu) ratios is reached around one pfu, and, to a first approximation, does not appear to increase with further increase in contents. This latter inference suggests that the for S (◻(S)-1) substitution on tetrahedral sulfur sites may be highly nonideal. When properly calibrated for sulfur fugacity based on experimental, spectroscopic, and theoretical studies, reactions such as Ag10Zn2Sb4S13=Ag10Zn2Sb4S12□OCT+1/2 S2 and Ag10Zn2Sb4(S11,□)TETS=Ag10Zn2Sb4S12□OCT will afford fahlore to serve as a petrogenetic indicator of sulfur fugacity in low-sulfidation environments. Until such calibrations are developed, the fahlore Ag/(Ag + Cu) ratios may nonetheless be used to define relative sulfur fugacities in rhythmically zoned fahlores from ore deposits such as Mangazeyskoye.
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Sulfur vacancies in fahlores from the Ag-Pb–Zn Mangazeyskoye ore deposit (Sakha, Russia)
- Marco E. Ciriotti
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Sulfur vacancies in fahlores from the Ag-Pb–Zn Mangazeyskoye ore deposit (Sakha, Russia)
Marco E. Ciriotti
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