HR: 0830h
AN: V51I-0393    [PDF]
TI: Preliminary Experimental Results on Phase Relations in the PbS-AgSbS$_{2}$-AgBiS$_{2}$ Ternary
AU: * Chutas, N I
EM: nichutas@u.washington.edu
AF: Department of Earth and Space Sciences, University of Washington Box 351310, Seattle, WA 98195 United States
AU: Sack, R O
EM: sack@geology.washington.edu
AF: Department of Earth and Space Sciences, University of Washington Box 351310, Seattle, WA 98195 United States
AU: Kress, V C
EM: kress@u.washington.edu
AF: Department of Earth and Space Sciences, University of Washington Box 351310, Seattle, WA 98195 United States
AU: Ghiorso, M S
EM: ghiorso@geosci.uchicago.edu
AF: Department of Geophysical Sciences, The University of Chicago 5734 S. Ellis Ave, Chicago, IL 60637 United States
AB: The presence of significant solid-solution of silver sulfosalts in galena in the PbS (galena)-AgSbS$_{2}$(miargyrite)-AgBiS$_{2}$ (matildite) ternary system suggests that ore in many important silver deposits may have been formed by retrograde unmixing of higher-temperature silver-bearing galena solid solution. We present results of evacuated silica capsule experiments designed to constrain the extent of Ag solid solution in galena and other important phase relations in this system over a range of temperatures relevant to ore formation. Experimentally-based phase diagrams in the AgSbS$_{2}$-PbS, AgSbS$_{2}$-AgBiS$_{2}$, and AgBiS$_{2}$-PbS binaries indicate complete solid solution above $420\deg$C. If these phase diagrams are correct and the topological features can be extended into the supersystem, they would imply that the galena structure (Fm-3m) extends across most or all of the ternary at high temperatures. Below $420\deg$C, a miscibility gap is observed on the AgSbS$_{2}$-PbS binary, approximately midway between the freieslebenite (PbAgSbS$_{3}$) and galena compositions. New experimental data indicate that this miscibility gap on the AgSbS$_{2}$-PbS binary extends well into the ternary. This miscibility gap appears to close at X$_{PbS}$ $>$ 0.60 and X$_{AgBiS2}$ $>$ 0.12 at about $350\deg$C, and X$_{PbS}$ $>$ 0.65 and X$_{AgBiS2}$ $\leq$ 0.06 at $400\deg$C. One experimentally determined tie line at $350\deg$C has equilibrium compositions of about X$_{PbS}$ = 0.823, X$_{AgBiS2}$ = 0.087, X$_{AgSbS2}$ = 0.090 and X$_{PbS}$ = 0.578, X$_{AgBiS2}$ = 0.145, X$_{AgSbS2}$ = 0.277. The presumption that the galena (Fm-3m) structure extends across the AgSbS$_{2}$-PbS binary at temperatures above $380\deg$C has not been established conclusively. If a single Fm-3m phase exits, it is extremely difficult to model the observed off-centered miscibility gap between AgSbS$_{2}$ and PbS using simple regular-solution theory. Alternatively, the observed miscibility gap may separate an Fm-3m phase on the galena side from a related, probably derivative, structure on the AgSbS$_{2}$ side. This second structure may reflect ordering of Ag$^{+}$ and Sb$^{3+}$ (Bi$^{3+}$) on cation sites. The alternate interpretation would imply a phase boundary extending from the miscibility gap possibly to the solidus, and would be expected to have significant influence on phase diagram topology and thermodynamic modeling in this system. Constraints on viable phase relations will be discussed along with preliminary models for relevant phases. Because many candidate symmetry transitions can be expected to be unquenchable, complete solution of this problem may require high-temperature calorimetry and in situ high-temperature x-ray structural refinements. Related issues in other parts of the ternary will also be discussed.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3630 Experimental mineralogy and petrology
DE: 3665 Mineral occurrences and deposits
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting