Priestley Toulmin
P. B. Barton Jr.
1966
<p><span>The equilibrium diagram for the </span>Fe<span>-</span>Zn<span>-</span>S<span> </span>System<span> has been worked out </span>in<span> détail from 580° to 850° C. Previous work on this </span>System<span> is proven to be seriously </span>in<span> error and températures heretofore estimated from the "</span>sphalerite<span> geothermometer" are without a sound quantitative foundation. </span>Sphalerite<span> solid solutions lie essentially along the FeS-ZnS join. Neither pyrite nor pyrrhotite takes up appréciable amounts of zinc. The FeS-ZnS binary solvus (équivalent to the assemblage </span>sphalerite<span> + stoichi-ometric FeS) is very steep, passing through 56 mole percent FeS at 850° C and 52 mole percent FeS at 580° C. It is not useful geologically because of the rarity of stoichiometric FeS as a primary minerai </span>in<span> </span>sphalerite<span>-bearing assemblages; however, the relatively large rôle played by total pressure on the binary solvus might permit the iron content of </span>sphalerite<span> to be useful </span>in<span> determining the pressure of équilibration of some météorites. Of greater géologie interest is the </span>sphalerite<span> + pyrite + pyrrhotite assemblage </span>in<span> which the composition of </span>sphalerite<span> changes from 13 mole percent FeS at 742° C to 19 percent at 580° C. Unfortunately, this equilibrium cannot be quantitatively extrapolated to lower températures from the presently available data and therefore quantitative geothermom-etry of pyrite + pyrrhotite + </span>sphalerite<span> assemblages is not feasible at présent. Thermochemical calculations prove that the iron content of </span>sphalerite<span> </span>in<span> pyrite + </span>sphalerite<span> assemblages is very sensitive to variations </span>in<span> température and fugacity of </span>S<span>2; this relationship provides an explanation of growth-zoned </span>sphalerite<span> as the resuit of fluctuations of sul-fur fugacity, and, </span>in<span> combination with other sulfide assemblages that define isobarically univariant </span>relations<span> between température and sulfur fugacity, forms a potential basis for quantitative geothermometry. </span></p>
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10.2113/gsecongeo.61.5.815
en
Society of Economic Geologists
Phase relations involving sphalerite in the Fe-Zn-S system
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