Sphalerite-wurtzite equilibria and stoichiometry

Evidence from natural occurrences and from published syntheses indicates that the sphalerite-wurtzite inversion is not an invariant reaction at 1020°C, 1 atm but is a univariant function of fs2 and temperature. Single crystals of zinc sulfide, grown in aqueous NaOH solutions which were selected to control fs2, have shown that a univariant boundary exists between sphalerite and wurtzite near 500 atm from 465 ± 4°C to 517 ± 2°C over a corresponding calculated fs2 range of 10−9.5 to 10−8.7 atm. Direct determination of fs2 for coexisting sphalerite and wurtzite, made by passing H2 + H2S mixtures over ZnS powder, gave fs2 of 10−5 atm at 890°C, between 10−5.5 and 10−6.4 atm at 800°C, and between 10−6.5 and 10−8.5 atm at 700°C. The fs2 -dependence of this phase change demonstrates that wurtzite is sulfur-deficient relative to sphalerite. Nonstoichiometry in zinc sulfide is also indicated by its color and by published luminescence studies, electrical measurements, and chemical analyses. Electrical measurements show the defects to be zinc vacancies in sphalerite and sulfur vacancies in wurtzite. The combined range of nonstoichiometry in ‘ZnS’ is on the order of 0.9 at per cent. The hypothesis that wurtzite might be formed metastably at low temperatures due to oxygen substitution for sulfur is untenable. No oxygen was detected by cell-edge measurements of sphalerite or wurtzite from the hydrothermal experiments. In the gas-mixing experiments in which wurtzite was produced from sphalerite below 900°C, oxygen was not present. Wurtzite is thermodynamically stable at lower fs2 than sphalerite. Above about 250°C, this stability field lies well outside of the normal sulfidation state encountered in ore-forming environments; however, at lower temperatures wurtzite may be deposited in highly reducing environments. Because it is stable throughout the geologically most important pH range within 2 or 3 units of neutrality, highly acid solutions are not necessary for its precipitation as advocated by Allen et al. (1914).