Phase relations were determined by monitoring the bulk resistance of Hg1−xS as a function of temperature, composition, and fugacity of sulfur, as well as by appearance–of–phase and DTA runs. Six invariant points involving vapor, cinnabar (cn), metacinnabar (mcn), and hypercinnabar (hcn) were found: (1) cn + mcn + Hg; (2) cn + mcn + S; (3) mcn + hcn + Hg; (4) mcn + hcn + S; (5) hcn + Hg + liquid; and (6) hcn + S + liquid, respectively at: (1) 345 ± 2°C, Hg1.00S(cn-mcn); (2) 315 ± 3°, Hg0.969S(cn)—Hg0.864S(mcn); (3) 481 ± 2°, Hg0.969S(mcn)—Hg0.967S(hcn); (4) 470 ± 2°, Hg0.946S(mcn)—Hg0.945S(hcn); (5) 804 ± 3°, Hg0.966S(hcn); and (6) 788 ± 2°, Hg0.959S(hcn). Hg0.953S(hcn) melted eongruently at 820 ± 3°C.

The crystallographic properties, optical properties, and density vary as a function of composition. Density measurements show that nonstoichiometry is caused by mercury vacancies and interstitial sulfur.

For the reaction 2Hg(l) + S2(g) = 2HgS(s), the ΔGR for stoichiometric cinnabar was found to equal −54,000 + 43.28T ± 500 calories. ΔGf,298.150 was found to be 11.9 ± 0.05 kcal and 11.3 ± 0.8 kcal for cinnabar and metacinnabar, respectively.

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