Mercury(II) Sulfide
Properties
| State | Solid |
| Color | Red (alpha, cinnabar) or black (beta, metacinnabar) |
| Solubility | Practically insoluble in water (Ksp ~ 10^-52) |
| Melting Point | 583 °C (alpha sublimes; beta converts to alpha) |
| Boiling Point | Sublimes around 583 °C |
About Mercury(II) Sulfide
Mercury(II) sulfide is the same compound viewed two ways: as cinnabar, the brilliant scarlet ore that was the source of basically all elemental mercury for two millennia and the pigment behind every red lacquer panel in the Forbidden City and every red robe in a Titian portrait — and as a textbook of mineral polymorphism. The hexagonal α-HgS (cinnabar) is thermodynamically stable, scarlet-red, and a direct-bandgap semiconductor at ~2.1 eV. The cubic β-HgS (metacinnabar) is metastable, black, has a narrower bandgap, and converts back to α on grinding or mild heating. The Ksp for HgS in water is around 10⁻⁵² — for context, that means the equilibrium dissolved [Hg²⁺] in a saturated HgS solution is roughly 10⁻²⁶ M, which is why HgS is the form mercury naturally settles into in anoxic lake sediments and why it stays geologically immobile until sulfate-reducing bacteria methylate it. The Spanish Almadén mine in Castilla-La Mancha worked the cinnabar deposit from Roman times until 2003 (a 2000-year run) and supplied something like a third of all mercury ever produced. Roasting cinnabar in air — HgS + O2 → Hg + SO2 — is how Pliny the Elder described mercury production in 77 AD, and it is essentially how it was still done industrially in the 20th century. The 2013 Minamata Convention on Mercury has now choked off most primary mining, and current mercury supply comes mostly from byproduct recovery at gold and zinc operations.
Where you'll encounter it
If you have ever seen a Pompeii fresco in person, the deep red walls in the Villa of the Mysteries are cinnabar pigment ground from HgS ore — the Roman painters knew the color was poisonous and reserved it for the wealthiest commissions. In a chemistry teaching lab, HgS shows up in qualitative analysis: add H2S to a Hg²⁺ solution and you get a black precipitate of β-HgS (metacinnabar), which slowly turns red on standing or warming. Geochemists who study mercury contamination in the Sudbury basin or the Idaho Silver Valley track HgS in lake-sediment cores as the relatively-immobile mercury sink — methylmercury production by Desulfovibrio bacteria converts a small fraction back into the bioavailable, neurotoxic form that biomagnifies up the food chain into walleye and loons.
Common Uses
- Historical primary ore for mercury production via roasting (HgS + O2 → Hg + SO2)
- Vermillion red pigment in oil painting, lacquerware, and historic frescoes (now obsolete)
- Traditional Chinese and Ayurvedic medicine ingredient (controversial, restricted)
- Black β-HgS precipitate as Hg²⁺ identifier in qualitative inorganic analysis
- Sediment-mercury reference compound in geochemistry and aquatic Hg-cycling research
Safety Information
GHS: Reproductive toxicant (Category 1B), Specific target organ toxicity (repeated exposure - CNS and kidneys, Category 1), Aquatic acute and chronic hazard (Category 1). Acute toxicity from intact HgS is unusually low because the extreme insolubility (Ksp ~10⁻⁵²) means almost no Hg²⁺ is bioavailable on swallowing. The danger is conversion: stomach acid plus dietary chloride, prolonged exposure to oxidizing agents, or intestinal microbiota can liberate Hg²⁺, and sulfate-reducing bacteria in sediments produce highly toxic methylmercury. OSHA PEL 0.1 mg/m3 (as Hg). Roasting cinnabar releases elemental Hg vapor and SO2 — never grind, heat, or roast in any quantity outside an industrial scrubber. Dispose as hazardous waste under Minamata Convention controls.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.