HgS adopts two structures, i.e. it is dimorphous. The more stable form is cinnabar, which has a structure akin to that for : each Hg center has two short Hg-S bonds , and four longer Hg---S contacts . The black form of HgS has the zinc blende structure.
Cinnabar is generally found in a massive, granular or earthy form and is bright scarlet to brick-red in color. It occasionally occurs, however, in crystals with a non-metallic adamantine luster. Cinnabar has a rombohedral bravais lattice, and belongs to the hexagonal crystal system, trigonal division. Its crystals grow usually in a massive habit, though they are sometimes . The twinning in cinnabar is distinctive and forms a penetration twin that is ridged with six ridges surrounding the point of a pyramid. It could be thought of as two scalahedral crystals grown together with one crystal going the opposite way of the other crystal. The of cinnabar is 2 - 2.5, and its specific gravity 8.998.
Cinnabar resembles quartz in its symmetry and certain of its optical characteristics. Like quartz, it exhibits birefringence. It has the highest power of any mineral. Its mean for sodium light is 3.08, whereas the index for diamond—a substance of remarkable refraction— is 2.42 and that for GaAs is 3.93.
See List of indices of refraction.
Generally cinnabar occurs as a vein-filling mineral associated with recent and alkaline hot springs.
Cinnabar is found in all localities that yield mercury, notably Almadén ; New Almaden ; Hastings Mine and St. John's Mine, Vallejo, California; Idrija ; ; , near Obermoschel in the ; Ripa, at the foot of the Apuan Alps ; the mountain Avala ; Huancavelica ;Murfreesboro, Arkansas ; and the province of Guizhou in China, where fine crystals have been obtained.
Cinnabar is still being deposited at the present day from the hot waters of Sulphur Bank, in California, and Steamboat Springs, Nevada.
Mining and extraction of mercury
Cinnabar was mined by the Roman Empire both as a pigment and for its mercury content , and it has been the main source of mercury throughout the centuries. Some mines used by the Romans remain active today.
To produce liquid mercury, crushed cinnabar ore is roasted in rotary furnaces. Pure mercury separates from sulfur in this process and easily evaporates. A condensing column is used to collect the liquid mercury, which is most often shipped in iron flasks.
Because of the high toxicity of mercury, both the mining of cinnabar and refining for mercury are hazardous and historic causes of mercury poisoning. In particular, the Romans used convict labor in their mines as a form of death sentence. The Spanish also used shorter term convict labor at the Almadén mines, with a 24% overall fatality rate in one 30 year period.
Abandoned mercury mine processing sites often contain very hazardous waste piles of roasted cinnabar calcines. Water runoff from such sites is a recognized source of ecological damage.
Cinnabar has been used for its color in the new world since the Olmec culture. Cinnabar was often used in royal burial chambers during the peak of Mayan civilization. The red stone was inserted into limestone sarcophagi, both as a decoration and, more importantly, to deter vandals and thieves with its well-known toxicity.
The most popularly known use of cinnabar is in Chinese carved lacquerware, a technique that apparently originated in the Song Dynasty. The danger of mercury poisoning may be reduced in ancient lacquerware by entraining the powdered pigment in lacquer, but could still pose an environmental hazard if the pieces were accidentally destroyed. In the modern jewelry industry, the toxic pigment is replaced by a resin-based polymer that approximates the appearance of pigmented lacquer.
In the Byzantine Empire, the Emperor and certain privileged bishops were allowed the exclusive right of signing their names with ink colored vermilion by the addition of cinnabar.
Other forms of cinnabar
*Hepatic cinnabar is an impure variety from the mines of Idrija in the Carniola region of Slovenia, in which the cinnabar is mixed with and earthy matter.
*Metacinnabarite is a black-colored form of HgS, which crystallizes in the cubic form.
*Synthetic cinnabar is produced by treatment of Hg salts with hydrogen sulfide to precipitate black, synthetic metacinnabarite, which is then heated in water. This conversion is promoted by the presence of sodium sulfide.
*Hypercinnabar, crystallise in the hexagonal form.