Metacinnabar

Chemical formula: Hg<sup>2+</sup>S<sup>2-</sup>

Metacinnabar is a black mercury sulfide, a rarer, cubic polymorphic variety of cinnabar, forming characteristic, lustrous crystals.

## Characteristics Metacinnabar is a mercury sulfide and a high-temperature, cubic polymorphic variety of the much more common, trigonal cinnabar. It typically forms small, well-developed tetrahedral crystals, often with modifications of other faces. It also occurs as granular aggregates, coatings, and earthy masses. Its color is uniformly black or grayish-black, and its luster is metallic to submetallic, giving it the appearance of small, shiny metallic particles on the host rock. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 3. Its most characteristic feature is its very high density, approximately 7.65 g/cm³, which makes specimens unusually heavy for their size. It is opaque, and its streak is black. It does not exhibit cleavage, and its fracture is uneven to subconchoidal. ## Colors and Varieties Metacinnabar is always black or steel-gray. It has no color varieties. Its name and existence are by definition linked to cinnabar – both minerals have the same chemical formula (HgS) but different crystal structures. Metacinnabar is the stable form at higher temperatures, while red cinnabar is stable at lower temperatures. ## History and Name The mineral's name, given in 1870 by G.E. Moore, comes from the Greek word "meta" (meaning "after," "beside," or "beyond") and the name "cinnabar," which directly refers to its chemical and polymorphic relationship with cinnabar. It was described based on specimens found at the Redington mine in Napa County, California, USA. ## Uses In places where it occurs in significant quantities together with cinnabar, it can be a subordinate ore of mercury. Due to its rarity and modes of occurrence, its main significance is scientific and for collectors.

Properties

Mohs hardness
3
Luster
Metallic to submetallic
Streak
Black
Density
7.6 - 7.7
Cleavage
None
Fracture
Uneven to subconchoidal
Transparency
Opaque
Crystal system
Isometric

Diagnostic features

## Identification Key diagnostic features of metacinnabar include its black color, black streak, metallic luster, and very high density (the specimen is surprisingly heavy). Its characteristic crystal forms – small, sharp tetrahedra – are also diagnostic. Occurrence in association with cinnabar, native mercury, or within mercury deposits is a strong diagnostic indicator. ## Distinguishing from Similar Minerals Metacinnabar can be confused with other black, metallic minerals: - **Sphalerite (marmatite variety):** Can be black and shiny, but has a much lower density and often exhibits excellent cleavage. - **Stibnite:** Has a similar luster but forms acicular or columnar crystals and has excellent cleavage along one plane. - **Acanthite:** Has a similar color and is also heavy, but usually forms different crystal habits and is sectile (can be cut with a knife). Density (specific gravity) testing is one of the best methods to distinguish metacinnabar from its imitators. ## Crystal Forms It crystallizes in the cubic system, most often forming tetrahedral crystals, sometimes with edges truncated by cube faces or other forms. It also occurs as granular aggregates, massive forms, and as black coatings and crusts on other minerals, especially on cinnabar.

Geological environment

## Genesis Metacinnabar is a hydrothermal mineral, forming under low-temperature and low-pressure conditions. It forms near the Earth's surface, in veins and pockets within sedimentary and volcanic rocks, often in association with hot springs and volcanic activity. It is the polymorphic form of HgS stable at temperatures above 344°C, but it can crystallize metastably at lower temperatures, often alongside cinnabar. ## Mineral Associations It most commonly co-occurs with its polymorph – cinnabar. Other typical associated minerals include native mercury, pyrite, marcasite, stibnite, realgar, orpiment, barite, calcite, chalcedony, and quartz. ## Localities The most famous localities, from which classic specimens originate, are in California, USA (e.g., Mount Diablo, Redington, New Almaden mines). Other important worldwide localities include Idrija in Slovenia, Almadén in Spain, Guizhou and Hunan regions in China, and Huancavelica in Peru.

Rarity

Not very common

For collectors

## Quality Criteria Specimens with sharp, well-formed, lustrous tetrahedral crystals are most highly valued by collectors. The larger and more isolated the crystal, the higher its value. The attractiveness of a specimen is significantly enhanced by its placement on a contrasting matrix, for example, on white quartz, dolomite, or in association with intensely red cinnabar. Purity and lack of damage are crucial. ## Popular Localities Historical localities in California (USA), such as Contra Costa County (Mount Diablo mine) and Napa County (Redington mine), are considered classic and provide the best specimens. Specimens from these locations are the standard for the species and command high prices in the collector's market.

Care and storage

## Cleaning **WARNING: This mineral is toxic.** Avoid creating dust. For dust removal, it is best to use only a soft brush or compressed air from a safe distance. Avoid wet cleaning to prevent the spread of potentially toxic particles. ## What to Avoid Metacinnabar contains mercury and is poisonous if ingested or if its dust is inhaled. Hands must be thoroughly washed after any contact with the mineral. It must not be heated, as it releases highly toxic mercury vapors. Avoid contact with acids. Store away from food and food preparation areas. ## Storage Specimens should be stored in closed, airtight containers (e.g., plastic boxes), away from children, pets, and individuals unaware of the hazard. Each container must be clearly labeled with the mineral's name and a toxicity warning. Storage in a well-ventilated area, separate from the main collection, is recommended.

External references

Sources

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