Calomel

Chemical formula: HgCl

Calomel is mercury(I) chloride, a rare and historically important mineral that darkens and decomposes when exposed to light.

## Characteristics Calomel is a rare halide mineral, being the natural form of mercury(I) chloride. It forms well-developed, tabular or prismatic crystals, often with an adamantine or vitreous luster. It usually occurs as coatings and crusts. It is very dense and heavy. A characteristic feature of calomel is its instability – under ultraviolet light, it darkens, and over time, it can decompose into liquid mercury and mercury(II) chloride. ## Physical Properties Calomel crystals are soft and sectile; they can be cut with a knife. The mineral has a hardness of 1.5-2 on the Mohs scale and a very high density, approximately 7.15 g/cm³. It exhibits strong birefringence, one of the highest known among minerals. The luster is most often adamantine, but can also be vitreous or earthy in the case of aggregates. ## Colors and Varieties Calomel is usually colorless, white, gray, yellowish, or brown. The coloration often results from the presence of inclusions of other minerals. No named varieties of this mineral are distinguished. ## History and Name The name "calomel" comes from the Greek words *kalos* (καλός) meaning "beautiful" and *melas* (μέλας) meaning "black". This name, seemingly contradictory to its light color, probably refers to its reaction with ammonia, which produces a black precipitate, or to the fact that it darkens in light. The mineral was known and used in medicine for centuries before it was formally described as a mineral species. ## Uses Historically, synthetic calomel was widely used in medicine as a purgative and disinfectant, but its toxicity led to the cessation of these practices. Currently, due to its unique optical properties (high birefringence), it is being studied for applications in specialized optical devices, such as polarizers. For collectors, it is primarily an interesting and rare specimen due to its history and chemical properties.

Properties

Mohs hardness
1.5-2
Luster
Adamantine
Streak
Pale yellowish white
Density
7.15
Cleavage
Good on {101}, poor on {001}
Fracture
Conchoidal
Transparency
Transparent to translucent
Crystal system
Tetragonal

Diagnostic features

## Identification Key diagnostic features of calomel include its very high density, low hardness (it is soft and sectile), adamantine luster, and tetragonal, tabular crystals. The reaction to light (darkening) is also characteristic, though destructive to the specimen. ## Distinguishing from Similar Minerals - **Cinnabar:** Cinnabar (HgS) has an intense red color and streak. Calomel is usually colorless to brown. - **Other Halides (e.g., Halite, Fluorite):** Calomel is many times denser and much softer than most similarly appearing halides. - **Cerussite:** Cerussite (PbCO₃) also has a high luster and density, but is harder (3-3.5) and has a different crystal habit and perfect cleavage. ## Crystal Forms Calomel crystallizes in the tetragonal system, forming characteristic, simple crystals with a tabular, thick-tabular, or short-prismatic habit. It often occurs as crusts, coatings, or earthy aggregates.

Geological environment

## Genesis Calomel is a secondary mineral, forming in the oxidation zones of mercury ore deposits. It forms as a result of weathering and alteration of primary mercury minerals, mainly cinnabar (HgS) and native mercury, in a chloride-rich environment. ## Mineral Associations It most commonly co-occurs with cinnabar, native mercury, eglestonite, terlinguaite, montroydite, calcite, limonite, and chalcedony. ## Localities The most important and classic localities, from which the best calomel specimens originate, are in the United States, in the Terlingua district in Texas. Significant occurrences have also been reported in Serbia (Avala), Spain (Almadén), Mexico (Guadalcázar area), and Germany (Palatinate).

Rarity

Rare

For collectors

## Quality Criteria The most prized calomel specimens are those with well-formed, sharp, and transparent crystals with a distinct adamantine luster. Large, undamaged crystals, especially on a contrasting matrix (e.g., with cinnabar), are exceptionally sought after. Color is also important – specimens with a pure, light yellow or honey color are more attractive than gray or brown ones. Due to the mineral's instability, specimens that show no signs of decomposition (darkening) are much more highly valued. ## Popular Localities The absolute classic and most desired source of calomel for collectors is the Terlingua district in Brewster County, Texas, USA. Specimens from there, often in association with other rare mercury minerals, represent the global standard for this species.

Care and storage

## Cleaning Calomel is extremely sensitive and toxic. Generally, it should not be cleaned. If absolutely necessary, a soft brush can be used to remove dust. Avoid water and any chemicals. ## What to Avoid - **Light:** Absolutely avoid exposure to sunlight and UV radiation, which cause it to darken and decompose. - **Water and Chemicals:** The mineral is sensitive to chemical reagents. Contact with water is not recommended. - **Temperature:** Avoid heating, which can accelerate decomposition and the release of toxic mercury vapors. - **Physical Contact:** Due to its toxicity, direct skin contact should be avoided. Always wash hands after contact. ## Storage Calomel specimens must be stored in complete darkness, in a sealed, airtight container (e.g., a "perky box"), to prevent light exposure and limit the potential release of mercury vapors. The container should be clearly labeled as containing a toxic mineral.

External references

Sources

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