Calomel - Encyclopedia

    Class : Halides
    Subclass : Chlorides
    Crystal system : Tetragonal
    Chemistry : HgCl
    Rarity : Rare

Calomel is a rare chloride found in the superficial oxidation zone of mercury deposits, especially in desert regions. it results from an alteration of cinnabar in an environment rich in chlorides : these can be brought by outcropping evaporites or by saline air if the deposit is close to the sea. In addition to cinnabar, calomel is practically always accompanied eglestonite, another chloride of mercury. It is a mineral whose etymology is uncertain, it can come from the Greek kalos (beautiful) and meli (honey) because of its sweet taste ; or melas (black) because the term calomel once referred to black mercury sulfide. The crystals are rare, often complex, rich in faces, most often square and tabular on {001} or elongated prismatic according to [001]. The mineral is grayish or yellowish-gray, turning black on exposure to light, with a strong adamantine luster. Of low hardness (1 to 2), it is very dense (more than 7), plastic and easily sectile. It fluoresces in a characteristic brick red. It is a very accessory ore of mercury.

Main photo : Calomel from Mariposa Mine, Texas, USA © Robert O. Meyer

Calomel in the World

Several deposits provided quality crystals ; few however exceed the centimeter. The German deposits of Moschellandsberg, the Yugoslavian deposit of Monte Avala, and the Spanish deposit of Almadén provided very beautiful specimens, notably the latter which produced crystals measuring 1 cm. In the United States, Terlingua (Texas) is the most famous, for the abundance and quality of its brilliant crystals of 5 to 8 mm associated with eglestonite. The Mexican mines, in particular that of El Doctor (Zimapan), sometimes showed calomel in abundance.

Photo on the right : Calomel from El Entredicho Mine, Spain © Borja Sainz de Baranda Graf

Calomel in France

This mineral is not present in the French underground.


Twins known on {112}, as contact or interpenetrating twin, often repeated and with irregular and hidden boundaries.

Fakes and treatments

No fake identified for this mineral species.

Hardness : 1.5 to 2
Density : 7.15
Fracture : Conchoidal
Streak : White-yellow

TP : Translucent to transparent
RI : 1.973 to 2.656
Birefringence : 0.683
Optical character : Uniaxial +
Pleochroism : Low
Fluorescence : Brick red

Solubility : Aqua regia

Magnetism : None
Radioactivity : None