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Calomel, Terlinguaite-222734.jpg
Amber calomel crystals and bright yellow terlinguaite on gossan matrix, 3 mm. across
Category Halide mineral
(repeating unit)
Strunz classification 3.AA.30
Crystal system Tetragonal
Crystal class Ditetragonal dipyramidal 4/mmm (4/m 2/m 2/m) -
Unit cell a = 4.4795(5) Å, c = 10.9054(9) Å; Z=4
Color Colorless, white, grayish, yellowish white, yellowish grey to ash-grey, brown
Crystal habit

Crystals commonly tabular to prismatic, equant pyramidal; common as drusy crusts,

earthy, massive.
Twinning Contact and penetration twins on {112},
Cleavage Good on {110}, uneven to imperfect on {011}
Fracture Conchoidal
Tenacity Sectile
Mohs scale hardness 1.5
Luster Adamantine
Diaphaneity Transparent to translucent
Specific gravity 7.5
Optical properties Uniaxial (+)
Refractive index nω = 1.973 nε = 2.656
Ultraviolet fluorescence Brick-red under UV
References [1][2][3]

Calomel is a mercury chloride mineral with formula (Hg2)2+Cl2 (see mercury(I) chloride). The name derives from Greek kalos (beautiful) and melos (black) because it turns black on reaction with ammonia. This was known to alchemists.[2]

Calomel occurs as a secondary mineral which forms as an alteration product in mercury deposits. It occurs with native mercury, amalgam, cinnabar, mercurian tetrahedrite, eglestonite, terlinguaite, montroydite, kleinite, moschelite, kadyrelite, kuzminite, chursinite, kelyanite, calcite, limonite and various clay minerals.[1]

The type locality is Moschellandsburg, Alsenz-Obermoschel, Rhineland-Palatinate, Germany.[2]


Calomel is used as the interface between metallic mercury and a chloride solution in a saturated calomel electrode, which is used in electrochemistry to measure pH and electrical potentials in solutions, In most electrochemical measurements, it is necessary to keep one of the electrodes in an electrochemical cell at a constant potential. This so-called reference electrode allows control of the potential of a working electrode.

(PDF) Reference Electrodes. Available from: [accessed Jul 09 2018]. [1] [4]



Calomel is a laxative and once was a common medicine, especially on the American frontier. It fell out of use at the end of the 19th century due to its toxicity. One victim was Alvin Smith, the eldest brother of Joseph Smith, founder of the Latter Day Saint movement.[5]


  1. ^ a b The Handbook of Mineralogy
  2. ^ a b c Calomel on Mindat
  3. ^ Calomel on Webmin
  4. ^ Kahlert, Heike (2010-09-01), "Reference Electrodes", Electroanalytical Methods, ISBN 978-3-642-02914-1. Springer-Verlag Berlin Heidelberg, 2010, p. 291, pp. 291–308, retrieved 2018-07-10 
  5. ^ Schmid, Jennifer. "Beautiful Black Poison". Weston A. Price Foundation. Retrieved 2017-10-05.