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Calomel, Terlinguaite-222734.jpg
Amber calomel crystals and bright yellow terlinguaite on gossan matrix, 3 mm. across
CategoryHalide mineral
(repeating unit)
Strunz classification3.AA.30
Crystal systemTetragonal
Crystal classDitetragonal dipyramidal 4/mmm (4/m 2/m 2/m) -
Unit cella = 4.4795(5) Å, c = 10.9054(9) Å; Z=4
ColorColorless, white, grayish, yellowish white, yellowish grey to ash-grey, brown
Crystal habitCrystals commonly tabular to prismatic, equant pyramidal; common as drusy crusts, earthy, massive.
TwinningContact and penetration twins on {112},
CleavageGood on {110}, uneven to imperfect on {011}
Mohs scale hardness1.5
DiaphaneityTransparent to translucent
Specific gravity7.5
Optical propertiesUniaxial (+)
Refractive indexnω = 1.973 nε = 2.656
Ultraviolet fluorescenceBrick-red under UV

Calomel is a mercury chloride mineral with formula Hg2Cl2 (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.[4]


Calomel was a widespread and popular medicine for administration to infants as a purgative to treat intestinal worms and "clear out noxious matter" but was used indiscriminately for a great number of ailments. It is tasteless and, mixed with a sweetener, was readily taken. Fumigation tents to supply calomel, heated on a metal plate, as a sublimate within children's lungs were a later method of delivery. As the mercury it contained had the effect of softening the gums, it was made the principle constituent of teething powders, until the mid-twentieth century.[5]

The compound 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 Church of Jesus Christ of Latter-day Saints.[6]


  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, Springer-Verlag Berlin Heidelberg, pp. 291–308, doi:10.1007/978-3-642-02915-8_15, ISBN 978-3-642-02914-1, retrieved 2018-07-10. PDF available.
  5. ^ Swiderski, Richard M. (2009). Calomel in America : mercurial panacea, war, song and ghosts. Boca Raton, FA: BrownWalker Press. pp. 37–9. ISBN 978-1-59942-467-5.
  6. ^ Schmid, Jennifer. "Beautiful Black Poison". Weston A. Price Foundation. Retrieved 2017-10-05.