Calomel

Calomel
Amber calomel crystals and bright yellow terlinguaite on gossan matrix, 3 mm. across
General
CategoryHalide mineral
Formula
(repeating unit)
(Hg2)2+Cl2
IMA symbolClo[1]
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
Identification
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}
FractureConchoidal
TenacitySectile
Mohs scale hardness1.5
LusterAdamantine
DiaphaneityTransparent to translucent
Specific gravity7.5
Optical propertiesUniaxial (+); very high relief
Refractive indexnω = 1.973 nε = 2.656
Birefringenceδ = 0.683
PleochroismWeak, E > O
Ultraviolet fluorescenceBrick-red under UV
References[2][3][4]

Calomel is a mercury chloride mineral with formula Hg2Cl2 (see mercury(I) chloride). It was used as a medicine from the 16th to early 20th century, despite frequently causing mercury poisoning in patients.[5]

The name derives from Greek kalos (beautiful) and melas (black) because it turns black on reaction with ammonia. This was known to alchemists.[3]

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.[2]

The type locality is Moschellandsburg, Alsenz-Obermoschel, Rhineland-Palatinate, Germany.[3]

  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b The Handbook of Mineralogy
  3. ^ a b c Calomel on Mindat
  4. ^ Calomel on Webmin
  5. ^ Davis, L. E (2000-07-01). "Unregulated potions still cause mercury poisoning". Western Journal of Medicine. 173 (1): 19. doi:10.1136/ewjm.173.1.19. PMC 1070962. PMID 10903282.