Calomel

Calomel
Amber calomel crystals and bright yellow terlinguaite on gossan matrix, 3 mm. across
General
Category	Halide mineral
Formula (repeating unit)	(Hg2)2+Cl2
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
Identification
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

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) from alchemists reaction with ammonia.

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.

The type locality is Landsberg, Alsenz-Obermoschel, Rhineland-Palatinate, Germany.

Medicine

Calomel is a laxative and once was a common medicine, especially on the American frontier. It fell out of use 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.