Calomel

Calomel
Amber calomel crystals and bright yellow terlinguaite on gossan matrix, 3 mm. across
General
Category	Halide mineral
Formula .mw-parser-output .nobold{font-weight:normal} (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	[1][2][3]

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]

Electrochemistry

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]

Medicine

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]

References

1. 
   ^ ^ab The Handbook of Mineralogy
   


2. 
   ^ ^abc 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.mw-parser-output cite.citation{font-style:inherit}.mw-parser-output .citation q{quotes:"""""""'""'"}.mw-parser-output .citation .cs1-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .citation .cs1-lock-limited a,.mw-parser-output .citation .cs1-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .citation .cs1-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/4/4c/Wikisource-logo.svg/12px-Wikisource-logo.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-maint{display:none;color:#33aa33;margin-left:0.3em}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}. 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.
   

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