Calomel

Calomel
Almost colorless and transparent calomel crystals from the "Mariposa Mine", Terlingua District, Brewster County , Texas, USA
General and classification
chemical formula	Hg 2 Cl 2
Mineral class (and possibly department)	Halides
System no. to Strunz and to Dana	3.AA.30 ( 8th edition : III / A.05) 01/09/08/01
Crystallographic Data
Crystal system	tetragonal
Crystal class ; symbol	ditetragonal-dipyramidal; 4 / m  2 / m  2 / m
Space group	I 4 / mmm (No. 139)
Lattice parameters	a  = 4.45  Å ; c  = 10.89 Å
Formula units	Z  = 2
Physical Properties
Mohs hardness	1.5 to 2
Density (g / cm 3 )	7.15 to 7.23
Cleavage	indistinct after {100} and {011}
Break ; Tenacity	shell-like
colour	colorless, white, white-yellow to gray-yellow, brown
Line color	White
transparency	transparent to translucent
shine	Diamond luster
Crystal optics
Refractive indices	n ω  = 1.973 n ε  = 2.656
Birefringence	δ = 0.683
Optical character	uniaxial positive

Calomel (also calomel ), outdated also known as horn mercury or under its chemical name mercury (I) chloride , is a rarely occurring mineral from the mineral class of halides . It crystallizes in the tetragonal crystal system with the chemical composition Hg ₂ Cl ₂ and develops either small, transparent to translucent, extensive crystals with a prismatic-tabular crystal habit and diamond luster or crust-shaped or earthy mineral aggregates .

Etymology and history

Kalomel was first mentioned in the records of 1608 by Beguin and 1609 by Oswald Croll . However, it is said to have been known to the ancient Tibetans. The mineral was not scientifically described until 1612 by Théodore Turquet de Mayerne († 1655), who gave the mineral the name "beautiful black". The name is a combination of the ancient Greek words καλός kalos for "beautiful" and μέλας melas for "black", according to legend, inspired by his black slave who knew how to prepare preparations of this substance well.

In fact, the name comes from the fact that the mercury (I) in calomel easily disproportionates into elemental mercury and mercury (II) . The mercury that forms is finely divided and thus ensures the black color. The disproportionation can be done by light or in aqueous solutions by raising the pH. For example, when ammonia solution is poured over calomel, finely divided black mercury and white mercury (II) amido chloride precipitate :

${\displaystyle \mathrm {Hg_{2}Cl_{2}+2\ NH_{3}\longrightarrow Hg+[Hg(NH_{2})]Cl\downarrow +\ NH_{4}Cl} }$

Calomel reaction with ammonia

The type locality is the Moschellandsberg near Obermoschel (Rhineland-Palatinate).

classification

In the now outdated, but still in use 8th edition of the mineral classification according to Strunz , the calomel belonged to the mineral class of "halides" and there to the department of "simple halides", where it formed an independent group together with kuzminite and moschelite .

The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), also assigns calomel to the “halides” class and there to the “simple halides without H ₂ O”. However, this section is further subdivided according to the molar ratio of cations (M) to anions (X), so that the mineral can be found according to its composition in the sub-section "M: X = 1: 1 and 2: 3", where it can also be found together with Kuzminit and Moschelit the "calomel group" named after him with the system no. 3.AA.30 forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns calomel to the class of "halides" and there to the department of the same name. Here he is also the namesake of the "calomel group" with the system no. 09.01.08 and the other members Kuzminit and Moschelit can be found in the subsection of " Anhydrous and hydrous halides with the formula AX ".

Crystal structure

Calomel crystallizes in the tetragonal crystal system in the space group I 4 / mmm (space group no. 139) with lattice parameters a = 4.45  Å and c = 10.89 Å as well as two formula units per unit cell .

properties

Yellow, earthy calomel from the "Mariposa Mine", Texas, USA (field of view: 5 mm)

Pure calomel is colorless. However, it can be white, white-yellow to gray-yellow or brown due to impurities or admixtures of non-formula ions , which gradually darken upon prolonged contact with air. Its optical dispersion is similar to that of diamond and its birefringence by far exceeds that of calcite , which is known for this property .

Compared with salt and nitric calomel is relatively insensitive in aqua regia but is soluble. If treated with soda solution , ammonia solution or other alkaline solutions, metallic mercury precipitates .

When heated to 400 ° C, the mineral changes directly from the solid to the gaseous state .

Education and Locations

Calomel on cinnabar from the Almadén mine , Spain

As a secondary mineral, calomel is formed through weathering of primary mercury minerals .

As a rare mineral formation, calomel could only be detected at a few sites. So far (as of 2011) around 80 sites are known. In addition to its type locality in Moschellandsberg, the mineral was also found in Germany in the “Daimbacher Hof” (formerly “Alte Grube” in Daimbach) near Mörsfeld , on Potzberg , in the “Frischer Mut” mine near Stahlberg and in the “Christiansglück” mine on Königsberg near Wolfstein in Rhineland-Palatinate.

Other sites include the “Chatsworth Mine” near Grassington in England , the “La Coipa Mine” near Diego de Almagro in Chile, the “Guilaizhuang Mine” near Pingyi in China, the Hérault department in France, and San Quirico in the Parma region and the “Levigliani Mine” near Stazzema in Italy, the “Ainoura Mine” in the Japanese prefecture of Nagasaki , at several locations in the Alai Mountains in the Kyrgyz region of Osh , in some regions of Mexico ; on the Kelyana river in the northern Muja Mountains and on the Ujuk in eastern Siberia and on the Mutnovskoe on Kamchatka in Russia's Far East, the “ Avala Mine” in Serbia, at several locations in the region around Košice in Slovakia, near Almería and Almadén in Spain, Neřežín in Czech Bohemia as well as in several regions of the United States of America (US).

use

Larger, mineable deposits of calomel are not known. For this reason it is of minor interest as mercury ore. Applications of the mineral are more of historical interest. Historical, medical applications can be found under → Mercury (I) chloride .

See also

• List of minerals
• Calomel electrode

literature

• Petr Korbel, Milan Novák: Mineral Encyclopedia (=  Villager Nature ). Nebel Verlag, Eggolsheim 2002, ISBN 978-3-89555-076-8 , p. 72 .
• Friedrich Klockmann : Klockmann's textbook of mineralogy . Ed .: Paul Ramdohr , Hugo Strunz . 16th edition. Enke, Stuttgart 1978, ISBN 3-432-82986-8 , pp. 487 (first edition: 1891).

Web links

Commons : Calomel  - collection of images, videos and audio files

• Mineral Atlas: Calomel (Wiki)

Individual evidence

1. ↑ ^{a b c d} Webmineral - Calomel
2. ↑ Calomel , In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF 60.8 kB )
3. ↑ Mindat - Calomel
4. ↑ Elias Altschul: Real Lexicon for homeopathic medicine, therapy, etc. Pharmaceutical preparation (p. 225, 226)
5. ↑ archive.org - Full text of Notes and Queries (1874)
6. ^ Entry on precipitates. In: Römpp Online . Georg Thieme Verlag, accessed on May 23, 2014.
7. ↑ Mindat - Number of localities for calomel
8. ↑ List of localities for calomel in the Mineralienatlas and Mindat