Calomel
Calomel is a mercury chloride mineral with formula Hg2Cl2 (see mercury(I) chloride). The name derives from Greek kalos (beautiful) and melas (black) because it turns black on reaction with ammonia. This was known to alchemists.
| Calomel | |
|---|---|
Amber calomel crystals and bright yellow terlinguaite on gossan matrix, 3 mm. across | |
| General | |
| Category | Halide mineral |
| Formula (repeating unit) | (Hg2)2+Cl2 |
| IMA symbol | Clo |
| 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 (+); very high relief |
| Refractive index | nω = 1.973 nε = 2.656 |
| Birefringence | δ = 0.683 |
| Pleochroism | Weak, E > O |
| Ultraviolet fluorescence | Brick-red under UV |
| References | |
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 Moschellandsburg, Alsenz-Obermoschel, Rhineland-Palatinate, Germany.
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