calomel

{{Infobox mineral | name = Calomel | boxwidth = | boxbgcolor = | image = Calomel, Terlinguaite-222734.jpg | imagesize = 260px | alt = | caption = Amber calomel crystals and bright yellow terlinguaite on gossan matrix, 3 mm. across | category = Halide mineral | formula = (Hg2)2+Cl2 | IMAsymbol=Clo | strunz = 3.AA.30 | dana = | system = Tetragonal | class = Ditetragonal dipyramidal 4/mmm (4/m 2/m 2/m) - | symmetry = | unit cell = a = 4.4795(5) Å, c = 10.9054(9) Å; Z=4 | color = Colorless, white, grayish, yellowish white, yellowish grey to ash-grey, brown | 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 = 1.5 | luster = Adamantine | streak = | diaphaneity = Transparent to translucent | gravity = 7.5 | polish = | opticalprop = Uniaxial (+); very high relief | refractive = nω = 1.973 nε = 2.656 | birefringence = δ = 0.683 | pleochroism = Weak, E > O | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence = Brick-red under UV | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = | references = }} 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.

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