Calomel Locality: Mariposa Mine (California Mountain Mine), Terlingua District, Brewster County , Texas , USA ( Locality at mindat.org ) Size: 3.0 x 2.5 x 1.0 cm. A rich, nearly solid mass of gemmy calomel crystals to 6mm with minor eglestonite and cinnabar, from this important and classic locale for the species. They turn opaque in sunlight as you normally see them, so lustrous crystals of this quality are most uncommon. — Photo: Robert M. Lavinsky · CC BY-SA 3.0
Calomel
Mercury(I) chloride — a mineral once dosed as medicine for centuries despite routinely poisoning patients with mercury.
Intro adapted from Wikipedia (CC BY-SA).
Calomel is an IMA-recognized mineral species in the halide class, with the chemical formula HgCl.
It crystallizes in the tetragonal crystal system (space group I4/mmm).
Key properties: Mohs hardness 1.5; specific gravity 7.15; Pale yellowish white streak; Adamantine luster; optically uniaxial.
IMA symbol Clo, grandfathered status — known before the IMA’s founding.
An uncommon secondary mineral formed through the alteration of other mercury minerals.
It is found associated with Mercury, amalgam, cinnabar, mercurian tetrahedrite, eglestonite, terlinguaite, montroydite, kleinite, moschelite, kadyrelite, kuzminite, chursinite, kelyanite.
Of uncertain origin; possibly from the Greek for beautiful and honey.