Horn silver

Chlorargyrite
Bromian chlorargyrite (embolite), Chañarcillo, Copiapó Province, Chile. Size: 5.0 x 4.7 x 1.0 cm.
General
Category	Halide
Formula (repeating unit)	AgCl
Strunz classification	3.AA.15
Crystal system	Isometric
Crystal class	Hexoctahedral (m3m) H-M symbol (4/m 3 2/m)
Space group	Fm3m
Identification
Color	Colorless when fresh; alters to bright chartreuse-green, light yellow, light green, grey, violet-brown on exposure to light
Crystal habit	Massive to columnar
Fracture	Irregular/uneven, sub-conchoidal
Tenacity	Sectile
Mohs scale hardness	1.5 - 2.5
Luster	Adamantine, resinous, waxy
Streak	White
Specific gravity	5.556
Optical properties	Isotropic
Refractive index	n = 2.071
References

Chlorargyrite is the mineral form of silver chloride (AgCl). Chlorargyrite occurs as a secondary mineral phase in the oxidation of silver mineral deposits. It crystallizes in the isometric - hexoctahedral crystal class. Typically massive to columnar in occurrence it also has been found as colorless to variably yellow cubic crystals. The color changes to brown or purple on exposure to light. It is quite soft with a Mohs hardness of 1 to 2 and dense with a specific gravity of 5.55. It is also known as cerargyrite and, when weathered by desert air, as horn silver. Bromian chlorargyrite (or embolite) is also common. Chlorargyrite is water-insoluble.

It occurs associated with native silver, cerussite, iodargyrite, atacamite, malachite, jarosite and various iron–manganese oxides.

It was first described in 1875 for occurrences in the Broken Hill district, New South Wales, Australia. The name is from the Greek, chloros for "pale green" and Latin for silver, argentum.