Challacolloite

Challacolloite, KPb₂Cl₅, is a rare halide mineral. It crystallizes in the monoclinic system (with space group P2₁/c^[2]) and occurs as white fumarolic encrustations on lava. It occurs as intergrowths with cotunnite.^[3]

Challacolloite
Honey yellow Challacolloite crystals from the famous locality of La Fossa Crater (Vulcano Island, Aeolian Islands, Sicily, Italy)
General
Category	Minerals
Formula	KPb2Cl5
IMA symbol	Chc[1]
Strunz classification	3.AA.55 (10th)
Dana classification	11.4.2.1
Crystal system	Monoclinic
Crystal class	2/m - Prismatic
Unit cell	878.2 ų
Identification
Formula mass	630.83 gm
Colour	Colourless to white
Crystal habit	Aggregate
Fracture	Brittle, subconchoidal fragments
Mohs scale hardness	2-3
Luster	Adamantine/Greasy
Streak	White
Specific gravity	4.77 (Calculated)
Density	4.77 g/cm3 (Calculated)
Optical properties	Biaxial (+), a=2.004, b=2.01, g=2.024, bire=0.0200
Refractive index	nα = 2.004 nβ = 2.010 nγ = 2.024
2V angle	67° (calculated)

It was first described from a finding at the Challacollo Mine, Iquique, Chile and thereafter identified in specimens from the 1855 Mount Vesuvius eruption and from the Kudryavyi volcano in the Kuriles and also from the Satsuma-Iwojima volcano in Japan.^[4] It was recognized as a valid mineral species by the IMA (International Mineralogical Association) in 2005.^[4]

Artificially grown KPb₂Cl₅ crystals are used for lasers.^[5]