Botallackite

Botallackite, chemical formula Cu₂(OH)₃Cl is a secondary copper mineral, named for its type locality at the Botallack Mine in St Just in Penwith, Cornwall. It is polymorphous with atacamite, paratacamite and clinoatacamite [Wikidata].^[2]

Botallackite
Botallackite from the Levant Mine, Cornwall
General
Category	Halide minerals
Formula	Cu2(OH)3Cl
IMA symbol	Blk[1]
Strunz classification	3.DA.10b
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/m
Unit cell	a = 5.717 Å, b = 6.126 Å, c = 5.636 Å; β = 93.07°; Z = 2
Identification
Colour	Shades of green
Crystal habit	Platy interlaced crystal crusts
Cleavage	{100} Perfect
Mohs scale hardness	Soft
Diaphaneity	Transparent to translucent
Specific gravity	3.6
Optical properties	Biaxial (+)
Refractive index	nα= 1.775, nβ= 1.800, nγ= 1.846
Birefringence	δ = 0.071
Pleochroism	Weak – blue green shades
Dispersion	r > v, strong
References	[2][3][4]

Botallackite crystallises in the monoclinic crystal system. It is mountain-green to green in colour, with one distinct to good cleavage.^[2]

Discovery and occurrence

It was first described in 1865 for an occurrence in the Botallack mine, Cornwall, England, and named for the type locality.^[2]

Botallackite forms in copper deposits exposed to weathering and salt water.^[2] It is reported from black smoker deposits due to reaction of primary sulfide minerals with seawater. It also occurs on copper bearing slag exposed to seawater. Minerals associated with botallackite include atacamite, paratacamite, brochantite, connellite and gypsum.^[3]