Pickeringite
Cobalt rich pickeringite from the Northern Slanské Mts, Prešov Region, Slovakia
General
Category	Sulfate mineral
Formula (repeating unit)	MgAl2(SO4)4·22(H2O)
IMA symbol	Pkg[1]
Strunz classification	7.CB.85
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/c
Unit cell	a = 20.8, b = 24.2, c = 6.18 [Å]; β = 95°; Z = 4
Identification
Color	Colorless, white; may be shades of yellow, green, or red due to impurities
Crystal habit	Rarely as acicular crystals, radial or matted aggregates; typically as incrustations and efflorescences
Cleavage	Poor on {010}
Fracture	Conchoidal
Mohs scale hardness	1.5
Luster	Vitreous
Diaphaneity	Semitransparent
Specific gravity	1.73–1.79
Optical properties	Biaxial (−)
Refractive index	nα = 1.476 nβ = 1.480 nγ = 1.483
Birefringence	δ = 0.007
2V angle	Measured: 60°
Solubility	Completely soluble in water
Common impurities	May contain iron, manganese or cobalt
References	[2][3][4][5]

Pickeringite is a magnesium aluminium sulfate mineral with formula MgAl₂(SO₄)₄·22(H₂O). It forms a series with halotrichite.

It forms as an alteration product of pyrite in aluminium rich rocks and in coal seams. It also occurs in pyrite rich hydrothermal ore deposits in arid regions. It forms in fumaroles and in caves. It occurs with kalinite, alunogen, epsomite, melanterite, copiapite and gypsum.^[2]

It was first described in 1844 as reflective for an occurrence in Cerros Pintados, Pampa del Tamarugal, Iquique Province, Tarapacá Region, Chile. It was named for American linguist and philologist John Pickering (1777–1846).^[3]