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Cobalt rich pickeringite from the Northern Slanské Mts, Prešov Region, Slovakia
Category Sulfate mineral
(repeating unit)
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
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 [1][2][3][4]

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.[1]

It was first described in 1844 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).[2]

Pickeringite in the Natural History Museum