Pickeringite
(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 in the Natural History Museum
(same H-M symbol)
c = 6.18 [Å]; β = 95°; Z = 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.[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]
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