Formula: MgAl2(SO4)4.22H2O
Hydrated normal sulphate, halotrichite group, forms a series with halotrichite and a partial series with apjohnite
Crystal System: Monoclinic
Specific gravity: 1.73 to 1.79 measured, 1.84 calculated
Hardness: 1½ to 2
Streak: White
Colour: Colourless to white, light yellow, pale pink, orange, grey, colourless in transmitted light
Solubility: Soluble in water, astringent taste.
Common impurities: Mn,Fe,Co

Hydrothermal environments

Pickeringite is a common secondary mineral formed by alteration of pyrite in aluminous rocks or in coal seams, also in the oxidised zone of pyritic hydrothermal mineral deposits, typically in arid regions, typically post-mining; it is also a fumarolic product, and formed in caves. Associated minerals include kalinite, alunogen, epsomite, melanterite, copiapite and gypsum (HOM).


At the Siglo Veinte Mine, Llallagua, Rafael Bustillo, Potosí, Bolivia, pickeringite occurs as a post-mining formation in old galleries, together with halotrichite (Minrec 37.2.148).

At the Smoky River, Peace River, Alberta, Canada, pickeringite occurs as thin veins and incrustations mixed with clay, along steep slumped banks of the stream valleys where marine shale forms the underlying bedrock. The pickeringite is formed by decomposition of the marine shale which contains iron sulphides. The shale is generally impervious to water but along steep river valleys the bedrocks become dislodged through slumping. This movement renders the beds more porous, resulting in oxidation of the sulphides and generation of heat. Apparently the water seeping through the loosened material is vaporised and rises carrying salts dissolved from the shale and precipitating them at or near the surface as a white incrustation (AM 17.401-403).

At Alcaparrosa Mine, Cerritos Bayos, Calama, El Loa Province, Antofagasta, Chile, metavoltine (?) occurs surrounded by a halo of copiapite in veins of pickeringite. The age relationships are metavoltine followed by copiapite with the pickeringite as the youngest mineral. Rostite occurs in tabular crystals associated with quenstedtite in vugs in coquimbite and with pickeringite. Quenstedtite and coquimbite are earlier minerals, and pickeringite is later. However, some pickeringite appears to be earlier (AM 23.669-760).

At the Queténa Mine, Toki Cu deposit, Chuquicamata District, Calama, El Loa Province, Antofagasta, Chile, pickeringite forms snow-white tufts of fibers on massive jarosite, together with chalcanthite and fibroferrite, as one of the last minerals to form (AM 23.669-760).

The type locality is Cerros Pintados, Tamarugal Province, Tarapacá, Chile.

The Huron River Shale Fire, Huron County, Ohio, USA. A surface outcrop of shale located on the Huron River caught fire in September 2009 and burned until March 2011. Fire-generated species that formed include pickeringite, as hairlike masses and long slender needles that are colourless or white. Like halotrichite, with which it forms a solid-solution series, it crystallises late in the paragenesis (R&M 92-3.256).

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