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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
Environments
Hydrothermal environments
Fumeroles
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).
Localities
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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