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Formula: Al2O3(SiO2)1.3-2.0.2.5-3.0H2O
Phyllosilicate (sheet silicate)
Crystal System: Amorphous
Specific gravity: 2.75 measured
Hardness: 3
Streak: White
Colour: White, pale blue to sky-blue, green, brown
Solubility: Gelatinises in hydrochloric acid
Common impurities: Ti,Fe,Mg,Ca,Na,K
Environments:
Volcanic igneous environments
Hydrothermal environments
Allophane is principally a weathering product of volcanic ash, and also a hydrothermal alteration product
of feldspar. It may be a precursor to
halloysite, but halloysite also alters to
allophane. Allophane
is frequently found with admixed halloysite,
imogolite,
limonite, opal,
gibbsite and
cristobalite, and occasionally with
chrysocolla or
evansite
(Dana).
Localities
The type locality is Gräfenthal, Saalfeld-Rudolstadt District, Thuringia, Germany.
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