Allophane

phyllosilicate

halloysite

Formula: Al2O3(SiO2)1.3-2.0.2.5-3.0H2O
Phyllosilicate (sheet silicate), allophane group
Specific gravity: 1.8 to 2.78
Hardness: 3
Streak: White
Colour: White, pale blue to sky-blue, green, brown
Solubility: Gelatinises in hydrochloric acid
Environments:

Sedimentary 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).
Common impurities: Ti,Fe,Mg,Ca,Na,K

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