Allophane

Allophane
General
Category Phyllosilicates
Kaolinite-serpentine group
Formula
(repeating unit)		 Al2O3·(SiO2)1.3-2·(2.5-3)H2O
Strunz classification 9.ED.20
Crystal system Amorphous
Identification
Color White, pale blue to sky-blue, green, brown, yellow
Crystal habit Crusts and masses
Cleavage None
Fracture Conchoidal
Tenacity Brittle
Mohs scale hardness 3
Luster Waxy to earthy
Streak White
Diaphaneity Transparent, translucent
Specific gravity 2.8

Allophane is an amorphous to poorly crystalline hydrous aluminium silicate clay mineraloid. Its chemical formula is Al2O3·(SiO2)1.3-2·(2.5-3)H2O. Since it has short-range atomic order, it is a mineraloid, rather than a mineral, and can be identified by its distinctive infrared spectrum and its X-ray diffraction pattern. It was first described in 1816 in Gräfenthal, Thuringia, Germany. Allophane is a weathering or hydrothermal alteration product of volcanic glass and feldspars and sometimes has a composition similar to kaolinite but generally has a molar ratio of Al:Si = 2. It typically forms under mildly acidic to neutral pH (5-7). Its structure has been debated, but it is similar to clay minerals and is composed of curved alumina octahedral and silica tetrahedral layers.[1] Transmission electron micrographs show that it is generally made up of aggregates of hollow spherules ~3-5 nm in diameter. Allophane can alter to form halloysite under resilicating aqueous conditions and can alter to form gibbsite under desilicating conditions. A copper containing variety cupro-allophane has been reported.

It forms waxy botryoidal to crusty masses with color varying from white through green, blue, yellow, to brown. It has a Mohs hardness of 3 and a specific gravity of 1.0.

It was named from the Greek allos - "other" and phanos - "to appear", as it gave a deceptive reaction in the blowpipe flame in old mineralogical testing.

References

  1. ↑ Smalley,I.J. 1979. A spherical structure for allophane. Nature 281, 339 only
  • Webmineral data
  • Mindat
  • Handbook of Mineralogy
  • Wada, K. (1989), Allophane and Imogolite in Minerals in Soil Environments, Soil Science Society of America, no.1, pp. 1051–1087.
  • Parfitt, R.L. (1990), Allophane in New Zealand. A Review. Australian Journal of Soil Research 28, pp. 343–360.


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