Allophane

Allophane
Allophane from Maid of Sunshine Mine, Courtland-Gleeson District, Cochise County, Arizona, US
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. 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.