Allophane
| Allophane | ||
|---|---|---|
Specific gravity | 2.8 | |
Allophane is an
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.[2] 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
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
- 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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