Allophane

Allophane is an amorphous hydrous aluminium silicate clay mineral. Its chemical formula is Al₂O₃·(SiO₂)_1.3-2·2.5-3(H₂O). Although it has a variable composition and although generally amorphous in structure, it has long been recognized as a mineral and can be identified by its distinctive X-ray diffraction pattern. It was first described in 1816 in Gräfenthal, Thuringia, Germany. Allophane is a weathering or hydrothermal alteration product of feldspars and other primary minerals and has a composition similar to kaolinite. A copper containing variety cupro-allophane has been reported.

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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

• Webmineral data
• Mindat