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Allophane

ALLOPHANE, one of the few minerals known only in the amorphous state. It is a glassy substance, usually occurring as thin encrustations with a mammillary surface; occasionally, however, it is earthy and pulverulent. The colour varies considerably. from colourless to yellow, brown, blue or green. Specimens of a brilliant sky-blue colour, such as those found formerly in Wheal Haniblyn, near Bridestowe in Devonshire, and in Sardinia, are specially attractive in appearance; the colour is here due to the presence of the copper mineral chrysocolla. The hardness is 3, and the specific gravity 1.9. Chemically, it is a hydrous aluminium silicate, Al2SiO5.5H2O. Allophane is always of secondary origin, resulting from the decomposition of various aluminous silicates, such as felspar. It is often found copper and iron. It was first observed in 1809 in marl at Grafenthal, near Saalfield in Thuringia; and has been found in lines fissures and funnel-shaped cavities. The name allophane was given by F. Stromeyer in 1816, from the Gr. allos, another, and faino, to appear, in allusion to the fact that the mineral crumbles and changes in appearance when heated before the blowpipe. Other names for the species are riemannite and elhuyarite, whilst closely allied minerals are carolathine, samoite and schrotterite (opal-allophane).

Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)

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