Online Encyclopedia

ALLOPHANE

Online Encyclopedia
Originally appearing in Volume V01, Page 699 of the 1911 Encyclopedia Britannica.
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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 Hamblyn, 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, Al,SiO5. 5H2O. Allophane is always of secondary origin, resulting from the decomposition of various aluminous silicates, such as felspar. It is often found encrusting fissures and cavities in mines, especially those of copper and iron. It was first observed in I$09 in marl at Grafenthal, near Saalfield in Thuringia; and has been found in some quantity in the chalk pits at Charlton in Kent, where it lines fissures and funnel-shaped cavities. The name allophanewas given by F. Stromeyer in 1816, from the Gr. iiXAos, another, and rpaivw, 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 schrStterite (opal-allophane).
End of Article: ALLOPHANE
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