1911 Encyclopædia Britannica/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 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, Al2SiO5. 5H2O. Allophane is always of secondary origin, resulting from the decomposition of various albuminous 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 1809 in marl at Gräfenthal, near Saalfleld 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 allophane was given by F. Stromeyer in 1816, from the Gr. ἄλλος, another, and φαίνω, 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 schrötterite (opal-allophane).