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) .