Imogolite, a Hydrated Aluminium Silicate of Tubular Structure

Abstract

IMOGOLITE, a gel-like hydrous aluminium silicate identified in several volcanic ash soils and other weathered pyroclastic deposits, has been shown by electron microscopy¹ to consist of bundles of fine tubes, each about 20 Å in diameter. Electron diffraction patterns^2,3 indicate a repeat distance along the tube axis of 8.4 Å, and a repeat distance perpendicular to this axis of 22 to 23 Å, corresponding to centre-to-centre tube separations. X-ray diffraction patterns are more diffuse, and differ principally in that the highest spacing observed for desiccated material is 18.4 Å. Two empirical formulae have been proposed: 1.5SiO₂·Al₂O₃·2.5H₂0 (ref. 2) and 1.1SiO₂·Al₂O₃·2.3−2.8H₂O (ref. 3); these two papers also advance tentative structures which assign the 8.4 Å repeat distance to a gibbsite-like chain or ribbon, although they differ in the nature of the postulated silicate anion. Neither accounts for the cylindrical structure later established.