Acid Dissolution of Akaganiéite and Lepidocrocite: The Effect on Crystal Morphology

R. M. Cornell1 and R. Giovanoli2
1 ETH Zentrum Zürich, Laboratory of Inorganic Chemistry CH-8092 Zürich, Switzerland
2 Laboratory for Electron Microscopy, University of Berne Freiestrasse 3, CH-3000 Berne 9, Switzerland

Abstract: The rate of dissolution of akaganéite in HCl increased with time over the bulk of the reaction leading to a sigmoid dissolution vs. time curve. The bulk of the dissolution of lepidocrocite could be described by the cube root law. Transmission electron microscopy examination of partly dissolved crystals of akaganéite showed that acid attack proceeded mainly along the [001] direction. Initially, the tapered ends of the crystals became squared, and as dissolution continued the lengths of the crystals decreased steadily. At the same time, the crystals were gradually hollowed out. Acid attack was most pronounced at the edges of the crystals of lepidocrocite and appeared to involve a disruption of the hydrogen bonds that link the sheets of octahedra making up the structure. Defects also acted as sites for preferential acid attack. Dissolution of multi-domainic crystals involved preferential attack along the domain boundaries, as well as at the edges of the crystals. Single-domain crystals were well developed, but appeared to contain internal imperfections, which promoted the formation of holes on the otherwise unreactive (010) faces.

Key Words: Akaganéite • Dissolution • Iron • Lepidocrocite • Morphology • Transmission electron microscopy

Clays and Clay Minerals; October 1988 v. 36; no. 5; p. 385-390; DOI: 10.1346/CCMN.1988.0360501
© 1988, The Clay Minerals Society
Clay Minerals Society (www.clays.org)