ACID DISSOLUTION OF AKAGANI~ITE AND LEPIDOCROCITE: THE EFFECT ON CRYSTAL MORPHOLOGY

Abstract--The rate of dissolution of akagan6ite in HC1 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 fpartly dissolved crystals of akagan6ite 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 cid attack. Dissolution of multi-domainic crystals involved preferential ttack along the domain boundaries, as well as at the edges of the crystals. Single-domain crystals were well developed, but appeared tocontain internal imperfections, which promoted the formation of holes on the otherwise unreactive (010) faces