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Atomic Force Microscopy of the Nacreous Layer in Mollusc Shells
Technical rept. 1 Jun 1993-31 May 1994
CALIFORNIA UNIV SANTA BARBARA DEPT OF CHEMISTRY
Pagination or Media Count:
We present atomic force microscopy observations of mature nacre, specifically the polygonal aragonite tablets which comprise the mineral portion of nacre, in two types of molluscs, a bivalve Atrina serrate and a gastropod Haliotis rufescens. By imaging in liquids it was possible to dissolve away the nacre layer by layer to reveal both the structure of a single, tablet and its relationship to vertically adjacent tablets. Atrina tablets inner face had a concave appearance the central depression was surrounded by elongate rings mimicking the orientation and aspect ratio of the unit cell rectangle. Unit cell orientations were generally aligned vertically and laterally between tablets of Atrina, although occasional twinning was also observed. Etching with HCI initially removed a top crust in bleached Atrina tablets, revealing a flatter base without the elongate rings. Further etching lifted off individual tablets to reveal underlying nacreous layers, showing no morphological registry between vertically adjacent tablets. The nacreous structure of Haliotis differed from Atrina in three ways 1 the tablets were flatter and showed no top crust or elongate rings 2 the positions of the central depressions approximately repeated between nacreous layers, showing that the presumed nucleation sites line up along a given stack and 3 the unit cell orientations were not preserved between laterally adjacent tablets but were between vertically adjacent tablets-an observation which could not be made by X-my crystallography
APPROVED FOR PUBLIC RELEASE