Electron microscopic investigations of the polymer/mineral composite material nacre

Abstract

Nacre of the mollusc Haliotis laevigata was investigated using transmission electron microscopy (TEM). Nacre, the inner iridescent layer of mollusc shells, is a typical example for a material which is formed by biomineralization processes. It is composed of the CaCO₃-polymorph aragonite and a small amount of about 5wt% organic matter [1]. The polygonal shaped aragonite platelets show a width which ranges from 5µm to 10µm and a thickness of about 500nm [2]. They are laterally arranged in layers and vertically in stacks. The aragonite crystal structure can be described by the space group Pmcn 62 with the c-axis perpendicular to the face of the platelets and thus to the surface of the shell [3]. The platelets are separated by layers of organic material, the organic matrix. This arrangement resembles a brick and mortar like structure. The combination of stiff crystalline material and soft organic material, as well as the layered structure increase the fracture toughness in comparison with pure (geological) aragonite.