Structural and microstructural characterization of the growth lines and prismatic microarchitecture in red abalone shell and the microstructures of abalone "flat pearls"

The structure of the growth lines in the shell of the red abalone, Haliotis rufescens, has been characterized by X-ray diffraction analysis and by scanning and transmission electron microscopy. The growth lines consist of a blocklike microstructure and a spherulitic microstructure, separated by a green organic matrix interlayer. The minerals in both the blocklike and spherulitic structures have been determined to be aragonite, the same CaCO3 polymorph as in the nacreous microstructure in these shells. The spherulitic structure is composed of radially distributed elongated crystals, whereas the blocklike structure involves crystalline aggregates with irregular shape. The individual aggregates are approximately single crystal, with orientations identical to that of the adjacent stack of tablets in the nacreous structure. The interfaces defining the transition from nacreous to blocklike microstructures are abrupt; on the other hand, the transition from spherulitic to nacreous microstructures shows more irregularity because of the occasional intergrowth of elongated crystals into the nacreous region. The microstructures of "flat pearls", produced by mineralization of an abiotic substrate (a glass cover slip) inserted between the mantle tissue and the growing edge of the shell in a live abalone, have also been studied. A thin calcitic CaCO3 layer is produced on the center of the glass substrate, which is soon covered by green organic matrix. This matrix extends beyond the calcitic region; that is, it can be secreted directly onto the glass. Mineralization of this green matrix layer involves the deposition of spherulitic aragonite, similar to that occurring in the native shell, which is then capped by nacreous aragonite. Thus, the microstructures within the flat pearls mimic very closely certain aspects of the microstructures within the growth lines of the native shell.