Structural Details as Clues to Understanding Nacre Formation

Abstract Mollusk nacre, the opalescent lining of shells such as abalone, has a fracture toughness about 3000 times greater than that of aragonite (CaCO3), the mineral of which it is almost entirely composed. Since it owes its fracture strength and toughness to its composite organic/inorganic microstructure, details of nacre formation provide clues for materials scientists hoping to mimic nacre's microstructure in fracture-resistant synthetic materials. TEM and SEM provide detailed structural information, which contributes to the understanding of nacre. An abalone shell was fractured to expose the stacked-plate morphology for imaging in a Philips XL30 FEG-SEM. Also, an ion-milled, cross sectional sample was prepared for imaging in a Philips CM200 FEG-TEM. While SEM shows tablets of aragonite with a highly uniform thickness of 0.5 μm, stacked in interdigitated columns (Fig. 1), cross-sectional TEM reveals organic layers, ∼10 nm thick, which separate the aragonite lamella (Fig. 2), consistent with previous studies.