Crystal Structure, Microhardness, and Toughness of Biomineral CaCO3

Shells have superior mechanical strength and high toughness compared with single crystals of the pure mineral. It is particularly interesting to elucidate how crystal structure and mechanical property are related to the binding between calcium carbonate and organic substance. We investigated the crystal structure, microhardness, and fracture toughness of inner, middle, and outer layers of the Meretrix lusoria shell at room temperature (RT) and after high-temperature heat treatment. The crystal lattice of the three layers with the aragonite structure expanded anisotropically from the synthesized aragonite. The inner and outer layers showed that the a and b axes expanded, while the c axis shrank. The anisotropic lattice strain of the three layers was reduced by annealing up to 473 K. The microhardness values of the outer, middle, and inner layers decreased with increasing heat treatment temperatures. The fracture toughness of the outer layer at 373 K was almost the same as the RT value, but increased at 473 K. The fracture toughness of the middle and inner layers increased at 373 K. Our findings will aid understanding the mechanism for the mechanical property of the shell.