Revisiting the role of calcite in Spondylus crassisquama shell

Spondylus crassisquama (princeps) is a bivalve mollusc inhabiting the coast of Central and South Americas. Its shell uses a two-layer armour system to protect the inner soft tissues from the environment. The external layer is composed of a calcite-based composite, and the internal layer is composed of an aragonite-based composite. For the first time, the structural characterisation of the S. crassisquama shell is reported. The results show similarities with those for other shells such as that of red abalone. As expected, microhardness tests demonstrate that the internal nacreous layer is stiffer and harder than the external calcitic layer. However, the amount of energy dissipated during the indentation tests of the calcitic layer is similar to the amount of energy dissipated by the nacreous layers. Despite the highly ordered structure of the nacreous layer, the characterisation conducted shows that the calcitic layer can also be used as a model for the development of bioinspired materials.