After presenting the major role of iron and its oxides in (bio)geochemistry and the environment, and in particular their involvement in numerous biological processes, we briefly discussed the processes by which iron oxides are formed in natural environments and in the laboratory. These processes depend mainly on acid-base, redox, photochemical and microbial-mediated phenomena. Among iron oxides and oxyhydroxides, there are a dozen structural types (magnetite, ferrihydrite, goethite, hematite, lepidocrocite, etc.), many of which are involved in biomineralization processes. In a second section, we review what is known about these processes in the context of the biominerals that form the teeth of grazing molluscs such as chitons and limpets. Chitons are marine molluscs that use highly mineralized, ultra-hard teeth to feed on algae stuck to rocks. To fulfill this function, chiton teeth must be hard and wear-resistant. Modern microscopic and spectroscopic analyses combined with finite element simulations have enabled us to better understand the biomineralization process, test ultrastructural features and analyze structure-mechanical property relationships in fully mineralized chiton teeth. The impressive mechanical properties obtained result from a hierarchically arranged chitin-mineral hybrid structure consisting of highly oriented nanostructured magnetite rods surrounding a soft core of organic-phase-rich iron phosphate. During mineralization, the rigid outer shell of the chiton stelleri root dentition undergoes four distinct stages of structural and phase transformation:
1) the formation of a crystalline organic matrix of α-chitin that forms the structural framework of the unmineralized layer;
2) synthesis of crystalline ferrihydrite aggregates along these organic fibers;
3) the subsequent solid-phase transformation of ferrihydrite into magnetite;
4) the progressive growth of magnetite crystals to form continuous parallel bars within the mature teeth.
The underlying α-chitin organic matrix appears to influence the density of the magnetite crystalline aggregate and the diameter and curvature of the resulting rods. These are likely to play a critical role in determining the local mechanical properties of mature radular teeth. By understanding the effects of the nanostructured architectures observed in chiton tooth, abrasion-resistant materials can be developed for machining, as well as functional coatings for medical equipment and implants.
