Non-stoichiometric magnetite and maghemite in the mature teeth of the chitonAcanthopleura hirtosa

Persistent polyamorphism in the chiton tooth: From a new biomineral to inks for additive manufacturing

Engineering structures that bridge between elements with disparate mechanical properties are a significant challenge. Organisms reap synergy by creating complex shapes that are intricately graded. For instance, the wear-resistant cusp of the chiton radula tooth works in concert with progressively softer microarchitectural units as the mollusk grazes on and erodes rock. Herein, we focus on the stylus that connects the ultrahard and stiff tooth head to the flexible radula membrane. Using techniques that are especially suited to probe the rich chemistry of iron at high spatial resolution, in particular synchrotron Mössbauer and X-ray absorption spectroscopy, we find that the upper stylus of Cryptochiton stelleri is in fact a mineralized tissue. Remarkably, the inorganic phase is nano disperse santabarbaraite, an amorphous ferric hydroxyphosphate that has not been observed as a biomineral. The presence of two persistent polyamorphic phases, amorphous ferric phosphate and santabarbaraite, in close proximity, is a unique aspect that demonstrates the level of control over phase transformations in C. stelleri dentition. The stylus is a highly graded material in that its mineral content and mechanical properties vary by a factor of 3 to 8 over distances of a few hundred micrometers, seamlessly bridging between the soft radula and the hard tooth head. The use of amorphous phases that are low in iron and high in water content may be key to increasing the specific strength of the stylus. Finally, we show that we can distill these insights into design criteria for inks for additive manufacturing of highly tunable chitosan-based composites.

A new biomineral identified in the cores of teeth from the chiton Plaxiphora albida

The hydrated iron(III) oxide limonite is reported for the first time as a biomineral. In situ laser Raman spectra of the tooth cores from major lateral teeth of the chiton Plaxiphora albida are compared with those of synthetic and mineral iron phosphates and iron oxides. Raman spectra measured on iron phosphate and iron oxide standard materials are shown to be easily distinguishable from one another. The central tooth cores of mature P. albida teeth do not show any evidence for the presence of a separate iron phosphate mineral. Rather, in each tooth a narrow band of the hydrated iron(III) oxide limonite is shown to separate the magnetite of the tooth surface from a central core region comprising both lepidocrocite and limonite. The high concentration of phosphorus in P. albida tooth cores, previously observed by energy dispersive spectroscopy, is not associated with a separate iron phosphate mineral, indicating that this element may be adsorbed onto the surface of the iron oxide minerals present. The failure to detect a separate iron(III) phosphate is discussed with reference to other chiton species that display high levels of iron and phosphorus in the cores of their mature major lateral teeth.