  Iron biomineralisation is critical for life. Nature capitalises on the
physical attributes of iron biominerals for a variety of functional, structural
and sensory applications. Although magnetism is an integral property of iron
biominerals, the role it plays in their nano-assembly remains a fundamental,
unanswered question. This is well exemplified by the magnetite-bearing radula
of chitons. Chitons, a class of marine mollusc, create the hardest biomineral
of any animal in their abrasion-resistant, self-sharpening teeth4. Despite this
system being subjected to a range of high resolution imaging studies, the
mechanisms that drive mineral assembly remain unresolved. However, the advent
of quantum imaging technology provides a new avenue to probe magnetic
structures directly. Here we use quantum magnetic microscopy, based on
nitrogen-vacancy centres in diamond, to attain the first subcellular magnetic
profiling of a eukaryotic system. Using complementary magnetic imaging
protocols, we spatially map the principal mineral phases (ferrihydrite and
magnetite) in the developing teeth of Acanthopleura hirtosa with submicron
resolution. The images reveal previously undiscovered long-range magnetic
order, established at the onset of magnetite mineralisation. This is in
contrast to electron microscopy studies that show no strong common
crystallographic orientation. The quantum-based magnetic profiling techniques
presented in this work have broad application in biology, earth science,
chemistry and materials engineering and can be applied across the range of
systems for which iron is vital.
