Buried organic-inorganic interfaces in a biological specimen have been imaged with three-dimensional nanoscale resolution and chemical specificity (Nature, DOI: 10.1038/nature09686). The study, which was conducted by Northwestern University materials scientists Lyle M. Gordon and Derk Joester, reveals that individual 5- to 10-nm-wide organic fibers in the teeth of chitons, a type of marine mollusk that chews through rock, have distinct chemical compositions and distributions. These findings suggest distinct functions in otherwise similar tooth structures. The work also demonstrates that atom-probe tomography, a technique typically employed to analyze metallurgical and semiconductor samples, can be used to elucidate hidden structures and chemical features of biological importance. As chiton teeth mature, a relatively soft apatite or iron phosphate core capped by a hard magnetite layer gradually fills and occludes a fibrous organic scaffold. By preparing 3-D chemical maps of the usually hidden fibers and their surroundings, the team observed that sodium and magnesium ions cluster within the fibers in small domains. Furthermore, despite the fibers’ micrometer-scale proximity to one another, some of them exclusively bind only one type of ion, the team says.