Bone mineral platelets are mesocrystals formed by monoclinic nanocrystals

Detailed multiscale models explain the complex hierarchical structure of bone but the crystal structure and precise organization of bone apatite nanocrystals are unknown. Crystal structure studies of bone mineral are difficult due to intimate structural intergrowth with organic matrix and small crystal size, (2-6 nm x 30-40 nm). Here, we present high resolution transmission electron microscope images of bone mineral platelets, which allow us to clarify both the crystal structure and morphology of bone mineral. We demonstrate that the mineral platelets are polycrystalline mesocrystals with flat faces formed by oriented attachment of several crystals parallel to the c-axis and flattened by the dominant (010) prism facets. Individual bone crystals have monoclinic symmetry, in contrast to hydroxyapatite, which is hexagonal. Interestingly, biomimetic apatite also exhibits monoclinic symmetry. These findings provide a plausible explanation for the observed flat platelet morphology of bone crystals, which plays a key role in collagen binding and is at odds with the hexagonal structure and morphology of geological apatites.