Abstract Bone is comprised of crystals of calcium phosphate surrounding collagen fibrils. The crystals occur in polycrystalline mineral platelets (MPs). The c-axes of the nanocrystal components of the platelets have an overall preferred orientation parallel to the collagen fibrils but which varies somewhat. In this contribution we show how dark-field (DF) image processing can be used to measure and visualize nanocrystal orientations over micrometer-sized areas. We recorded selected area electron diffraction patterns from ion-beam milled TEM sections of the femora of Wistar rats cut parallel to the collagen fibrils. Diffraction arcs of the 002 reflection represent the variation in the orientation of c-axes of the nanocrystals. Performing DF imaging of electrons from five consecutive, discrete portions along these arcs, we created a series of images. Using a different color coding for each portion, we constructed a merged DF image, which allows intuitive visualization of gradual change of nanocrystal orientation. We show that many complete platelets are built of crystals whose c-axes vary in orientation by up to 26°. In some regions, stacks of platelets have essentially uniform c-axis orientation. These could be “tesselles” which make up only small fraction of volume of bone. Elsewhere adjacent MPs may differ widely (up to 50°) in orientation.
Bibko et al. (2026) studied this question.