Diffractive X-ray Tomography of Oriented Mineralized Structures Doctoral thesis in Hierarchical Materials
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- Institutt for fysikk 
Many materials around us in everyday life have a hierarchical structure, meaning that defining structural features are found at multiple length scales, often ranging from the molecular to the macroscopic scale, together providing the overall physical properties. Bone, cartilage, and shale rocks are examples of such materials – important to understand for their relevance as biological building blocks and construction materials, but difficult to characterize with conventional microscopic imaging techniques. Limitations of these conventional techniques include complicated sample preparation procedures, difficulties of obtaining three-dimensional (3D) images of larger volumes and limited possibilities of imaging with orientational contrast. In this thesis, the application of new X-ray microscopy methods to the hierarchical materials bone, cartilage, and shales is investigated. New methods to investigate the structural properties of bone and cartilage can lead to an increased understanding of bone growth, help cure diseases such as osteoporosis and osteochondrosis, and aid in the development of new implants. Shales are sedimentary porous rocks studied for the production of oil and gas, and are candidates to be used as low-permeable cap rocks in carbon capture and storage technology. Several mechanical properties of bone, cartilage and shale are due to the orientation of crystalline structures found at the micro-scale. Oriented collagen fibrils provide mechanical stiffness in bone and resistance to tensile load in both bone and cartilage. Clay minerals in shale orient relative to the compaction loading directions during sedimentary processes, and knowledge of the clay mineral orientation can be used to predict fracture behavior. X-ray diffraction computed tomography (XRD-CT) is an increasingly popular imaging technique that allows 3D mapping of sample interiors without de iii iv Abstract structive sample preparation. A recent extension of XRD-CT; X-ray diffraction tensor tomography (XRDTT), also allows the preferred orientation of crystallites to be obtained. The main focus of this thesis is the application XRD-CT to bone, cartilage and shale. By applying XRD-CT to fossil samples, a tibia of a tetrapod Discosauriscus austriacus and the humerus of a fish Eusthenopteron foordi, it is demonstrated that muscle attachments can be located nondestructively, providing a useful tool for paleontologists as these fossils are rare and sample sectioning should be avoided. XRDTT was also used to study bone growth in samples from a femoral condyle obtained from a young pig. The chemical composition of the mineralized phase and distinct patterns of bone mineral hydroxyapatite (HA) orientation could be mapped nondestructively in 3D close to the bone-cartilage interface, providing novel information about HA organization in the developing bone and in the surroundings of cartilage canals. In a third study, Pierre shale was studied by using XRDTT. By analyzing registered XRD-CT and attenuation- contrast computed tomography (CT) tomograms, Pierre shale samples were found to contain oriented clay and high density mineral inclusions. Clinochlore and illite clay mineral orientation maps were reconstructed in 3D. Localized high-attenuating regions were identified to contain pyrite and clinochlore. A complementary work in a fourth study included in this thesis suggests a remedy to some of these problems: X-ray 3D coherent diffraction imaging (CDI) was performed to map the details of the clay mineral matrix, pyrite inclusions and nanoscale pore distributions not resolvable with XRD-CT. These studies represent novel applications of recently developed X-ray micro scopy techniques to hierarchical materials, where much effort has gone into refining and adapting these physics- and computation-heavy methods for their practical use. As a result, new knowledge has been gained about the mineralization in developing bones, and the capability of diffraction-contrast orientation mapping to infer the muscle attachment locations in fossils. Promising avenues have been demonstrated to unravel the complex structures of shales.