XR-µCT for Earth material research
Thanks to the financial support from the Canada Foundation for Innovation, the Ontario Research Fund, and the Faculty of Science at Carleton University, we are running Canada's first high-resolution X-ray micro-computed tomography (XR-µCT) laboratory for Earth material research. The laboratory allows us to obtain quantitative microstructural and textural data that are essential to unravel fundamental atomic-scale processes that govern the formation of rocks. Because our instrument uses higher-energy X-rays and longer dosage times than conventional X-ray tomography facilities, we can non-destructively study the interior of solid matter at ultra-high resolution.
Figure 1: Calculated XR-µCT cross section of schist (Grt - garnet, Bt - biotite, Qtz- quartz, Ilm - ilmenite, Ms - muscovite, St - staurolite).
The key components of the XR-µCT system include (1) a 40-130 keV micro-focus (< 5 µm) X-ray source, (2) a distortion-free 2240 x 2240 pixel flat-panel X-ray sensor, (3) a micro-positioning sample stage that can handle also heavy (several kg) objects, and (4) a data acquisition, reconstruction and image processing system that renders a 3D image of the sample interior. While the sample is illuminated by an X-ray cone beam, the sample rotates around its vertical axis so that hundreds of angular views can be detected. Each view is a magnified projection image and corresponds to a calculated cross-section through the sample monitoring the details of its internal structure. These details reflect the 2D spatial distribution of X-ray attenuation of the sample interior which is directly related to variations in density and chemical composition.
Figure 1 is a XR-µCT cross-section through a rock cylinder drilled out of a garnet mica schist. The different grey values correspond to different degrees of X-ray attenuation and can be used to distinguish between garnet, biotite, ilmenite, muscovite, and quartz. Note that some muscovite grains pseudomorph staurolite. The diameter of this cross-section is ca. 3 cm, pixel size is ca. 15 µm.
Figure 2: Numerical model of the 3D distribution of garnet in the scanned schist. The matrix of the rock has been made transparent.
Figure 2 is the numerical reconstruction of the scanned rock cylinder with garnet crystals shown in red. The rest of the rock has been replaced in this model by a transparent matrix. The animation in Figure 3 shows the largest garnet crystal of the scanned rock volume. Its position in the rock, as well as its shape (note that it does not exhibit isometric but hexagonal symmetry!) needs to be considered to prepare a physical cross section required for chemical, crystallographic or other analyses.
Figure 3: The largest garnet crystal of the scanned rock volume. Its position and (hexagonal!) shape needs to be considered when preparing physical sections for chemical and other analyses.