The flux reaction schematics and the crystal structure of rhombohedral KBiS 2 , along with the tomography setup and both absorption contrast and diffraction contrast tomography data are all shown in Figure . tube, embedded in flux and is optically opaque. The unit cell of the crystalline product KBiS 2 is shown in Figure (c). The density-contrast reconstruction of the entire tube in the tomographic region is shown in Figure (f) with a density cutoff chosen so that only the Bi-containing crystals are visible, while the flux is transparent. Needle-and plate-shaped crystals with some dimensions greater than 1 mm are evident, and powder diffraction of replica samples, with all crystals ground together, confirmed they are the same phase (Figure ). Initially, no fit to the Laue patterns could be obtained by DCT using reported crystal structures of phases in the K-Bi-S system: KBiS in the Inorganic Crystal Structure Database (ICSD), which indexed the DCT Laue data with a high completeness value. After our analysis was complete, this phase was also reported in powder form. and G2, have reflections that precess as expected (marked as short arrows) as the sample is rotated by 10 • over 5 frames. These reflections are primarily of {110} and {114} character, as indexed in Figure in Supporting Information. The ability to index the central grain, despite the presence of additional peaks and absorption from nearby grains, is a testament to the robustness of the grain reconstruction algorithm. The grains G1 and G2 belong to a single needle with nominally the same crystal orientation as can be seen in Figure With the phase identified and orientation established, a few insights into the crystal growth mechanisms are apparent. Tomographic slices are viewed in absorption contrast mode in Figure , primarily focused on the long needles in the center of the sample, which, due to their unbroken lengths and protruding dendrites, can be assumed to be among the first large grains to form. Secondary and tertiary dendrites can be seen attached to the primary needle crystals that originate from the bottom of the reaction tube, with this convergence most apparent in Figure (b). The cross-sectional views of the needles in Figure (d) and (e) reveal hollow centers that likely arise from screw-dislocation-driven spiral growth when the dislocation strain field energy is balanced by the creation of an internal surface energy.