Fluid flow in porous media plays a central role in environmental and industrial systems, with important consequences for reactive transport processes. We have recently shown that steady laminar flows through transparent bead packs produce chaotic microscale trajectories, like turbulent flows but at low Reynolds numbers. This fundamentally changes the current paradigm of reactive transport modeling as chaos enhances microscale chemical gradients and controls mixing rates. Yet, proving the existence of chaotic mixing in rocks, key for reservoir and storage applications, is an outstanding challenge. This proposal seeks beamtime to conduct the first experiments to evidence chaotic mixing in rocks, for which ESRF-ID19 is the only beamline allowing the required acquisition of fast, highly- resolved, and low-noise microscale solute gradient images. This new dataset will be transformative in the current understanding of a broad range of reactive transport processes.