Although calcium carbonate (CaCO3) has been the most studied mineral in geochemistry, its nucleation pathway has never been structurally resolved directly in an aqueous environment due to its relatively low solubility. The effects of external parameters, such as the addition of organic molecules, on its structural evolution during nucleation remain even more elusive. Consequently, before attempting to understand the effect of additives on CaCO3 nucleation, we need to ensure a sufficient understanding of the system by itself. In this proposal, we aim to elucidate the nucleation pathway of CaCO3 by a new experimental approach that allows probing the nanostructural evolution of very diluted systems. In addition, we will investigate the influence of additives on the nucleation of CaCO3. Our method is based on a specific data collection strategy for diluted solutions during in situ time resolved high energy X-ray scattering (HEXS) in combination with a titration set-up.