We report the first complete crystal structure determination of the rare mineral znucalite, a zinc uranyl-carbonate, using advanced three-dimensional electron diffraction (3D ED) and powder X-ray diffraction. This uranium mineral, previously known only from powdery aggregates and with ambiguous symmetry and composition, was found to be monoclinic with space group P2₁/m and a layered structure. We found that it consists of double sheets of zinc, coordinated octahedrally and tetrahedrally by oxygen atoms, linked together by carbonate groups and hydroxyls. These sheets are separated by thick interlayers that host uranium, calcium, and water molecules. The uranium is present as uranyl ion (UO₂²⁺), and the carbonate groups show several non-equivalent environments, as confirmed by Raman spectroscopy. The refined formula, based on both structure and chemical analyses, is Zn₁₀Ca₀.₈₂₈[UO₂]₀.₈₂₈[CO₃]₄(OH)₁₅.₃₁₂(H₂O)₅.₄₈, which matches well with electron microprobe data. The study also provides new insights into the environmental significance of znucalite, showing it can immobilize uranium in the circumneutral aqueous solutions and estimating its solubility product, which is essential for predicting uranium mobility in contaminated sites. This work also demonstrates how 3D ED enables the structure characterization of minerals that cannot be studied by conventional X-ray diffraction due to their tiny crystal size.