The occurrence of trace amounts of uranyl in naturalcalcite has posed a long-standing problem in crystal chemistrybecause of speculation that the size and shape of theuranyl ion may preclude its incorporation in a stable latticeposition in calcite. This also defines an importantenvironmental problem because of its bearing on thetransport and sequestration of uranyl released from nuclearfacilities and uranium mining operations. Calcite is anearly ubiquitous mineral in soils and groundwater aquifers.X-ray absorption spectroscopy and X-ray fluorescencemicroprobe studies of uranium in relatively U-rich ∼13 700-year-old calcite from a speleothem in northernmost Italyindicate substitution of uranyl for a calcium and two adjacentcarbonate ions in calcite. These new data imply thaturanyl has a stable lattice position in natural calcite,indicating that it may be reliably sequestered in calciteover long time scales.