The Tulul Al Hammam area in central Jordan is an advantageous natural analogue site to study long-term U(VI) retention in ~ 1 Ma old U-bearing combustion metamorphic marbles with clinker-like mineralogy exposed to prolonged supergene alteration for at least ~ 100 kyr. The marbles contain abundant grains of high-temperature (ca. 800–850 °C) primary double Ca-U(VI) oxides (mainly Ca3UO6 and CaUO4), which are commonly replaced by hydrated calcium uranates with various impurities (Si, Fe, Al and F). A more hydrous natural analogue of X-phase (Ca2UO5·2-3H2O) occurs as a predominant secondary U compound after primary Ca-U(VI) oxides. The phase was studied by single-crystal XRD, SEM/EDX and electron microprobe (EPMA) analyses and Raman spectroscopy. It is a non-crystalline phase with a specific finger-like microtexture consisting of thin (no wider than 1–2 μm) lamellar particles. Its Raman spectrum shows a single strong band at 706–713 cm−1, sometimes coexisting with up to three weak diffuse bands (ν ~ 390, ~ 540 and 1355–1400 cm−1). The find of the natural X-phase (Ca2UO5·2-3H2O) is evidence of its long-term stability in a natural environment. It proves explicitly that the compound Ca2UO5·nH2O is a solubility-limiting phase in aged cements. The results have implications for geological disposal of radioactive wastes.