Natural analogue approaches to prediction of long-term behaviour of Ca2UO5 center dot 2-3H(2)OX-phase: case study from Tulul Al Hammam site, Jordan

摘要

The Tulul Al Hammam area in central Jordan is an advantageous natural analogue site to study long-term U(VI) retention in similar to 1 Ma old U-bearing combustion metamorphic marbles with clinker-like mineralogy exposed to prolonged supergene alteration for at least similar to 100 kyr. The marbles contain abundant grains of high-temperature (ca. 800-850 degrees 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 center dot 2-3H(2)O) 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 mu m) lamellar particles. Its Raman spectrum shows a single strong band at 706-713 cm(-1), sometimes coexisting with up to three weak diffuse bands (gamma similar to 390, similar to 540 and 1355-1400 cm(-1)). The find of the natural X-phase (Ca2UO5 center dot 2-3H(2)O) is evidence of its long-term stability in a natural environment. It proves explicitly that the compound Ca2UO5 center dot nH(2)O is a solubility-limiting phase in aged cements. The results have implications for geological disposal of radioactive wastes.