Unusual combustion-metamorphic phosphorus-rich varicolored marbles of the Tulul Al Hammam area in central Jordan, 60 km east of the Dead-Sea – Jordanian Transform, bear abundant accessory grains and crystals of primary double Ca-U(VI) oxide minerals (calcium uranates). The Ca-U(VI) oxides are associated with typical pyrometamorphic minerals, such as sulfate-bearing fluorapatite, fluorellestadite, spurrite, fluormayenite, dorrite, brownmillerite, srebrodolskite, periclase (Zn-, Ni-, Co-, and Cu-rich), lime, and U-rich lakargiite. The primary Ca-U(VI) oxide minerals have large compositional ranges, with 45.07 to 82.54 wt.% UO3 and 16.66 to 56.05 wt.% CaO corresponding to CaO-to-UO3 molar ratios from 1 to 6. The marble-hosted calcium uranate minerals are most often Ca3UO6 or Ca2UO5 existing as individual crystals (5 to 25 µm), while CaUO4 is a rarer phase found as small inclusions in fluorapatite/fluorellestadite. These compounds were identified by SEM microscopy, electron microprobe analyses, and Raman spectroscopy. The existence of other calcium uranate minerals (Ca4UO7, Ca5UO8, Ca3U2O9, and Ca6UO9) has been inferred from SEM and electron microprobe analyses of single grains (<10 µm). The mineral assemblages correspond to high-temperature (ca. 800–850 °C) and ambient pressure conditions of spurrite-merwinite metamorphic subfacies. The Ca-U(VI) oxide minerals formed during Quaternary combustion metamorphism of U-rich bituminous calcareous sediments (oil shales of the Maastrichtian-Paleocene Muwaqqar Chalk Marl Formation, Belqa Group). Numerous pseudomorphs after cracked and hydrated calcium uranate grains with a wide range of impurities (Si, Fe, Al, and F) may be due to chemical weathering and hydration of the primary calcium uranate minerals. A more hydrous analogue of the so-called x-phase (Ca2UO5•1.5–3H2O) was found as a main secondary phase after the primary Ca-U(VI) oxide minerals. The unusual mineralogy and chemistry of the varicolored marbles can provide clues to the behavior of trace elements in concrete, their stability in radioactive waste repositories, and to purification of metals under dry annealing conditions.

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