Luminescence of uranium-bearing opals: Origin and use as a pH record

• Fluorescence is typical of uranyl speciation.
• Uranyl occurs either as phosphate or hydroxo-polynuclear complexes trapped onto the opal internal surface.
• Data can be used as a pH record of opal formation.

Fluorescence of minerals has been long used for U exploration. It is proposed here that opal fluorescence can be used as a probe of the pH of the formation solution, bringing constraints for modeling U speciation and sequestration at the Earth's surface. We present a study of fluorescence spectroscopy of U in opals resulting from low-temperature (29 °C) alteration of mineralized rhyolitic lavas (Nopal I U deposit, Sierra Peña Blanca, Mexico). These opals show green fluorescence with a concentric distribution at microscopic scale, which is unambiguously assigned to uranyl groups (oxidized form of U). Spectra appear typical of uranyl in opal as reported in literature for other localities according to peak positions. When considering also the lifetime of fluorescence spectra (with time resolved laser fluorescence spectroscopy), fingerprinting indicates that uranyl occurs either as phosphate or hydroxo-polynuclear complexes trapped onto the opal internal surface. Data are indicative of a pH of opal formation around 8, as derived from both conditions of laboratory experiments and reference to calculated diagrams of speciation. This pH value is consistent with the Nopal geological formations that were potentially in contact with the aquifer involved in the hydrothermal process. In addition, the microscopic zoning of U suggests that the speciation has been stable since opal formation more than 50 ka ago, which allows the record of pH that prevailed at that time.