Coffinite, USiO₄, Is Abundant in Nature: So Why Is It So Difficult To Synthesize?

Abstract

Coffinite, USiO4, is the second most abundant U(4+) mineral on Earth, and its formation by the alteration of the UO2 in spent nuclear fuel in a geologic repository may control the release of radionuclides to the environment. Despite its abundance in nature, the synthesis and characterization of coffinite have eluded researchers for decades. On the basis of the recent synthesis of USiO4, we can now define the experimental conditions under which coffinite is most efficiently formed. Optimal formation conditions are defined for four parameters: pH, T, heating time, and U/Si molar ratio. The adjustment of pH between 10 and 12 leads probably to the formation of a uranium(IV) hydroxo-silicate complex that acts as a precursor of uranium(IV) silicate colloids and then of coffinite. Moreover, in this pH range, the largest yield of coffinite formation (as compared with those of the two competing byproduct phases, nanometer-scale UO2 and amorphous SiO2) is obtained for 250 °C, 7 days, and 100% excess silica.

Coffinite formation from UO2+x

“…These results clearly indicated the formation of the neoformed coffinite. 2− proposed by Mesbah et al 15 UO 2 (cr) is not allowed to be present in the calculations. Symbol correspond to experimental data at equilibrium (≥ 100 days).…”

“…5 at 217; 254 and 339 days), the surface of altered grains was covered by small particles embedded in a gel. Some of these particles exhibited a bipyramidal morphology characteristic of coffinite 15 .…”

“…A detailed investigation of meteoric roll-front deposits in the Athabasca basin, suggest an estimated temperature of coffinite precipitation in the uranium front of no greater than 50 °C 13 . Even though laboratory experiments report coffinite formation at 150–250 °C 14 , 15 , it appears that these elevated temperatures are not required to form coffinite in nature.…”

“…Although coffinite is abundant in uranium ore deposits, its synthesis has been a major challenge since its initial description as a mineral in 1955 16 . A number of investigators have sought to obtain pure synthetic coffinite, but only a few have succeeded 14 , 15 , 17 – 21 . Synthetic coffinite was always obtained under hydrothermal conditions.…”

Incipient formation of zircon and hafnon during glass alteration at 90°C

“…The stabilization of USiO 4 , coffinite, a mineral isostructural to zircon and hafnon, has been recognized to be facilitated by the formation of uranium silicate aqueous complexes, acting as a precursor of U silicate colloids. The external surface layer of the alteration gels of Zr/Hf‐bearing glasses may play the role of such precursors in the incipient formation of zircon and hafnon at low temperature.…”