Abstract

Uranyl carbonates are one of the largest groups of secondary uranium(VI)-bearing natural phases being represented by 40 minerals approved by the International Mineralogical Association, overtaken only by uranyl phosphates and uranyl sulfates. Uranyl carbonate phases form during the direct alteration of primary U ores on contact with groundwaters enriched by CO2, thus playing an important role in the release of U to the environment. The presence of uranyl carbonate phases has also been detected on the surface of “lavas” that were formed during the Chernobyl accident. It is of interest that with all the importance and prevalence of these phases, about a quarter of approved minerals still have undetermined crystal structures, and the number of synthetic phases for which the structures were determined is significantly inferior to structurally characterized natural uranyl carbonates. In this work, we review the crystal chemistry of natural and synthetic uranyl carbonate phases. The majority of synthetic analogs of minerals were obtained from aqueous solutions at room temperature, which directly points to the absence of specific environmental conditions (increased P or T) for the formation of natural uranyl carbonates. Uranyl carbonates do not have excellent topological diversity and are mainly composed of finite clusters with rigid structures. Thus the structural architecture of uranyl carbonates is largely governed by the interstitial cations and the hydration state of the compounds. The information content is usually higher for minerals than for synthetic compounds of similar or close chemical composition, which likely points to the higher stability and preferred architectures of natural compounds.

Highlights

  • Uranyl carbonate phases play a very important role in all processes related to the nuclear fuel cycle

  • Comparison of crystal–chemical characteristics of isotypic natural and synthetic compounds can give an answer to the environmental conditions of mineral growth

  • This observation allows us to assume that the formation of natural uranyl carbonates does not need any specific environmental conditions, which was recently suggested for uranyl selenites [127] and uranyl sulfates [126]

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Summary

Introduction

Uranyl carbonate phases play a very important role in all processes related to the nuclear fuel cycle. This conjunction starts from U deposits, where uranyl carbonate minerals form during the direct alteration of primary U-bearing rocks (containing uraninite, etc.). In dissolved form, uranyl carbonates can play an important role in U release to the environment. It should not be forgotten that uranyl-carbonate mineralization has been described among the alteration products of the “lavas” that were formed during the accident at the fourth reactor of the Chernobyl nuclear power plant in 1986 [6,7]. The amount of synthetic structurally characterized uranyl carbonates is inferior to natural phases but can give an idea of the crystallization conditions present in the environment

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