Abstract
Metamictization effects have been investigated in zircon, thorite, uranothorite, and thorogummite using X-ray absorption spectroscopy at Zr-K, Th-LIII, and U-LIII edges. Extended X-ray absorption fine structure (EXAFS) spectra of metamict samples are characterized by a major contribution due to the O nearest neighbors with some contributions from next-nearest neighbors (Si and Zr in zircon, Si in thorite). In zircon, Zr-O distances decrease by ≈ 0.1 Å while the coordination number of Zr decreases from 8 to 7. In contrast, the eightfold coordination of Th in crystalline thorite is preserved in metamict thorite. Si second neighbors around Zr or Th are generally observed in metamict samples with distances close to those measured in crystalline phases. No other contribution to EXAFS is observed in thorite, but Zr-Zr distances are observed in zircon. They decrease by ca. 0.3 Å as a function of zircon metamictization. Minor elements have been studied in one metamict zircon (Näegi, Japan): Hf and Th are eightfold coordinated whereas unusual sixfold coordinated U(IV) is observed. The location of the actinide polyhedra in metamict and annealed zircon cannot be precisely characterized with the available EXAFS data. In the thorite samples investigated, U occurs as uranyl groups which often contribute to a bright color (“orangite”).
Metamictization processes are characterizedby a loss of medium range order. There is no evidence for decomposition into crystalline oxides. Furthermore, metamict and glassy silicates do not exhibit the same local structure around Zr and Th. The structural interpretation of EXAFS data must take into account the creation of O vacancies arising from a displacement or tilting of the SiO4 tetrahedra during metamictization of zircon-like structures. If the cation can take a lower coordination number (as in the case of Zr), a coordination change allows the local structure to be partly maintained during metamictizalion. If not, as for Th, the local structure is rapidly destroyed. Significant weathering is responsible for the presence of U as uranyl complexes in metamict thorium silicates, probably enhanced by radiation damage. Intense weathering is also responsible for the decomposition of thorogummite into simple oxides.
