57Fe Mössbauer annealing studies of fully metamict gadolinite from Ytterby (Sweden) have been carried out in air from 373 to 1473 K. The Mössbauer spectrum of the untreated sample, dated at 1795 Ma and absorbed α-dose 1.07 × 1016 α-decay/mg, is characterized by two broad peaks solely from Fe2+ components in octahedral coordinations. The corresponding quadrupole splitting distribution (QSD) shows two distinct maxima at 1.59 and 2.16 mm/s, which reflect the post-metamictization distributions of Fe2+ positions up to heating at 773 K. Changes in the Mössbauer hyperfine parameters are observed only after heating to 873 K, at which point the first Fe3+ component from the oxidation of Fe2+ appears. The relative contribution of Fe3+/∑Fe reaches a maximum of 0.54 at 1073 K and then decreases with increasing temperature. A new Fe3+ doublet is observed in the samples heated from 1173 to 1473 K. This doublet is characterized by extremely high quadrupole splitting ranging from 2.51 to 2.66 mm/s, implying extreme distortion of the coordination octahedra in partially and fully crystalline gadolinite. The transition from metamict to crystalline structure begins at 1133 K where the Mössbauer spectrum is represented by one Fe2+ doublet and one Fe3+ doublet. Both doublets suggest a uniform transitional octahedral site over the whole structure. With increasing annealing temperature, the line widths of the Fe2+ doublet decrease, and, at high temperatures, an asymmetry of the absorption peaks is observed. The sample becomes completely recrystallized after annealing at 1373 K, and at this point the Mössbauer spectra are nearly exact superpositions of the pure Fe2+ and Fe3+ (present in minor proportion) in synthetic gadolinites.

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