Abstract
57 Fe Mossbauer annealing studies of fully metamict gadolinite from Ytterby (Sweden) have been carried out in air from 373 to 1473 K. The Mossbauer spectrum of the untreated sample, dated at 1795 Ma and absorbed α-dose 1.07 × 10 16 α-decay/mg, is characterized by two broad peaks solely from Fe 2+ 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 Fe 2+ positions up to heating at 773 K. Changes in the Mossbauer hyperfine parameters are observed only after heating to 873 K, at which point the first Fe 3+ component from the oxidation of Fe 2+ appears. The relative contribution of Fe 3+ /∑Fe reaches a maximum of 0.54 at 1073 K and then decreases with increasing temperature. A new Fe 3+ 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 Mossbauer spectrum is represented by one Fe 2+ doublet and one Fe 3+ doublet. Both doublets suggest a uniform transitional octahedral site over the whole structure. With increasing annealing temperature, the line widths of the Fe 2+ 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 Mossbauer spectra are nearly exact superpositions of the pure Fe 2+ and Fe 3+ (present in minor proportion) in synthetic gadolinites.
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