This paper presents the results of 57Fe Mössbauer spectroscopy, SEM-EDS analysis, gamma-ray spectrometry, and X-ray diffraction (XRD) patterns for an unknown metamict phase (UMP) from a syenite pegmatite at Tuften quarry, southern Norway. The sample exhibits 232Th and 238U activities of 137 and 2.6 Bq g–1, respectively, and a calculated total absorbed α-dose of 8 × 1015 α-decay mg–1. Its chemical composition falls generally between chevkinite-(Ce)–perrierite-(Ce) and allanite-(Ce)–ferriallanite-(Ce) mineral compositions. The Mössbauer spectrum of an untreated UMP sample can be fitted to two Fe2+ and Fe3+ quadrupole doublets assigned to octahedral coordination with a relative Fe2+/ΣFe ratio of 0.11. A sample of the UMP was also annealed in argon for one hour at 1273 K. Powder of the completely recrystallized sample was subjected to XRD analysis and indexed to the P121 space group with unit-cell dimensions of a 8.179 Å, b 14.16 Å, c 4.291 Å, and β = 96.71°. The corresponding Mössbauer spectrum is characterized by the presence of three quadrupole doublets also assigned to Fe2+ and Fe3+ in octahedral coordination with a relative Fe2+/ΣFe ratio of 0.15. One of the Fe3+ doublets shows extremely high quadrupole splitting of 2.60 mm s–1, implying extreme distortion of the coordination octahedra.

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