57Fe Mössbauer Spectroscopy and X-ray Diffraction of Annealed Highly Metamict Perrierite: Activation Energy and Recrystallization Processes

Abstract

This paper presents the results of 57Fe Mössbauer spectroscopy and X-ray diffraction analysis of highly metamict perrierite (REE,Ca,Th)4(Fe2+,Mg)2(Ti,Fe3+)3Si4O22 after annealing in argon from 673 to 1273 K for one hour. Radioactive elements in metamict minerals damage crystal structure on geologic time scales primarily due to recoil nuclei from α-decay of 238U, 232Th, 235U, and their daughter products. Metamict minerals are widely used in geochronology and can serve as natural analogs for the study of radiation effects in high-level nuclear waste. Analyses were performed on fragments of a perrierite sample collected from granitoids near Amherst, Virginia (USA). Electron microprobe and gamma-ray spectrometry recorded Fe concentrations of 4.7 wt.% and Th and U concentrations of 0.64 and 0.06 wt.%, respectively. The calculated total absorbed α-dose was 7.8 × 1015 α-decay mg−1. The Mössbauer spectrum of the untreated sample can be fitted to two Fe2+ and two Fe3+ doublets in octahedral coordination with a relative ΣFe2+/ΣFe of 0.63. For samples annealed at 1173 K and 1273 K, spectra show a decrease in the total contribution of Fe2+ to 0.58 due to dehydroxylation associated with the simultaneous oxidation of post-metamict Fe2+ to Fe3+. In the examined perrierite, Fe2+ occurs in structural positions B and C(1). The broad, predominant Fe3+ doublet observed in the spectrum of the unannealed sample splits into two components at 973 K interpreted to represented positions C(1) and C(2) in the perrierite structure. The Mössbauer spectra show a prominent decrease in the width of the high-energy absorption peak representing Fe2+ components with increasing temperature. The variation in the width of this peak versus the annealing temperature seems to be an indicator of thermally induced recrystallization. Based on the exponential dependence of the derivative function of the parameter with the inverse temperature and using an Arrhenius plot, an activation energy (EA) of 0.73 eV was determined for thermally-induced recrystallization. Corresponding XRD data show progressive recrystallization with increasing annealing temperature. The XRD pattern of the fragment annealed at 1273 K indicates that highly metamict perrierite recrystallized to the pre-metamict state that can be indexed to the C2/m space group.