Mössbauer Spectroscopy of Metamict Allanite From Ytterby (Sweden) Annealed at 973, 1073, and 1173 K in Argon

The effect of annealing at temperatures of 973, 1073, and 1173 K on fully metamict allanite-(Ce), (Ce,Ca,REE)2(Fe2+,Fe3+)(Al,Fe3+)2O[Si2O7][SiO4](OH) was studied using Mössbauer spectroscopy and powder X-ray diffraction. The Mössbauer spectra of the untreated sample and the samples annealed at 973, 1073, and 1173 K can be fitted to three Fe2+ doublets and one Fe3+ doublet. The relative contribution of ∑ Fe2+/∑ Fe in the Mössbauer spectra increases from 0.72 for the untreated sample to 0.86 for the sample annealed at 1173 K. The X-ray diffraction pattern of the fragment annealed at 1173 K indicates that the fully metamict allanite-(Ce) recrystallized into an apatite-like phase, and not into the crystalline phase of this mineral.