Study On High Temperature Phase Transformation Of Metamict Fergusonite

The long-existing natural metamict minerals contain significant quantities of radioactive elements as a solid solution that lead to their transition from the crystalline state to the so-called“metamict”,or aperiodic.With the development of X-ray diffraction analysis technology,it is easier to understand and recover their original crystal structure,the amorphous state can be preserved in a material for long periods of geological time,so that metamict minerals are considered as the natural analogues of aged ceramic nuclear waste forms.The metamict fergusonite sample of this study comes from Jiangsu province,China.The original sample is completely amorphous due to the relatively high content of radioactive elements(the content of U was 5.50 wt.%and the content of Th was 3.15 wt.%).In this study,the original sample was broken into 5 small pieces,and separate pieces of the same grain of metamict fergusonite were annealed in air at different temperature:700?,800?,1000?,1200?.Several modern tests technologies such as X-ray diffraction,Raman spectroscopy,scanning electron microscopy,and electron microprobe were used to study its high-temperature phase transition behavior and to extract its lattice structure information,then discover the existence of rare earth elements in the samples and the information for understanding the mineral formation and transformation mechanism.Based on the experimental results of the electronic microprobe and the analysis results of the X-ray diffraction experiments,the chemical formula of the fergusonite is calculated as:(Y.Ca.W.U.Yb.Th.F.Si.Nd.Ce.Pb.).(Nb.Ti.)O.The fergusonite sample at room temperature is amorphous,and the recrystallization process is appearedin the samples annealed at high temperature.As the annealing temperature was rised,the fergusonite sample occured a continuous phase transformation.From the results of X-ray powder diffraction and Raman spectroscopy experiments,it was preliminarily determined that there were three different phases T'(special tetragonal system),T(tetragonal system),M(monoclinic system)in the fergusonite samples between700?and 1200?.The specific phase transformation process is:T'phase is appeared at the annealing temperature of 700?in the fergusonite sample,then a phase transformation T'-T is occurred between 700?and 800?;another T-M phase transformation is discovered in the fergusonite sample between 800?and 1000?.The existence of two phase transformation critical temperature t₁ and t₂ in this sample can be identified.Scanning electron microscope results show that after heating to 800°C,there are more microstructures on the dehydrated sample surface;the higher the temperature,the larger the fracture is.These characteristics may be related to the phase transformation process of the mineral.That is,the water molecules inside the sample is related to the metamictization phenomenon of the sample,but the detailed mechanism still needs to be further studied and determined.Based on the previous study of metamict fergusonite,it is acknowledged that the rare earth element Y(Yttrium)is more enriched at high temperatures.With the occurrence of phase transformation,the crystallinity of the mineral also becomes higher,and the content of the rare earth element Y displays small fluctuations.The rare earth element content is highest at the annealing crystallization temperature of 700?,which is enriched in the T'phase.As the temperature increases,the rare earth element content is the lowest during the T'phase transformation to T phase at 800?,which further indicates that the recrystallization of minerals is accompanied by the migration process of rare earth elements.