Investigations into the chemistry of metamict minerals


Roman V. Bogdanov


Saint-Petersburg University, St. Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg, 199034 Russia; B.P.Konstantinov Petersburg Nuclear Physics Institute NRC «Kurchatov Institute», 1, Orlova roscha mcr., Gatchina, Leningrad region, 188300, Russia



Abstract:

Metamict minerals, which were designated as such at the end of the 19th century, have been known to scientists for around 200 years. But it was not until the turn of the 21st century that intensive research was begun into the mechanisms behind their formation, their properties and the possibilities of their practical use. The main characteristic of such minerals is the absence of a crystal lattice, or, to be more exact, the loss of their crystalline properties as the result of auto-radiolysis triggered by the nuclear decay of nuclides belonging to the radioactive families of uranium and thorium. An understanding of the processes that determine the metamict disintegration of crystalline structures and also a knowledge of the fundamental properties of metamict substances offer a key to dealing with challenging issues in the nuclear power industry: developing materials for the blanket or first wall of a fusion reactor, synthesizing matrix material to immobilize highly radioactive waste, selecting preservative matrices for weapon-grade plutonium, etc. Metamict minerals are naturally-occurring analogues to nanostructures, and the preeminent focus of researchers in recent decades has been on exploring the structural changes in the atomic subsystem of metamict materials. Studying the electronic subsystem, in other words the chemical processes in metamict structures, has been accorded much less attention. But nothing could be more important, since it is this factor that determines the redox condition of actinides in matrix materials and their mobility upon contact with the liquid phase. It is also critically important to study the thermo-chemical behavior of nuclides in matrix materials since, as a result of the alpha-disintegration of actinides, the temperature in matrices can rise by several hundred degrees. In our investigations, attention was given to three chemical aspects of the metamict state: 1) developing a methodology to identify metamict minerals. The problems here are obvious, since researchers do not have two key parameters at their disposal in order to identify them: there is no crystal lattice and they do not know the original chemical composition of the mineral. 2) studying the thermo-chemical processes in a metamict mineral, more precisely the changes in the valence state of uranium and cerium in the course of isochronal annealing of the mineral from room temperature up to 900 degrees Celsius. The specified f elements are thought to be analogous to the minor actinides (Np – Cm). 3) detecting the differences in thermo-chemical behavior between the uranium isotopes U-238 and U-234. This is an important question since the light isotope of uranium reflects the behavior of radiogenic products during alpha-disintegration of the actinides in the corresponding matrix material. The proposed review report will contain a brief summary of our findings. This work is supported by the grant of RSF grant No. 14-31-00022.