Composition, Geochemical Alteration, and Alpha-Decay Damage Effects of Natural Brannerite

Abstract

To investigate the long-term alteration behavior of brannerite, we have undertaken a study of twelve natural samples from a range of geological environments. Our results indicate that seven of the samples exhibit only minor alteration, usually within veinlets or around the rim of the sample. The remaining five samples consist of variable amounts of unaltered and altered brannerite. SEMEDX analyses of unaltered areas indicate that the chemical formulae may deviate from the ideal stoichiometry. The U content ranges from 0.45 to 0.88 atoms per formula unit (pfu). Maximum amounts of the other major cations on the U-site are 0.48 Ca, 0.22 Th, 0.14 Y, and 0.07 Ln (lanthanide = Ce, Nd, Gd, Sm) atoms pfu. The Ti content ranges from 1.86 to 2.10 atoms pfu. Maximum values of other cations on the Ti-site are 0.15 Fe, 0.14 Si, 0.09 Al, 0.06 Nb, 0.04 Mn, and 0.04 Ni atoms pfu. Altered regions of brannerite contain significant amounts of Si and other elements incorporated from the fluid phase, and up to 40-90% of the original amount of U has been lost as a result of alteration. SEM-EDX results also provide evidence for TiO₂ phases, galena, and a thorite-like phase as alteration products. Electron diffraction patterns of all samples typically consist of two broad, diffuse rings that have equivalent d-spacings of 0.31 nm and 0.19 nmi, indicating complete amorphization of the brannerite. Many of the grains also exhibit weak diffraction spots due to fine-grained inclusions of a uranium oxide phase and galena. Using the available age data, these samples have average accumulated alpha-decay doses of 2-170 × 10¹⁶ alphas/mg. Our results indicate that brannerite is subject to amorphization and may lose U under certain P-T-X conditions, but the overall durability of the titanate matrix remains high.