Impact of actinide-induced crystal defects on REE-bearing titanite reactivity

Abstract

Titanite is a widespread accessory silicate which, in some deposits, hosts rare earth elements (REEs) at levels where it can be considered an important carrier. As titanite also accommodates Th and U at concentrations up to 2000 ppm, evidence of metamitctization, a process where radioactive decay of the actinides progressively damages the crystalline structure, is often observed. The current study aims to investigate how the extent of metamictization within a natural titanite specimen may impact its reactivity, principally in the context of REE recovery. Detailed structural characterization revealed the presence of crystalline domains coexisting with sectors having a dominant amorphous component. Compositionally, the density of irradiation-induced defects, leading to amorphization, shows a strong positive correlation not only with the abundance of actinides, but also with the REE concentration. Heat treatment of titanite performed at temperatures ranging from 400 to 800 °C indicates that, while the compositional zonation is maintained, thermal annealing of the defects results in progressive recovery of the crystalline structure. After treatment in sulfuric acid at 200 °C, the habit of reacted titanite grains is highly irregular, indicating distinct decomposition rates for specific sectors, where the more refractory regions coincide with the REE-depleted crystalline domains. A more detailed visualization of the impact of crystal properties on titanite reactivity, obtained by exposing polished grains to a 50% NaOH solution at 50 °C, reveals that, for pristine titanite, the extent of etching observed on crystal surfaces is controlled by the density of the actinide-induced defects, mainly concentrated in the REEs- and Th-enriched amorphous domains. On the other hand, for grains that were initially heat-treated, the etching features are much weaker, emphasizing that thermal annealing renders the crystals less reactive. From these observations, selective REE recovery, where only the more accessible amorphous titanite domains need to be decomposed, seems possible as long as the ore pre-treatment does not involve a high-temperature stage.