Microstructural analysis of pitchblende in the Miaoershan area using TEM lattice imaging

Abstract

This paper aims to reveal the formation process of pitchblende by analyzing its microstructural features on the nanoscale and atomic scale. Herein, we selected pitchblende samples from drilling cores from the Miaoershan area, southern China, as the research objects. The microstructural features of pitchblende particle cores were analyzed using high-resolution transmission electron microscopy (HRTEM), electron diffraction, energy spectrum (EDS) and electron microprobe analysis (EMPA), combined with density functional theory calculations. The results of this study reveal that the pitchblende grain boundaries are not clear, as they exhibit a cloudy distribution and some preferred orientations under HRTEM. The selected area electron diffraction (SAED) analyses show discontinuous ring patterns that are coherent with the presence of a preferred orientation. The HRTEM results reveal that pitchblende is a complex aggregate composed of nanocrystals with a diameter of 3–7 nm and unit cell parameters of a0 = 0.556 nm. The elements in the pitchblende are characterized by an irregular and scattered distribution of Ca, As, S, Fe, Mn, and Si. We assume that the formation of pitchblende is related to hard-base fluid in a reduced environment containing As and S. We used density functional theory calculations to obtain the pitchblende crystal structure model and simulated the positions of the U and O atoms to provide a reference for further analysis of the pitchblende crystal structure. We conjecture that uranium ions of U4+, U5+ and U6+ may coexist in pitchblende (stoichiometry U3O7).