Nanoscale Characterization of Earth & Environmental Materials: Exploring New Occurrence and Processes

A central challenge in Earth and environmental sciences is identifying and characterizing trace elements—often present at ppm levels—at atomic resolution. Recent advances in transmission electron microscopy (TEM), particularly high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), allow direct visualization of heavy atoms due to strong atomic-number (Z) contrast. By combining HAADF-STEM with STEM-EDX, EFTEM, and high-resolution TEM, we have significantly expanded the analytical power of electron microscopy in geochemistry.

One major achievement was the first atomic-scale characterization of lead (Pb) in 4.4–3.1 Ga zircon from the Archean Yilgarn Craton, Australia. We identified two distinct forms of Pb: (1) nanoscale domains (~3 atom%) structurally substituting for Zr⁴⁺ in crystalline zircon, and (2) Pb concentrated within amorphous regions created by radiation damage. These results demonstrated that radiation-induced amorphization forms fast diffusion pathways, fundamentally refining models of Pb mobility and the reliability of zircon geochronology.

We also achieved the first direct imaging of radioactive cesium (Cs) atoms in environmental materials. In Cs-rich microparticles from the Fukushima Daiichi accident, we identified zeolitic pollucite inclusions containing up to 36 wt% Cs₂O. Atomic-resolution HAADF-STEM imaging along the [111] zone axis revealed ordered arrays of Cs atoms, consistent with multi-slice image simulations. These results demonstrate that locally enriched Cs reacted with siliceous materials during reactor meltdown through volatilization and condensation processes. Despite intense beta radiation doses (10⁶–10⁷ Gy), the pollucite structure remained crystalline, indicating remarkable radiation tolerance. This breakthrough provides fundamental insight into the structural state and long-term behavior of radioactive Cs in nuclear accident environments.

Beyond these discoveries, we have visualized uranium nanocrystals in atmospheric particles, gold nanoparticles in sulfide minerals, rare arsenic-bearing nanophases, and radiation damage textures in candidate nuclear waste form minerals.

Our ongoing research integrates atomic-resolution TEM with NanoSIMS and synchrotron-based micro–X-ray analyses to enable simultaneous structural, chemical, and isotopic characterization across scales. By pushing analytical boundaries, we aim to uncover fundamental geological processes and establish new methodological frontiers in nanogeoscience and environmental mineralogy.