SUB-NANOMETER DETECTION OF HYDROGEN IN MINERALS USING VIBRATIONAL ELECTRON ENERGY LOSS SPECTROSCOPY

The circulation of hydrogen is crucial for understanding the evolution and dynamics of Earth's habitable environment. In the solid part of the Earth, hydrogen can be stored within the crystal structures of various mineral phases. As significant storage mechanisms occur at the atomic scale, probing methods with sufficiently high resolution are essential. However, achieving sub-micron resolution for hydrogen measurements remains exceptionally difficult.

Infrared (IR) spectroscopy has extreme sensitivity to H₂O at ppm level. Nevertheless, the relatively large beam size necessitates sample sizes of tens to hundreds of micrometers. While micron scale resolution can be achieved in Raman spectroscopy, intensely focused laser beams can damage samples. These limitations are significant for analyzing individual minerals in multi-phase samples and sub-micron-scale mineral inclusions.

Recent advancements in Scanning Transmission Electron Microscopy (STEM) have enabled vibrational electron energy loss spectroscopy (vibEELS). which allows for the detection of vibrational modes of hydrogen and lattice modes with spatial resolutions of < 1 nm. Moreover, vibEELS can be conducted with the beam positioned outside sample grains (aloof configuration), thereby avoiding damage to samples.

At Arizona State University, we are developing workflows to conduct vibEELS on minerals using the NION UltraSTEM, along with data analysis methods to extract structural and quantitative information. In this presentation, we will showcase vibEELS of hydrogen in various classes of minerals: hydrous minerals (brucite and serpentine), nominally anhydrous minerals (ringwoodite and stishovite), and amorphous phases (hydrous silica glass and synthetic silicate melt). The results show OH and H₂ modes consistent with known IR and Raman active modes. Additionally, we robustly observed lattice modes, which can be used for phase identification simultaneously with the hydrogen measurement.

This technique will introduce new measurement capabilities, including water partitioning between minerals, structural analysis using single crystal spectroscopy, and nanometer resolution hydrogen content mapping. Its use of extremely small sample amounts will enable hydrogen measurements from samples returned from space missions and small mineral inclusions from a range of Earth's environments.