Solid State Geochemistry: High Resolution Transmission Electron Microscopy of Minerals

Arizona State University, Scottsdale AZ

Investigators

Abstract

Buseck EAR-0003533 The goal of this project is to examine chemical and structural deviations from ideality in geologically and environmentally significant minerals. Such deviations can hold important clues to geological processes and to the geological past. Electron diffraction, electron spectroscopy, high-resolution TEM imaging, computer simulation of images, and calculation of structure models will be combined to solve problems of micro- and nanocrystalline minerals. The serpentine minerals are important constituents of ultramafic rocks underlying the oceanic basins. They are likely significant carriers of water into the mantle during subduction; the release of that water promotes partial melting and magmatism in the overlying mantle wedge, leading to volcanism. Subducted serpentines also influence the development of earthquakes at depth. Given their tectonic importance, it is important to obtain as much fundamental knowledge as possible about them. The proposed research will resolve structural problems regarding antigorite and chrysotile serpentine, with potential geophysical implications. Garnets have nominally cubic symmetry, but many display non-cubic properties. Preliminary HRTEM and XRD measurements suggest cation ordering as the source of their lower than cubic symmetry. These data will be refined and, if confirmed, be used to produce a model of chemical ordering to explain their anomalous characteristics. Many minerals that are of great environmental importance form at or near Earth's surface. Knowledge of the structural states of poorly crystallized materials is important for understanding weathering, heavy-metal deposition, and soil formation. These low-temperature assemblages typically occur in intimate, heterogeneous mixtures of fine-grained minerals. Using electron nanodiffraction (END), it is possible to study volumes of a few nm3 - many orders of magnitude less than by any other diffraction method. END will be used to investigate the hydrous ferric oxide minerals that are collectively called ferrihydrite and their biological cousin ferritin and materials such as clays that suffer rapid radiation damage during electron microscopy.