Hydrous Components in Nominally Anhydrous Phases

California Institute Of Technology, Pasadena CA

Investigators

Abstract

Hydrogen exists as a minor component in numerous crystalline substances as molecular water and hydroxide ions, the so-called hydrous components. Minerals with these components include those that make up the Earth and synthetic industrial analogues that are of technological importance. The hydrogen in Earth’s minerals account for the vast majority of our planet’s hydrogen and has a profound effect on the behavior of Earth’s interior. Similarly, hydrous components can significantly modify the material properties of synthetic materials, such as semiconductors. Despite the clear importance of hydrogen in these contexts, there remain two fundamental uncertainties: 1) where does the hydrogen incorporate into crystal structures and 2) at what concentrations hydrogen is present. This work will address both issues through a comprehensive approach of both laboratory experiments and computer simulations. This will include development of analytical techniques for hydrogen detection, detailed spectroscopic studies, and interpretation of experimental data from quantum mechanics calculations. In addition to continuing work on previously studied silicate minerals (the most abundant minerals in the earth), the research will also focus on binary oxide minerals. Binary oxides are not only geologically relevant, but also have extreme relevance in many established and emerging technologies, ranging from solar cells, to lasers, touch screens, pigments, semiconductors, and flat screen displays. Hydrogen impurities have been shown to play a key role in these settings. Thus, this research will not only be important for understanding the inner workings of our planet, but will also be significant for the development of new technologies. It has long been understood that hydrous components (OH- and H2O) in nominally anhydrous minerals (NAMs) are of great importance in earth science. These phases, which include abundant mantle phases such as olivine, garnet, pyroxene and ringwoodite, are likely the largest reservoir for hydrogen in the Earth. The trace hydrogen in NAMs often has outsized effects on a wide range of their material properties, from melting points, to mechanical deformation, thermal and electrical conductivity, color, radiation stability. Despite the clear significance of hydrogen in NAMs, there persist two fundamental issues. First is the quantification of hydrogen concentrations in these phases. Numerous techniques have been attempted or utilized over the last few decades including nuclear profile analysis and SIMS measurements, but the technical challenges related to measuring trace hydrogen concentrations in NAMs have proven difficult to overcome. Second is the identification of specific hydrogen sites in NAMs. Although some defect sites have been identified in specific NAMs such as garnets, there is little recourse for establishing the detailed defect structures in most phases. Our proposed solution to these issues is a holistic approach that considers both established and emergent techniques for measuring trace hydrogen in minerals, and quantum mechanics (density functional theory) calculations. These research components will be linked through infrared spectroscopy of oriented crystals, a technique that has proven fundamental in the study of NAMs. Our work will initially focus on simple oxide minerals such as rutile and stishovite, a group of phases that has typically been underrepresented in research into NAMs, but whose study could greatly benefit the field as a whole, with additional benefits to the development of technological materials. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.