Atomic-level structure of silicon and aluminum in natural and synthetic minerals
EMSL Project ID
2416
Abstract
Silicon and aluminum are ubiquitous in Earth?s crust. They are often associated with organic matter in high concentrations, as in petroleum source and reservoir rocks, and in low concentrations, as in biofilms in igneous and sedimentary rocks. Silicate solids are known and exploited for their ability to concentrate and convert organic materials. Yet the inverse relationship, that of organic matter on the structure of the silicate solids, is largely inferred and mostly unknown. Further siliceous rocks host the earliest and best-preserved microfossils. Fundamental questions about early Earth could be addressed through study of these ancient rocks but many biological, chemical, and physical signatures are lost during diagenesis. It is possible, and seems likely, that living organisms would impart some structural signature to silica that may be preserved in the geological record. Atomic defects (silanol groups, non-bridging oxygen) affect the reactivity, energetics, and oxygen exchange rates of silicate minerals. Understanding the origin and persistence of these defects would provide insight into the differences between diagenetic and primary quartz and hydrous silica phases. Rapidly crystallizing phases would be expected to exhibit more of these defects and may show rapid exclusion of accompanying network modifiers during very early diagenesis. Certain types of defects may be characteristic of the presence of organic matter. We suggest these phenomenon are related and that the nature of this relationship is dictated by the atomic environment of silicon and accompanying ions. Herein, we propose to use a combination of field and laboratory studies to investigate the ultrastructure of silica over a range of thermal environments using solid-state CP-MAS NMR. We will focus on natural solids that are nominally pure silica phases, specifically very recent silica-rich, thermal-spring sediments. Thermal deposits contain high concentrations of silicon, and low concentrations of other elements. Organic matter content is expected to vary from low to moderately high, depending on the temperature and type of deposit: high temperature thermal deposits have low organic carbon contents while organic carbon content progressively increases with decreasing temperature. The organic matter is expected to be a combination of living microorganisms and, at least at high temperature, dissolved organic matter scavenged from solution. Hence a record of the thermal and geochemical environment of formation will be preserved in solid form along the cooling gradient. This signature has the potential to distinguish thermal and chemical environments through structural distinctions in authigenic and diagenetic minerals. Samples of modern and ancient deposits from Yellowstone National Park, Nevada, Kamchatka, and Australia are available at the University of Montana and will be examined. Dr. Nancy Hinman will have primary responsibility for the contextual framework for sample selection and spectroscopic data interpretation. Graduate student, Elizabeth McKenzie, will have primary responsibility for collection of NMR spectra and microscopic characterization. Collaborators, Drs. James McKinley and Herman Cho, both of Pacific Northwest National Laboratory (PNNL), will assist with electron microscopy and NMR spectroscopy, respectively. The analyses will be performed at the PNNL Environmental and Molecular Science Laboratory in Richland, WA.
Project Details
Project type
Capability Research
Start Date
2002-01-03
End Date
2004-09-29
Status
Closed
Released Data Link
Team
Principal Investigator