Probing silicate minerals to explore mechanisms behind age-dependent dissolution rates: An NMR study of feldspars
EMSL Project ID
48321
Abstract
A method developed to quantify hydroxyl (OH) sites on silicates is applied here for the first time to feldspar grains from soils in a glacial chronosequence. Illuminating the interplay between accumulation of surface OH sites and time primary silicate minerals have spent in the weathering environment could be a step towards mechanistically identifying the reasons mineral dissolution rates are observed to decrease with time. This is of interest in the environmental sciences, because silicate dissolution has an influential role in Earth's climate. With a better understanding of the molecular-scale controls on dissolution, we can potentially address unanswered questions surrounding climate change, and perhaps contribute to emerging geoengineering research in accelerated weathering. In this study, we have sampled soil from discrete deposits--with ages ranging 10ka to 200ka--along glacial chronosequences in the Sierra Nevada and Colorado Rocky Mountains. Feldspar grains were picked from these soils and treated with (3,3,3 trifluoropropyl) dimethylchlorosilane (TFS). This molecule selectively binds to Q3 OH sites on silica and alumina tetrahedra--sites wherein one tetrahedral anion forms a hydroxyl at the surface during dissolution and the other three remain coordinated with the crystal. Nuclear magnetic resonance (NMR) spectroscopy is a sensitive, non-destructive analytical method, and we propose to use it here to quantify 19F in TFS on our treated feldspar samples. OH concentrations would be inferred from these data, and normalized to BET specific surface area. Treated samples would also be analyzed via X-ray Photoelectron Spectroscopy (XPS) to examine secondary clay phases on mineral grain surfaces. Imagery with Scanning Electron Microscopy (SEM) would also greatly benefit this study, by illuminating the extent of morphological surface features that affect weathering, such as etch pits. Samples taken through this series of analyses at EMSL would be subsequently returned to the University of Southern California, for elemental analysis to assess time-dependent mineralogical evolution. Preliminary results from NMR experiments we have already completed show a positive correlation between Q3 OH site count on feldspars and the exposure age of their host deposits. Elemental analysis confirms that mineralogical differences among samples did not dominate this trend. Our preliminary results confirm that the NMR procedure is sensitive enough to quantify concentrations of OH sites as found on feldspar surfaces, and to distinguish variability in natural samples. It is our hope that continued investigation using the NMR method will shed light on the molecular-scale processes that drive interface-limited systems.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2014-10-01
End Date
2015-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members