Joint Chemistry-EPS Colloquium: Brian Phillips

Interrogating impurities in carbonate minerals with NMR spectroscopy

The calcium carbonate minerals, ubiquitous in Earth's sediments, typically contain a multitude of impurities at trace to minor concentrations. These impurities can yield physicochemical information about the environment in which the mineral formed. High-resolution microanalytical and imaging methods now enable chemical and isotopic analysis at sub-millimeter resolution, enabling fine temporal resolution of chemical and isotopic variability useful as paleoenvironmental proxies. In some cases however, the nature of such impurities and their relationship to the mineral structure is unknown, making uncertain the resiliency of the trace element record to alteration and robustness of proxy relationships. NMR spectroscopy offers unique capabilities for investigating such questions, through both spectral fingerprinting and probing interactions between impurity and the host structure. I will present the results of recent studies using these methods that touch on the nature of phosphorus in calcite and aragonite, the coprecipitation of large organic molecules in calcite, and the distribution of boron species in calcite. Our results show that the carbonate mineral structure can accommodate impurities as incongruent defects, but also that heterogeneity in the mode of incorporation occurs that may contribute to proxy offsets.