Solid State NMR Investigations of Silicate Weathering
Open Access
- Author:
- Davis, Michael Christopher
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 10, 2009
- Committee Members:
- Karl Todd Mueller, Dissertation Advisor/Co-Advisor
Karl Todd Mueller, Committee Chair/Co-Chair
Alan James Benesi, Committee Member
Albert Welford Castleman Jr., Committee Member
Brian Dempsey, Committee Member
David J Wesolowski, Committee Member - Keywords:
- Kinetics
NMR
Quartz
Forsterite - Abstract:
- While many kinetic studies have focused on dissolution and precipitation reactions under far from equilibrium conditions, few studies have focused on the near equilibrium region. Here we present near equilibrium dissolution and precipitation results for quartz and forsterite. For quartz, dissolution was expected to follow a simple first order kinetic model, but quartz studies at pH 8-10 and 125-200 °C indicate a more complex dissolution mechanism not easily normalized by solution chemistry. Nucleation theory, which has been used by Dove to describe silicate dissolution, is used along with surface evolution models proposed by Bandstra and Brantley to describe near equilibrium kinetics. Unlike quartz, understanding forsterite dissolution is complicated by incongruent magnesium-to-silicon release, leached layer formation, and the precipitation of a secondary magnesium-rich phase. 29Si magic angle spinning (MAS) and 1H-29Si cross polarization (CP) MAS nuclear magnetic resonance (NMR) studies of forsterite have identified a polymerized silicon rich leached layer on the forsterite surface for experiments conducted under acidic conditions at high temperature. 25Mg MAS, multiple quantum (MQ) MAS, quadrupolar Carr-Purcell-Meiboom-Gill (QCPMG), and 1H-25Mg CP QCPMG experiments have also been utilized to characterize the two nonequivalent magnesium sites in forsterite and examine the changing magnesium environment as a result of dissolution. A NMR interrogation of hydrated albite glasses is also presented. The presence of water in silicate systems has direct influence on the physical properties of melts including transition temperature, viscosity, conductivity, and density. Therefore, studying the role of water in aluminosilicate glasses is essential in understanding a wide variety of geochemical processes. NMR is a sensitive probe of both local structure and dynamics, and in this study 27Al MAS and MQMAS NMR experiments were performed on hydrated albite glasses created from quenched silicate melts. Optimized simulations of the 2D MQMAS spectra were performed and the extracted NMR parameters (chemical shift, quadrupolar coupling) were compared with literature values in making site-occupancy assignments. These results coupled with iterative fits to MAS spectra indicate a complex bonding environment which may include unreacted albite glass (Q4-Al), aluminum terminated with a single hydroxyl group (Q3-Al), or aluminum coordinated to a protonated oxygen linking Al-O-Si bonds (Q4-Al(Obr)). We conclude that the mechanism of depolymerization contributes to the structural and physical properties of hydrous albite glass. In addition, using an empirical relationship from the literature and the integrated intensities from MAS lineshapes, local bonding arrangements were quantified as a function of hydration. In addition, studies of strontium in apatite and carbonate samples were also conducted. 87Sr QCPMG NMR results indicate the presence of two nonequivalent strontium environments corresponding to the two available sites for substitution in the crystal structure. Strontium-calcium carbonate (CaxSr1-xCO3) solid solutions were synthesized and investigated with 87Sr QCPMG NMR. Multiple strontium sites have been observed by 87Sr QCPMG NMR but the observed resonances were broadened by second order quadrupolar broadening causing significant overlap between different sites. Future studies will try to elucidate the number of sites present by combining ab initio calculations with simulations of the strontium lineshapes using the computer package SIMPSON.