Minor element distribution and site preference in geological and technological crystals [electronic resource]
Keywords: Geological and Environmental Sciences.

This thesis consists of an introductory chapter, three chapters utilizing paramagnetic NMR to investigate dilute concentrations of transition metals or lanthanides, a chapter using NMR to directly observe incompatible trace elements in mantle minerals, and a final concluding chapter. It presents both new data and new approaches to investigating low concentrations of paramagnetic and diamagnetic species in inorganic materials. The observations and conclusions presented further develop our understanding of minor element distributions and site preferences in geological, optical, catalytic, and cement minerals. Chapter 1 frames the thesis by introducing the broad value of understanding trace elements as well as the challenging nature of investigating low concentrations of elements. A brief overview of the primary experimental technique, NMR, is given. A complete description of paramagnetic NMR, in which the effects of unpaired electron spins can be observed in the spectra of neighboring ions, follows, including formulae necessary to predict paramagnetic interactions. Chapter 2 presents published work using 29Si NMR to investigate forsterite (Mg2SiO4) containing 0.05 to 5 % of the Mg2+ replaced with Ni2+, Co2+, or Fe2+. In marked contrast to the typically narrow single resonance of the pure material, the materials with paramagnetic dopants display 4 to 26 additional small, fast relaxing peaks. By comparing the forsterite peaks with those of similarly structured monticellite (CaMgSiO4) and integrating the peak areas, we determined the transition metals' site preference, which can occupy either the M1 or M2 forsterite sites. In forsterite, Ni2+ occupies only M1, Fe2+ occupies M1 and M2 roughly equally, and Co2+ occupies both M1 and M2 in an approximately 3:1 ratio. These findings for low concentrations support expectations from previous studies of olivines that use other methods (e.g. XRD) with much higher transition metal cation contents. This work also demonstrates some of the limits of paramagnetic NMR, as spectra with very low (e.g. 0.1%) concentrations of Mn2+ or natural concentrations of Fe2+ produced low information content spectra. In chapter 3, which also describes published work, we observe paramagnetic shifts produced by 0.3 to 2.2 mol % NiO or CoO in CaO or MgO using 17O, 25Mg and 43Ca NMR. Systematic paramagnetic shifts were identified for both dopants (Ni2+ or Co2+) and both bulk materials (CaO and MgO), which in some cases could be assigned to Ni2+ or Co2+ with specific relationships relative to the observed nucleus. Using a "random mixing" peak area model, we compared the observed spectra with modeled spectra and determined that Co2+ and Ni2+ are randomly distributed within MgO but avoid other transition metals in CaO. Chapter 4 demonstrates the approach of using paramagnetic NMR to investigate the dopant distribution in optical materials, as a technique to complement optical spectroscopy. Paramagnetic shifts are identified for Ce3+, Nd3+, Yb3+, Tm3+, and Tm3+-Cr3+ in 27Al and/or 89Y NMR spectra of Yttrium Aluminum Garnet (YAG). In some cases identified peaks are assigned to specific lanthanide-observed nuclide spatial relationships, and dopant lanthanides appear to follow a random mixing model. In the 27Al spectra, we identify peak splitting of the furthest shifted AlO6 peak resulting from two Nd3+ neighbors, potentially indicating magnetically coupled spin states. We identify systematic changes in the spectra that relate to known lanthanide parameters, such as larger shift distances when the expectation value of electron spin is larger. Chapter 5 investigates diamagnetic systems with low concentrations (70 to 7000 μg/g) of added incompatible Al3+ or Sc3+ using 27Al or 45Sc NMR with the ultimate goal of constraining the site preference and substitution mechanisms of Al3+ in forsterite. With the support of the SIMPLISMA algorithm, trace Al3+ species are identified in MgO, CaO, larnite, clinoenstatite, and forsterite. As an analog to Al3+, Sc3+ is similarly investigated in MgO and forsterite, and is found to prefer exclusively six coordinated sites, with a relatively simple single site preference. In marked contrast, the spectra of Al3+ in forsterite display an AlO4 site and at least three AlO6 sites. The identified peaks indicate that at least two substitution mechanisms affect Al3+ incorporation in forsterite. In larnite (the natural mineral name of the Portland cement component belite), Al3+ is detected only in SiO4 sites. Synthesized enstatite samples, primarily composed of clinoenstatite, display aluminum distributions similar to previous orthoenstatite work. In periclase, Al3+ and Sc3+ is observed in the cubic six coordinated Mg site, and a secondary distorted AlO6 species is also observed. Chapter 6 summarizes the previous chapters' findings and comments on the broader implications of this work.