Solid state NMR study of minerals and glasses : application of off-resonance mutation spectroscopy

Abstract

In this thesis solid state NMR experiments on a variety of minerals and glasses are described with the aim of studying the dependence of their structure on the chemical composition. Chapter 1 introduces the reader to the subject. Chapter 2 describes 23Na and 27Al NMR experiments on a number of alkali fluoroaluminates. These substances consist of AIF6 octahedra which can be linked together to form a variety of structures, similar to the way in which Si04 and AI04 tetrahedra are linked together in aluminosilicates. The 23Na and 27Al chemical shifts correlate well with the type of structure. AIF3 is a network structure and has the most negative 27Al chemical shift (-16 ppm). The chemical shift increases from -3 and -1 ppm in the sheet structure chiolite (NasAI3F14) to 0 ppm in cryolite (N~AIF6)' The 23Na chemical shift is dependent of the number of fluorines coordinating Na: Na in an NaF6 group has a more positive chemical shift than Na in NaFs. Additionally, 27Al chemical shifts for oxides are shown to be at least 20 ppm less shielded than those in similar fluorides. Chapter 3 studies pentameric (Si,Al)s016 groups with (almost) straight intertetrahedral AI-O-Si angles in a zunyite with excess Al and in harkerite. The excess Al in the zunyite lattice is shown to occupy the central tetrahedral Si(l) site. The 27Al chemical shift correlation with the AI-O-Si angle is extended to 1760 Chapter 4 reports the first application of off-resonance nutation spectroscopy to five alkali aluminosilicate glasses. These glasses have a chemical composition similar to that of a granite, with the alkali's replaced by Li, Na, K, Rb, and Cs. Off-resonance nutation spectroscopy, newly developed at the HF-NMR facility in Nijmegen, proves to be very powerful in determining the quadrupole interaction and subsequently the isotropic chemical shifts for Al in these glasses. The quadrupole interaction and the chemical shift decrease when the size of the charge-balancing counterion increases. The correlation of the 27Al chemical shift with the AI-O-Si angle for framework aluminosilicate structures (see also chapter 3) is used to show, that the size of the alkali ion determines the size of the rings building up the glasses. Since the quadrupole interaction for Al is linearly correlated with the polarising power of the alkali ion, the aluminium and the alkali ions are believed to be closely bound. This is supported by a theoretical ionic model predicting this correlation. Chapter 5 investigates the use of corundum (a-AI20 3 ) as a standard in quantatative NMR measurements. As AI is a quadrupolar nucleus, a number of requirements have to be fullfilled in order to obtain reliable results. Most importantly, only the central transition is excited by using a very short rf pulse in order to be in the region of linear response. The intensities of five natural and synthetic corundum samples are compared. Despite the fact that all experimental conditions are identical and in the linear region, the intensities differ considerably. A synthetic corundum obtained by heating Al(OH)3 has the highest intensity, close to the expected value, but the other samples have much too low intensities. Relaxation effects are shown to be responsible for this. It is proposed, that only in case of a wellcharacterized corundum sample without impurities a-A120 3 can be used as a standard for quantitative NMR measurements.