Nuclear Magnetic Resonance Spectroscopy of Geological Materials
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the basic concepts of solid-state NMR, and applications to crystalline and glassy silicates^7 as well as NMR at high temperature have been reviewed recently.8 NMR is unusual in its range of applicability because it provides information on the short- and intermediate-range structure around nuclides of most of the elements, and it can often be highly quantitative. It is particularly useful in characterizing the intermediate-range structure of disordered or amorphous solids, including glasses. Some NMR line shapes, such as fully averaged liquid lines and quadrupolar and chemical shift patterns in solids, are well understood. However, as for other techniques, interpretation of the broadening of these patterns by disorder often relies on assumptions of Gaussian distributions that may be difficult to test. In addition, much interpretation of spectra relies on empirical correlations among structure and chemical shift, and thus can be subject to errors and ambiguities. However, theoretical approaches to predicting spectra are progressing rapidly.9 The following examples describe crystalline and glassy solids, and molten silicates. NMR studies of crystalline silicates have been of major importance in defining thermodynamically crucial structural details such as ordering state, and in calibrating techniques for unknown structures. Silicate glasses have long been studied by geochemists as quenched equivalents of the magmas that are the precursors of igneous rocks. Although a glass records the structure of the liquid only at the glass transition temperature (usually many hun-
dreds of degrees below the melting temperatures of interest in most of petrology), studies of glasses remain a necessary starting point. NMR on glasses has recently been extended to in-situ, high-temperature studies of the liquids themselves.
Short-Range Structure The isotropic chemical shifts (8^) for a number of well-studied nuclides, usually determined for solids from magic angle spinning (MAS) spectra, are controlled primarily by variations in the extent of paramagnetic deshielding of the nucleus. Several of these nuclides, such as ^Si and 27 Al, are of particular interest in inorganic geochemistry and materials science. In general, structural changes that induce more ionic bonding and a greater positive charge on the nucleus result in decreased chemical shifts (lower resonant frequencies, usually plotted farther to the right in spectra). If the first-neighbor anions are all oxygen atoms, the greatest effect of this sort is caused by changes in the coordination number. For example, the s Si chemical shift for [4]-coordinated Sipi™) ranges from about —65 to about —120 ppm, and for [6]-coordinated cation (SiVI) from about -180 to -220. Halfway between should lie Siv, which is known from a few organic molecules loand was recently discovered in silicate glasses.11"13 The most common coordination number for Si in the Earth is six. A major constituent of the lower mantle of the Earth is, for example, the perovskite-structured phase of MgSiO3. However, with
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