Nuclear Magnetic Resonance of Geological Materials and Glasses
Authors: Moran Grainne , Howe Russell F. 
Published: September 2006

This article concerns the application of nuclear magnetic resonance (NMR) spectroscopic techniques to the analysis of geological materials and glasses. It includes inorganic minerals and glasses, but does not cover soils and clay minerals to any extent. The first part presents an overview of solid‐state NMR experiments applicable to inorganic solids. The range of nuclei accessible by NMR and the particular requirements for obtaining good spectra of more difficult nuclei, including quadrupolar nuclei, are discussed. The experimental conditions under which solid‐state NMR can provide quantitative analytical measurements are also considered.

NMR spectroscopy provides element‐specific speciation and structural information including coordination environment and bond distances. It is particularly valuable for the analysis of amorphous and partially crystalline materials not amenable to structure determination by X‐ray diffraction. NMR can detect and quantify materials containing multiple phases and can be used for in situ monitoring of phase transitions. It provides qualitative and quantitative speciation information for framework nuclei in glasses and is also capable of characterizing the dynamic behavior of mobile ions and solvent through relaxation, diffusion and chemical exchange experiments. Thus it has become an important technique for the study of hydrous minerals and gels and for ion‐conducting glasses.

The sensitivity of NMR varies considerably, depending on the nucleus observed, its abundance and its chemical environment. NMR sensitivities and detection limits cannot compete with elemental analytical techniques such as X‐ray fluorescence. In addition, quantitation in solids requires careful control of experimental parameters and careful calibration to avoid artifacts. However, the selectivity of NMR as an analytical technique is unsurpassed and the development of specialist pulse sequences to measure specific local interactions in solids makes it particularly well suited to the analysis of geological materials and glasses.

NMR imaging enables structural information to be resolved in three dimensions, although resolution in solid‐state imaging is limited to tens of micrometers at best owing to the broader line widths and lower signal‐to‐noise ratios compared with liquids. However, capabilities in this field are being extended all the time and the application of NMR imaging techniques in materials analysis is expanding steadily. An alternative to conventional imaging is NMR force microscopy, which shows potential for surface mapping at higher resolutions.

The applications section is organized on the basis of the materials being analyzed. Within each section, NMR techniques are classified according to the observed nuclei. The emphasis is on more recent applications and on developments leading to enhanced resolution and sensitivity. Silicates and aluminosilicates represent the largest class of applications, followed by phosphate, borate and other oxide glasses. Sol–gel materials and the reactions leading to gel formation are also discussed, as are hydrous materials more generally. The other important areas of application reviewed are minerals, ceramics, high‐temperature melts and ion‐conducting glasses. Finally, the applications of NMR imaging in this field, although sparse at present, are potentially very great. This is illustrated by two major areas of application to date, namely the imaging of bone minerals and cement and concrete.