Focused Ion Beam (FIB) combined with SEM and TEM: Advanced analytical tools for studies of chemical composition, microstructure and crystal structure in geomaterials on a nanometre scale

Abstract

In order to make meaningful interpretations of trace element and isotopic information from geomaterials, prior characterization of the material of interest with respect to structural and chemical homogeneity, radiation damage, and microstructure is often required. Transmission electron microscopy (TEM) is a powerful analytical tool that may be used to fully characterise a wide range of synthetic and natural materials. The focused ion beam technique (FIB) is an ideal tool for TEM sample preparation that allows for the fabrication of electron-transparent foils with typical dimensions of 15 × 10 × 0.150 µm from any region of interest (i.e., site-specific) and in virtually any material. FIB sample preparation consumes only a small volume of material (approximately 2300 µm 3 for a standard foil with the dimensions 15 × 10 × 0.150 µm), leaving most of the material essentially unaffected by the sampling process. Modern TEM facilitates the measurement of chemical composition with unsurpassed spatial resolution (e.g., nanometer scale). High-resolution imaging and electron diffraction provide important complimentary structural information. It is the combination of crystal structure information and chemical composition that allows for the complete characterization of the volume of interest. High-resolution elemental mapping, or line scans, may be used to acquire complete energy dispersive X-ray analysis (EDX) spectra of a material pixel by pixel. Subsequent evaluation of the data allows for the correlation of the EDX spectrum with the respective elemental map or line scan with an image, thus illustrating spatially resolved chemical composition variations at the 0.5–1.0 wt.% level. Frequently, minerals enclose sub micrometer- or even nanometre-sized inclusions of minerals, fluids, or silicate melt, which are of great interest to geoscientists. Nanoinclusions (e.g., in diamond or olivine) are of particular interest because they may have preserved their original crystal structure thus providing a geobarometer recording the pressure conditions at the time of formation of the inclusion. FIB/TEM are appropriate techniques to sample and subsequently define the chemical composition and the structural state of mineral inclusion on the nanometre scale. The combination of FIB with an SEM allows for 3D information to be obtained from samples including: 3D imaging (e.g., phase distribution and volume of the individual phases in symplectites); 3D distribution of elements in a specific volume (3D elemental maps); and three dimensional texture analysis using electron backscatter diffraction (EBSD).