instrumental techniques applied to mineralogy and geochemistry

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TEM in Geology. Basics and applications 23 In conclusion, a TEM is a complex machine that simultaneously combines a powerful microscope with crystalline-lattice resolution, a diffractometer, and a chemicalanalysis device. More than its high spatial resolution, this combination of techniques is the real reason for its powerful capability for geological research. Samples Although a universal electron microscope is possible, transmission and scanning/microprobe capabilities are, in practice, separate techniques. One of the reasons is the different characteristics of the samples involved. TEM samples must be as thin as possible. Three means of preparation are the most common for geological samples: 1) Ion milling. This technique allows the extraction of a small area from a thin section, prepared with adhesive easily melted by heat. A copper ring is affixed to the selected area with the aid of a microscope, the rock is cut around the ring, and the round piece of sample removed from the thin section by heating. The copper ring with the attached piece of rock is placed into the ion mill, which bombards the centre of the ring with a beam of ions or uncharged atoms (Fig. 2). The area surrounding the hole produced in the centre of the sample contains sectors thin enough to be studied by TEM. 2) Fine powder deposited on a holey-carbon film. This is the simplest method, very useful when neither a particular orientation of the sample nor especially thin areas are necessary. The disadvantage for petrographic studies is that information on the location of the studied grain in the rock is lost. It is a very useful method for chemical analysis of fine-grained materials. FIGURE 2. Basics of ion-milling preparation of samples. A round piece of rock, removed from the thin section by heating, is placed in the ion mill, which bombards the centre of the ring with a beam of ions or uncharged atoms (left). Sectors thin enough to be studied by TEM surround the hole thus produced (right).
24 Fernando Nieto 3) Ultramicrotomy. This is a method widely used in bio-medical sciences. Suitable minerals either have to be relatively soft, like many layer silicates, or very fine-grained. The sample can be encased within a suitable epoxy and then sliced, like ham in a delicatessen, producing extremely thin wafers, which are deposited in a grid. Electron diffraction The Ewald sphere, which allows prediction of the directions of the diffracted beams, is far larger for electrons than for X-rays as a consequence of the difference in wavelength between the two kinds of radiations (Fig. 3a). Therefore, the radius of the sphere with TEM is so large and the crystal so thin (causing the diffraction spots to become spikes) that the pattern is essentially tangent over a large region of the reciprocal space and many diffraction spots can be recorded without any sample motion. In combination with the strong diffraction effects that occur with electrons, this means that planes through reciprocal space can be viewed in real time. In contrast to X-ray diffraction, diffracted electrons have a greater probability of themselves being scattered as they pass through the sample. Dynamical diffraction gives rise to diffracted intensities that are difficult to interpret quantitatively. Another consequence of dynamical effects is that weak reflections such as those “forbidden” by symmetry commonly appear. These dynamical effects are commonly undesirable, particularly if we want to extract any information from the intensities of the spots, but in some cases may be welcome as they allow easy identification of the symmetry from the “forbidden” reflections. By simple geometric considerations, it can be demonstrated that the d hkl spacing of a given spot can be calculated by dividing a constant by the distance to the central spot in the diffraction pattern. This camera constant is the product of the wavelength of electrons by the so-called camera length, which, in diffraction, plays a role equivalent to magnification in the images. Nevertheless, the camera constant can be affected by a number of factors, including distortions, and it is far from its theoretical value; therefore, precision and accuracy are clearly worse than for X-ray diffraction, but can be approximated to 0.1% if distortion is corrected by the Capitani et al. (2006) method and some basic guidelines are respected (Mugnaioli et al. 2008). As a diffraction pattern is a representation of the reciprocal space, in a similar way to X-ray diffraction, shorter distances represent longer d spacings.
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