2.2 Methodology and sample preparation
2.2.6 Sample preparation for microstructural investigations
For microstructural analyses via EBSD, reflected light microscopy, or geochemical analyses by means of electron microprobe, samples need to have a special preparation procedure.
Initially, a representative sample is dry-sectioned from the drilling core by means of a diamond saw. After sectioning, the samples were ground in order to produce a planar surface and to remove the deformation layer at the surface induced during sectioning. For this study, samples were manually ground (dry) on a grinding instrument, using 60-, 120-, 500-, and 1000-grit SiC abrasive papers. Then, the samples were polished with diamond spray (3.0 and 1.0 µm) on a low-napped synthetic cloth. For the final polishing step, a low-napped synthetic velvet cloth was selected, and a 0.25 µm diamond spray was used as polishing medium. The time required for polishing depends on the particular sample, but was generally quite short, with each abrasive size being used not longer than 2 minutes to minimize effects of preferential polishing and etching rates of halite compared to anhydrite. The polished samples were used for the bromide and strontium content determinations by means of electron microprobe described in sections 2.2.2 and 2.2.4.
For microstructural investigations via EBSD and reflected light microscopy, samples need to be etched after polishing. This procedure reveals grain boundaries and dislocation substructures within the grains in an excellent way. Especially for EBSD, etching is necessary to remove abrasive damage induced by polishing, as the crystal lattice of the top 10-50 nanometers needs to be strain-free and clean from contamination. Otherwise, the resulting EBSD patterns will be of poor quality or not visible at all. Several etching mediums were tested to find out, which of them provides optimal results. These include slightly undersaturated NaCl solution, 0.2 µm water-free silica suspension, distilled water, and diluted HCl, with a mixture ratio of H2O : HCl = 5:1. In general, etching has to be very short (not more than 10 s), as otherwise topographical differences on the surface will be generated due to preferential etching of halite compared to anhydrite, which would impair the quality of EBSD patterns. After etching, the samples need to be rinsed with a substance that has a relatively high evaporation rate. This enables quick removal of the etching medium and normally avoids the precipitation of new halite crystals on the sample surface. In this study, samples were rinsed with either butanol or methanol, with the latter having produced higher quality surfaces, presumably due to its higher evaporation rate. Finally, the samples were dried in a jet of warm air and stored under dry conditions.
For microstructural investigations, rock salt samples from various locations (Morsleben, Teutschenthal, Remlingen, and Gorleben) and different stratigraphic units (Muschelkalk (Middle Triassic); Stassfurt rock salt, Leine rock salt, and Aller rock salt (Zechstein, Upper Permian) were prepared.
Fig. 2.8: Examples of etched sample surfaces with high quality in respect to the visualization of grain boundaries and substructures.
Fig. 2.9: Examples of etched sample surfaces with lower quality showing fluid inclusion-rich grain boundaries and impurities within the grains. Due to evaporation of the fluids released during etching, the surface is partly coated with minute halite crystals, and thus substructures are less visible.
Etching with distilled water and rinsing with methanol generally yields very good results in respect to the visualization of grain boundaries and substructures (Fig. 2.8).
However, the sample surface quality is strongly dependent on two factors: (1) the specific properties of the investigated rock salt samples, and (2) the quality of grinding and polishing.
One major problem related to the sample itself seems to be the presence of impurities or fluid inclusions within the grains or at the grain boundaries, respectively (Fig. 2.9). For example, grain boundaries are opened by the etching procedure and cannot be sealed quickly enough;
consequently, some fluids will remain at the boundaries and evaporate later. Because of this, parts of the sample surface will be covered by a layer of minute halite crystals, or by droplets of released fluids around the grain boundaries (Fig. 2.9). Some samples show open grain boundaries (Fig. 2.9), which is probably due to partial breaking-up of the rock salt as a consequence of the drilling procedure or through the stress relief of the sample that is caused
by the uplift of the core. The preparation of the Kristallbrocken was especially problematic due to the inclusions within this halite type. Most of these inclusions are filled with sulphate crystals and brine (Fig. 2.10A). During polishing, the sulphate crystals fall off and scratch the sample surface, and the brine released during etching leads to minute halite crystals on the surface (Fig. 2.10B). Another problem presumably concerns manual grinding and polishing.
If the sample surface is not perfectly flat, a pattern of ridges can be observed after rinsing with methanol (Fig. 2.11).
Fig. 2.10: Photomicrographs of Kristallbrocken surfaces after etching showing substructures and solid inclusions. Areas around the inclusions can be coated with minute halite crystals (Fig. 2.10A) or droplets of released fluids (Fig. 2.10B). Both features impair the surface quality and make EBSD measurements difficult.
Fig. 2.11: Photomicrographs of sample surfaces after etching showing a pattern of ridges.
Despite all these phenomena, the microstructures of the samples can be studied quite well via reflected light microscopy, but for EBSD investigations they are very problematic, since it is very difficult to get good EBSD patterns and nearly impossible to enable automated measurements. Furthermore, the quality of etching and thus EBSD analysis seems to be highly dependent on the sample provenance and its specific properties rather than the etching procedure. Such problems have not been reported from other studies concerned with EBSD analyses on rock salt samples; however, most of these studies dealt with synthetic rock salts
(e.g., Trimby et al. 2000a, b; Pennock et al. 2005, 2006), and these have generally lower water contents and fewer impurities than natural rock salts.
Another technique to visualize grain boundaries and deformation-related microstructures is gamma-irradiation (Urai et al. 1987; Garcia Celma & Donker 1996, as cited in Schléder 2006). This technique involves gamma-irradiation of the sample slabs at constantly elevated temperatures (e.g., at 100°C), with dose rates varying between 1 kGy/h and 4 kGy/h and up to a total dose of about 4 MGy (cf. Schléder 2006). After this procedure, thin sections are prepared from these samples, which are then etched using slightly undersaturated NaCl solution following the method of Urai et al. (1987). Due to the irradiation with high doses of gamma ray, the internal structure (grain boundaries, subgrain boundaries, growth bands, and zonations) of the samples is revealed, which can be used for the investigation of deformation mechanisms. For technical reasons, it was not possible to use this technique for this thesis.
In this study, EBSD was used to measure the orientation of a set of grains in a rock salt shear zone sample from the Morsleben salt mine (see Fig. 5.2C in chapter 5.1.2). To allow for a good statistical basis, the aim was to measure a lot of grains, because although the sample is from a shear zone, the grain sizes are still in the range of mm. Unfortunately, automatic measurements were hardly possible due to the poor sample surface quality and thus insufficient EBSD pattern quality. A total of 150 measurements could be carried out on about 30 grains. The results show no crystallographic preferred orientation in this sample.
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