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- This step is done as a “finishing” step.
- Here you will remove the last few microns from the section, taking away any damage from the diamond wheel and getting the material to the correct thickness.
- By doing this step you are also ensuring that the section will take a good polish later on.
- Ready a glass plate on a flat surface.
- Mix water and 600 grit Silicon Carbide such that it has a muddy consistency, done earlier.
- Spread the Silicon Carbide and water slurry evenly across the glass plate.
- Take the thin section and place it face down on the glass plate.
- Apply direct downward pressure with your fingers on to the section and move the section in circular motions.
- To check the thickness of the section take it off the plate, rinse and dry it and then make use of the micrometer as outlined earlier.
- You should check that you are grinding down evenly by measuring each corner of the section as well as the center.
- This should all be the same value.
- If one side is thicker than the other, adjust where you apply pressure on the slide and also the position of the slide when hand grinding.
- You should not only make use of the micrometer to check the thickness, it should be used in conjunction with the microscope and a Michel – Levy colour chart.
- Practical way of using the Michel – Levy colour chart (see figure below) : This chart uses the observed interference colours of certain minerals to judge the thickness of the thin section. Using quartz as a reference mineral, if it is observed that quartz is displaying intereference colours (in cross-polarised light) that are first order blue/purple, it means that the quartz grains are approximately 60 microns thick. They would need to be ground down further to around ±30 micron. As you grind down the sample the colour of quartz should change from a purple to bright orange to a bright yellow to dull yellow and eventually a dull grey to white. It must be noted that quartz cannot be always used as your indicator mineral, as it is not present in every rock. The principles still applies to other minerals though.
- Once your indicator mineral is the correct colour and the thin section is the correct thickness (which is check again using the micrometer) the grinding is done.
- There is a chart posted on the cork board in thin section, with common minerals listed.
- Michel-Levy colour chart (right) and quartz grain (left) that displays the change of interference colors as the thickness of the grain decreases. The purple is the thick part of the grain and the white is the correct color.
- Example of a thin section that is too thick (left), and the resultant interference colours and one that is the correct thickness (right). Thin section is of granite.
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