Thin Sections

How to make rock thin sections

Basic Equipment:
  • quality glass slides (1" x 3" are easy to obtain) and rectangular cover slips (to cover a large portion of the slide e.g. 20mm x 50mm);
  • small diamond saw;
  • a square of plate glass for grinding (or a lap); 
  • coarse, medium, and fine silicon carbide grit powders (400, 600, and 1200);
  • colourless low viscosity epoxy (e.g. epoxy 330);
  • Petrographic (Polarising) microscope (the most important bit, most difficult bit and the most expensive)

Preparing the Block
  1. break the rock sample into a size small enough to cut on the diamond saw. 
  2. cut the rock into a rectangular block with dimensions smaller than the slide. only one flat surface is needed, this will be the side to bond to the glass slide.
  3. cut some coarse grit and water onto the glass plate (not the slide) to make a slurry
  4. grind the flat sample side on it to make it very flat and smooth using a figure-8 pattern and cover all of the glass plate to prevent the surface being unevenly warn
  5. when the rock surface looks smooth, clean the glass plate and grind it again with the medium grit and then finally clean it again and grind with the finest grit, to make a very smooth and flat sample surface
Mounting the block

  1. Allow the sample to dry (you can use a gentle heat on a hot plate (say 60 degrees C.)
  2. after the sample is dry, mix a little epoxy and put a large drop in the centre of the slide, making sure that there are no bubbles in it (it needs to become perfectly clear).
  3. carefully set the smooth side of the rock sample onto the glass slide. You want the epoxy to spread evenly across the slide under the sample, so press it down, but gently.
  4. set the sample aside so the epoxy will cure
  5. with a sharp blade, carefully clean off any hardened epoxy residue from the glass slide.
Section thickness
  1. cut off as much of the rock sample from the slide as possible (a jig of some kind will help a lot). Aim for a millimetre of rock on the slide. Save the rock chip in case you need to try again.
  2. return to the glass plate, hold the slide with the rock side down with a few fingers in the centre of the back of the slide, grind the sample thin, using 600 grit. The goal is exactly 30 microns (0.03 mm) of even thickness across the entire slide.
  3. using the petrographic microscope with the polarizing lenses crossed (crossed nicols) check it over and over again each time you grid.
  4. The best mineral to use as a thickness gauge is feldspar, which is present in most volcanic rocks and many sedimentary ones. As the sample gets thinner, the interference colours ( birefringence) of feldspar crystals will go from bright colours to yellow to medium-light gray. Gray is a good indicator but much thinner will be too thin. If quartz is present as well, it will be orange or deep yellow when a little too thick, becoming straw colour when the right thickness is obtained. Use an optical mineralogy reference chart to help.
  5. as the desired thickness is approached you will see more and more light passing through the slide. Take lots of car because it is very easy to over grind (and you have to start all over again!).
  6. To finish, epoxy a cover slip onto the rock, being careful to once again make the epoxy free of bubbles.
  7. label the slide.
Practice is the key to getting this right. The first ones will often have many mistakes, bubbles, uneven thickness, too thin, etc. but never mind, each one done means the next one will be better.

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