Remove a carbon coat from thin sections with methanol

tl;dr: methanol is great but don’t let it kill you

Thin sections, oh glorious thin sections! They are little slices of truth, windows into the processes that shape all rocks. And for a variety of reasons, geologists do terrible things to them. Thin sections are subjected to staining, acid etching, laser beams and more. In my case, my samples are all subjected to the electron microprobe for in-situ mineral analysis. The first step in this process is to coat each thin section in carbon. Samples are placed in a carbon evaporator, which creates a vacuum (down to at least ~10^-4 torr) and coats the sample in a ~20nm thick carbon layer. This is great for analytical work, but it dulls everything in plane polarized light, can mask birefringence colors, and just forget about trying reflect light microscopy.

Carbon coated lunar basalt 70135,64 looks mostly “normal” in plane-polarized light (though a bit dim). Most major phases are easily identifiable (plag = plagioclase; px = pyroxene), but reflected light is necessary to identify opaques…

Same area of lunar basalt 70135,64 shown in reflected light. Not only can you still not identify the opaque phases, but you’ve lost the boundaries between plagioclase and pyroxene!

It’s just a big gray mass of cracks. If you need to take a second look at your sample, that carbon coat will just have to go! Despite being only 20nm thick, the coating is surprisingly resilient. You must also take care not to damage the thin section, so what do you do? There are various ways to approach carbon coat removal, and I thought it would be useful to highlight two common methods: grit and methanol

Removing a carbon coating with Al-polish

Use a fine grit Al-oxide powder and water to polish off the carbon coat. This quote from a mineral forum post suggests several ways to remove a carbon coat, including:

The simplest way to remove a carbon coat or a gold coat is via a 1 micron Al2O3-H2O slurry (1 part Al2O3 to 8 parts H2O) on a Buehler Polishing Cloth ( Catalog No. 40-7218 Microcloth with adhesive for a 8 inch wheel) aka “moleskin”. Gentle rubbing of the thin section or grain mount by hand on the mole skin polishing cloth (on a flat surface) with a generous amount of the slurry will completely remove the coating both on the surface of the mineral grains as well as in between the cracks and grain boundaries. This is due to the the action of the very fine short hairs of the moleskin. The Al2O3 can be easily removed under a running tap (preferably distilled water) or else (if fussy) in distilled water for five minutes under ultrasound. Afterwards the thin section or grain mount should be dried using a soft cloth or a kleenex wipe.

I really like the idea of using a polishing cloth, as it is rather difficult to remove carbon from low areas (e.g., cracks) with only a kimwipe. Removing Al2O3 buildup in these areas can also prove difficult. In addition, this method gets messy, and I am always worried about over-polishing and losing the sample. That is why I prefer to use methanol.

Removing a carbon coating with methanol

Methanol is also an effective carbon coat remover, with one caveat: Methanol is extremely toxic! You should already be wearing gloves during cleaning to prevent your gross human oils from transferring to the thin sections, but in the case of methanol it is an absolute necessity to have proper hand protection. I also prefer to work under a fume hood to prevent inhalation (and because it smells wicked strong). Methanol is my preferred method because it is clear and evaporates rapidly, making for an easy assessment of the status of carbon removal and leaves almost nothing to clean up.

Before, during, and after carbon coat removal. Methanol preferentially removed carbon from the sample, so it was cleaned before the surrounding epoxy. The thin section is 1″ diameter.

How I removed this carbon coat with methanol:

  1. Wear safety gloves
  2. Place the thin section on a flat surface
  3. Moisten a kimwipe with a drop or two of methanol
  4. Hold the thin section in place and rub the kimwipe in a variably circular motion, applying gentle pressure
  5. Regularly change the face of the kimwipe being rubbed on the thin section. This minimizes the risk of loosened material scratching the surface.
  6. Keep the kimwipe moist but not too juicy with methanol
  7. Check your progress in reflected light. The image below is the nearly-cleaned thin section, with a couple trouble spots to finish up. Checking your progress early and often is the best way to get your eye in on carbon coat removal. You won’t know when it’s all gone if you have no idea what it looked like before you started!

A couple of minutes work on 70135,64 and the carbon coat is nearly gone. Now we can see the opaque phases in 70135,64 are mostly ilmenite, with some exsolution troilite (and unlabeled Fe-metal and possible melt inclusions) popping out that we would have missed previously.

70135,64 after methanol removal of the carbon coat. Notice the change from the previous picture – we uncovered a new exsolution feature in the lower left ilmenite grain. The ugly methanol droplets are also almost all gone, and the remaining “bubbles” are melt inclusions.

A few swipes with a methanol-dipped kimwipe is also a quick way to remove all those annoying loose particles surrounding laser ablation pits.