FMF - Friends of Minerals Forum, discussion and message board

[Tom Mazanec]

I read these things are 30 microns thick.
Wow, that's just over a thousandth of an inch.
How can you slice a stone to that kind of thinness?

[Mike Wood]

You can't. The stone can be sliced to approx. 1mm thickness, mounted on a glass slide, then ground down to near transparency - at 0.03mm (30 microns) thickness. This can be done by hand, with varying sizes of 'grit'. Never tried it myself though.

[Pete Richards]

They are called petrographic thin sections (or just thin sections, though they're usually made to study rock samples). Typically, a sample is broken down or cut to the right size to fit on a microscope slide. Sometimes the rock is impregnated with epoxy to give it added strength. A flat surface is ground on the rock with a diamond lap or a piece of plate glass and carborundum grit. The flat surface is glued to the microscope slide with epoxy. Most of the rock is cut away using a diamond saw and an adjustable holder that feeds the specimen and glass through the blade to make a cut parallel to the glass, leaving perhaps 0.5 to 1 mm thickness of rock. This can then be ground thinner by hand on a glass plate, but that takes a long time, a lot of elbow grease, and usually produces a slide of uneven thickness. The alternative is to use a diamond-studded grindstone mounted similar to the cutoff saw to gradually grind the rock thinner and thinner, stopping frequently to check the thickness. When the thin section is finished, a cover slip is often glued on top to protect the specimen, or it may be polished, especially if it will be studied using a scanning electron microscope or a microprobe.

That's just a sketch of the process, and there are variants. But in all cases the rock is glued to a glass slide or other supporting material, and special equipment is used to remove all but ~30 microns of the rock, producing a rock slice of uniform thickness.

[Jordi Fabre]

[Roy Starkey]

Apologies Jordi - I just posted it without thinking - will do better in future!

[Susan Robinson]

I remember making a few thin sections in the petrology and petrography course I took while in college. We ground down the thin section on a rotating lap. The fun part was looking at the thin section through the petrographic microscope and seeing the incredible colors and patterns of the minerals in polarized light. Perhaps if anyone has any of those images, they could post a few on this site, so people could see what fantastic "abstract art" the thin sections create through the scope.

Susan Robinson

[Jesse Fisher]

Per request, a few photomicrographs from petrographic thinsections.

[Tom Mazanec]

[John S. White]

Thought you might like to see this. It is a video from the optics exhibit created by Olaf Medenbach for the 2014 Munich Show. What you see here is a very large thin section of an igneous rock on a revolving stage so that you can view the slide in plain light and in polarized light. Sorry I am not sure what the rock type is except that the large crystals are a feldspar.