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About Thin Sections


thin section plane-polarized light

Olivine grain in thin section under plane-polarized light

UCLA Petrographic Workshop

In geology school, you learn how to break rocks properly, how to measure angles and directions in the field with the Brunton compass, how to climb safely. You learn how to get permission to enter private property, how to pan for gold, how to identify dozens of minerals by sight and when to take them instead to the laboratory.

And in the lab, you learn to use the petrographic microscope. That's how I fell in love with thin sections. Maybe you've seen some striking images of minerals in thin section, looking like some wildly colored abstract painting. What's up with those?

Making Thin Sections

A thin section is a hair-thin slice of rock, mounted on a microscope slide. Making one is an exercise in old-fashioned, hands-on science. It starts as you clamp a stone in a vise and cut a slice from it with a diamond saw, a slow-motion version of the butcher's meat slicer. You take the slice, trim it to a small wafer, then polish one face of it on an iron lapwheel covered with carbide grit. You fix that face to a glass slide with hard resin. Then you grind the other side down to about 30 micrometers thickness, fix a thin glass cover slip on it with more resin, and that's your thin section—a translucent sliver of stone, thinner than paper.

They have machines that do this, but my school didn't have one for student use. But if the great geologist G. K. Gilbert did it this way a century ago, then I could too.

Viewing and Analyzing Thin Sections

The petrographic microscope looks like something from Dr. Frankenstein's lab, a large apparatus with twin eyepieces and several extra attachments. As a kid I played with a microscope, but looking down the barrel of this instrument for the first time gave me vertigo, much like the experience of seeing the Moon through a good telescope.

To the naked eye, a thin section is nearly transparent, and that's true under the microscope too. Even dark minerals like garnet or olivine are clear. The stage with the thin section on it can be rotated, like turning a camera lens, and as you do that some of the minerals wink bright, then dark. That's because underneath the stage is a polarizing filter, and any mineral that itself is polarizing interferes with that polarized light. You can demonstrate the effect yourself with polarizing sunglasses by tilting them sideways against the sky, which is also polarized. Where polarizers cross each other, you get darkness, and where they both have the same direction you get brightness.

The real fun begins when you flip in the microscope's second polarizer, which is mounted above the stage and is crossed relative to the first polarizer. Then bright turns to dark across the whole field of view, and the different minerals take on interference colors that sometimes are quite vivid. The explanation takes us into mineral optics, a very deep field for which some details can be found elsewhere.

Basically, those colors tell the observer a lot about a mineral's identity and composition, and the angles that can be measured using the rotating stage tell more. For good measure, there's also a very thin wedge of quartz that the microscopist can slide in and out of the light path to gauge the effect on the interference colors.

The whole experience—turning the stage, working the polarizers and wedge, and consulting color charts—is much like working an old-fashioned slide rule. (If that makes no sense, ask your grandparents.) And just as with the slide rule, a practitioner with good hands and a good eye can do great work without electronics. The difference is that making and studying thin sections is still an essential skill today.

There are other old-fashioned skills that are still part of geologists' education, or some of them anyway. Sketching is one. Here's an article that shows some lovely art from geologists' field books of the 19th century. Another very ancient technique—flint knapping—dates from deepest prehistory. Geologists don't do that . . . crazy amateurs and some archaeologists do.

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