Rock — Through the Looking Glass

Technician Luan Heywood finds me in the core lab. “Hey, I’m making a thin section. Would you like to come see how I do it?”

Of course! When I looked at one of these ultra-slim pieces of rock, glued to a slide, under the microscope a couple of weeks ago, I was enchanted. Thin sections are very beautiful…

A thin section under the microscope [Credit: Anna Glüder]
…especially in polarized light!

The same thin section in polarized light [Credit: Anna Glüder]
I’m very curious to see how Luan makes them.

Scientists look at thin sections under a microscope to determine the mineral composition of a rock. On Expedition 382, the rocks are “dropstones” deposited by Antarctic icebergs that melted along ago, and our team will use them to discover where in Antarctica the rock came from. This will help them determine which parts of Antarctica were most prone to melting during warm periods in the past.

Icebergs carry rocks and dirt from Antarctica, called iceberg-rafted debris, which deposits on the seafloor as the iceberg melts. [Credit: Marlo Garnsworthy]
When a scientist requests a thin section, Luan takes a “billet” of rock—a small piece suitable for gluing to a slide.

A billet of rock [Credit: Marlo Garnsworthy]
She has freeze-dried the billet over some hours to make sure it is completely dry. If not, any dampness might interfere with the glue.

[Credit: Marlo Garnsworthy]
Safety glasses on! Luan starts by cutting and polishing the side of the rock she’ll stick to the slide. She uses a grinder, changing to sandpaper of ever finer grit until the rock is smooth and shiny. Then she uses a two-part epoxy glue and sticks the polished side to a slide. She waits to be sure the glue is dry before the next step.

Suction keeps the slide attached to the plate. The blade grinds through the rock, and the excess rock is saved for further analysis.

The piece attached to the slide is very thin, but it’s nowhere near thin enough yet! The rock must be almost translucent before it will be useful to the scientist who requested the thin section.

Luan uses the grinder, stopping now and then to test the thickness of the rock with a special measuring device along the way. She’s aiming for 30 microns—that’s 30/1000 of a millimeter, or about half the width of a human hair.

When she thinks it’s getting close, Luan looks at the thin section under a microscope. She uses the appearance of the rock and her knowledge of how various minerals look under polarized light to judge when the thin section is ready. If the section is not yet thin enough, it will look quite colorful under polarized light. She patiently explains how a polarizer works and how the minerals in the rock bend rays of light. She uses a fine grit aluminum oxide powder and water to patiently polish the rock until it’s glossy. Then she measures it again and checks it under the microscope.

[Credit: Marlo Garnsworthy]
Voilà! The thin section is complete.

Luan Heywood and a completed thin section [Credit: Luan Heywood]


Education & Outreach Officer for Exp 382 & Exp 390 I am a children's book author, an illustrator, and an editor, writing teacher, and science communicator. Outreach Officer for Antarctic research cruise NBP17-02, IODP Expeditions 382 & 390.
More articles by: MGarnsworthy

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