Rocks and Meteorites Through a Microscope: A Web Tour and Exercise!

While a great deal of information can be gathered from rocks through hand specimen examination, the observational work of a geologist is incomplete without the careful examination of samples through a microscope.

Microscopic petrography of rocks is done to characterize rock Textures and Fabrics, just as is done with hand samples. However, we can also use the microscope to make detailed identifications of constituent minerals based on their Optical Properties (i.e., how minerals refract or reflect light). To see the optical characteristics of minerals, we have to pass light through them, so what we do is cut a very, VERY thin slice of rock (about 30 mircometers thick, called a Thin Section). To look at these Thin Sections, we use a Polarizing Microscope.

We polarize the light to assure that all the light waves which reach the thin section are vibrating in the same direction. As the light passes through the thin section, the different minerals serve to both refract (or bend) the light, and often to re-polarize the light, so the light waves leaving the section are no longer aligned. By inserting a second polarizer oriented at 90 degrees to the first, we cut off all lightwaves which have not been repolarized (that means we see black under crossed polars). But any light which has been re-polarized by passing through a mineral in the thin section will show up as a strong color, which means that many minerals (though not all of them) will be brightly colored when you look at them.

Kyanite Gneiss, a common Metamorphic rock. The large, round, pinkish crystals that go dark under crossed polarizers are Garnets. Biotite mica is brown in plain light, and reddish brown with crossed polars, while kyanite is colorless in plain light, but pink to blue with crossed polars

Both this coloration, called Birefringence, and the Index of Refraction are distinguishing characteristics of minerals, and can be used to identify them.

While y'all may gain a passing familiarity with how some common minerals look through a petrographic microscope, that isn't our primary goal. Mostly, I want you to look at how the mineral grains are put together in a rock (or its Texture) and how they are arranged (called Fabric), because these features can tell us a lot about how the rock came to be.

This exercise has two parts: first, I want you to look at some common terrestrial rocks as we see them through a microscope, and then we'll look at some meteorites. What we're going to do is compare them texturally, so y'all can see for yourself how a meteorite specialist comes up with a model for the origins of an extraterrrestrial rock.

Link to Page on Terrestrial Rocks under a Microscope.

link to Page on Meteorites under a Microscope

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