When looking at rocks, we have one obvious problem. Most rocks are rather opaque.
So when you want to analyze you can forget about that lumpy old microscope… But geologists have known a trick since 1858. It’s called a thin-section. You cut down your sample to a mere thickness of 30 micrometer, which is comparable to the average thickness of a human hair (although this varies strongly).
Now you can actually have light from a microscope shine through your material and if you polish it good enough with some diamond paste, the image gets clear enough to be analyzed by specialists.
When looking at the picture below, you see all the great colors and one might think this is fake. But actually this is just another trick to get the optical properties of the crystals making up your rock sample. You use polarized light.
When looking at waves of sunlight there is almost no order of the waves. They just oscillate in any direction and so does light from a light bulb. But perfectly polarized light oscillates in only one direction. You know this from 3D-movies like Avatar. When you go to the cinema you get these ?Real3D?-glasses. Each each glass (left and right) is a different polarization filter, so the 3D effect can be created from two slightly different images.
So when we but our thin-section between two polarization filters and rotate them to just the right angle, we get some very interesting properties of our sample. See the picture below. It’s a thin-section of gabbro.Gabbro is a large coarse-grained black rock that forms when magma cools down beneath the Earth’s surface. But under just the right angle you see the minerals making up this rock sample.
Have a look at the clear parallel lines on the right. Those are plagioclase feldspar. These are abundant silicate minerals in Earth’s crust that help petrologists a big deal in identifying the evolution of a rock sample. The bluegreen areas are clinopyroxenes that are a different form of silicate minerals that are a main component of the Earth’s mantle. The width of the entire (micro-)photo is 0.5cm.
Photo credit: Wikipedia Commons user Siim Sepp, 2006 under License GDFL