For the web site of the week, I have chosen Igneous rocks in thin section. In order to identify minerals and interpret the history of a rock, geologists will often make a thin section. A thin section is made by slicing and polishing a rock until it is only about 30 micrometers thick (human hair is typically 40-120 micrometers thick). This rock slice is mounted on a glass microscope slide, and viewed using a petrographic microscope, which is a light microscope with a rotating stage. The geologist can then view the slice of rock with either ordinary light or polarized light.

The image here is of a rock called diorite using polarized light. Diorite is similar to granite, but it has less quartz. The prominent mineral grain in the center is plagioclase, which appears whitish in ordinary light, but displays this distinct black and white banding when viewed with polarized light. As the geologist rotates the stage of the microscope, the bands change back and forth between black and white. The angles at which they do so can be used to estimate the sodium and calcium content of the mineral, which varies depending on the chemical composition of the source magma and the history of the rock.

As an undergraduate and graduate student in geology, these thin sections kept me mesmerized for hours. More of the beauty of God’s creation, to be enjoyed like a landscape or sunset.

Grace and Peace