Rocky IV


April 1, 2008

Tips and tricks for learning the inner secrets of rocks, but don’t tell Grammy!

Boy, am I in trouble now. Our grandkids, Donovan and Delaney, and their Mommy and Daddy, are visiting us. A couple of days ago, I showed the kids Rocky, my pet rock. Without thinking, I licked the surface of the rock so my saliva would enhance the color of the rock, and the kids could appreciate the pretty bands of different colored minerals that make Rocky a gneiss (pronounced nice). The bands consist of light-colored feldspars and quartz, and dark colored mica and amphiboles.

We have some gravel along the grassy areas in our yard where the dogs play. Next thing I knew, Donovan and Delaney were licking the rocks to see them change colors. Cool, except Grammy and Mommy were watching.

Geologists do a number of things to learn the inner secrets of rocks. They lick them. They crack them open to see fresh mineral surfaces. They cut them and polish the surface, just like granite or marble countertops. They cut them into extremely thin slices that you can see through and look at them under special polarizing microscopes. They even coat them with a thin layer of gold and look at the surface under a scanning electron microscope (SEM).

That is what you see in the accompanying SEM micrograph (see Figure 1- right). Mica, the main mineral in this SEM image, is in almost every mineral kit you can buy. You probably remember it as the mineral that can be split into sheets so thin that you can see through them. But if you thought you had split the mica into a single sheet, you were not even close.

This SEM micrograph is looking at the edge of some mica sheets from a very tiny part of a dark band of minerals on Rocky’s surface. From this view point, the mica sheets look like the edge of a stack of papers. The white bar at the top of the photograph is 20 microns long. To put that into perspective, a human hair has a diameter of about 70 microns. So the end of a human hair would completely cover this SEM micrograph.

Also in the micrograph are some grains of feldspar (with smooth sides) and quartz (the ragged grains) that are nestled between some of the mica sheets. Each mineral in this rock recrystallized at a specific temperature and pressure. For example, in the center left of the image, you can see where the mica sheets (which had already crystallized) were distorted by the feldspar crystal that recrystallized later, at a lower temperature and pressure than existed when the mica recrystallized.

But enough about Rocky’s inner secrets. I need to tell Donovan and Delaney that they should not lick rocks. They should spit on them. Do you think that will make Grammy and Mommy happy?

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