Wednesday, December 31, 2014

What a Geologist Sees - Part 16

Before you conclude that I have completely lost my mind, it is an old toaster, circa 1930s.

I included this photo in this series to show the sheets of muscovite mica behind the center electrodes. This illustrates one of the uses of muscovite, as an electrical insulator. You can also find mica used in the newer, vertical-type toasters.

[While showing this to one of the teachers at my junior college campus, he remarked that there is an organization devoted to collecting old toasters. I asked him whether the toaster collectors got "toasted" during their social events at their shows.]

In general, if you have a rock with small, aligned mica flakes, it is probably a metamorphic rock, such as a schist. If the mica flakes are larger than 3/4 inch across, it is probably an igneous rock.

Because of the "flaky" nature of mica, large pieces are known as "books" and they usually occur in irregular igneous intrusions called "pegmatites". Mica books 5 feet in diameter have been mined in Georgia pegmatites in the past. If memory serves me correctly, mica has been mined in the Ball Ground, Thomaston, and La Grange areas of Georgia, as well as several places in western North Carolina. Pegmatites also include many other interesting minerals.

Synthetic mica has been in production for more than 50 years, making mining mica less of a necessity.

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