Rock Of The Week For 2008-07-23

This month we feature a rock that’s not really a rock. Instead it’s a pure mineral. Usually, a rock is made up of more than one mineral, like in granite. Or if only one mineral is present, like in limestone, it’s made up of many individual grains of that mineral. This month’s rock is a single crystal of a mineral called muscovite. Muscovite is a type of mica mineral and like all micas is very platy. It’s made up of smooth, thin sheets that seem like they are just stuck together, which in some ways is exactly true. The bonds that connect the molecules from one sheet to another are much weaker than the bonds that connect the molecules within a sheet. This means that the mineral easily flakes off pieces of itself. The piece of muscovite that Sam E. sent to us is so thin (about 1 mm or just under 1/16 of an inch) that light can pass through it. It’s very fragile because of this and would easily break if not handled carefully. But its large size (about 4 inches long) means that it grew over a long period of time inside a magma chamber or possibly deep underground in a “metamorphic zone” where the pressures and temperatures produced by tectonic forces were just right to grow a big crystal like this. If you analyze the spectrum using either the igneous or metamorphic library, the results include more than just one mineral. That’s because the one muscovite mica spectrum in our library is not exactly the same as the one from Sam E. So spectra from other minerals are used to help improve the “fit”. But the reality is that Sam’s rock is really a beautiful example of a single crystal of muscovite. Now we can add it to the library and have two examples of muscovite. Thanks for the nice piece, Sam!

Thanks for sending in such a cool rock!

Check out our previous rocks of the week: