Monday, July 27, 2009

Naming Igneous Rocks

The names we give igneous rocks are based on their chemical compositions. The relative amounts of just three main minerals; quartz, plagioclase feldspar, and potassium feldspar are all you need to know to start naming igneous rocks. In addition to these three light-colored, felsic minerals, the abundance of dark, mafic minerals can also help you distinguish one type of igneous rock from another.

Making a First Guess

The chart above will help you get started. Once you know you have an igneous rock, look at the texture to decide if it is intrusive or extrusive. Then use this chart to make your first guess based on how dark (mafic) or light (felsic) your rock appears.

Name That Rock!

To be sure you've named your rock correctly you need to compare the amounts of plagioclase feldspar, potassium feldspar, and quartz and plot it on the chart on the left. It takes a little practice to get used to a triangular graph!

Try this: suppose your rock is coarse-grained, so you know it's intrusive. It has 40% quartz, 30% potassium feldspar, and 30% plagioclase feldspar, it's called granite.

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