Identifying Igneous Rocks

What are rocks?

Common minerals.

Sedimentary and metamorphic rocks.

Igneous Rocks

The relative amounts of just three main minerals; quartz, plagioclase feldspar, and potassium feldspar are all you need to know to start identifying 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.

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

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 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.

--Taken from USGS Geology in the Parks.