GEOL 3235 : Introduction to Petrography

Laboratory 3a - Igneous rocks

The rocks for this first exercise in Igneous Petrography are stored in the wooden trays.

First, ask yourself what kind of rock is this? Are the grains interlocking? What minerals are present? Are there any structures that suggest something about the origin of the rock?

Prepare a Table with the following columns -- Sample Number -- Texture -- Feldspar(s) -- Quartz -- Other Minerals -- Name.

FIRST, look at each of the 30 rocks and write down the texture -- Phaneritic, Aphanatic, Porphyritic with a Phaneritic Groundmass, Portphyritic with an Aphanatic Groundmass. Work together and quiz each other. When you are finished with textures, look at each rock and, if it contains feldspar, determine the type(s). Estimate the percentage of the total rock occupied by the feldspar(s). Do this for all of the samples. Repeat by determining whether each rock contains quartz and, if it does, estimate the percentage of quartz present in the total rock. At the same time you may want to note other minerals that are present.

DO NOT try and name the rock UNTIL you have completly filled out the columns.

You will have 3 laboratory periods to complete this exercise.