The rocks for the second exercise in Igneous Petrography are stored in the cardboard boxes marked I-40.
Prepare a Table with the following columns -- Sample Number -- Texture -- Feldspar(s) -- Quartz -- Other Minerals -- Name.
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?
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 2 to 3 laboratory periods to complete this part of the exercise.
When you are satisfied with your work, concentrate on samples 11, 13, 16, 19, 22, 28, and 33. "Imagine" that you are in the field. For each sample write a "complete description" -- rock name, mode (list of minerals present and estimated percentages) and your "best guess" as to the genesis of the sample. This part of the exercise should be completed in 1 or 2 laboratory periods.
When you are finished with the I40 tray, find me and I will give you a few more samples to work on -- complete descriptions.