EOSC 221
Introduction to Petrology
Winter 2001

LAB 1 - Introduction to the Petrographic Microscope;

Relief and Refractive Index; Isotropic Minerals

(Differs slightly from official lab)

Goals:


1) To become familiar with the function, adjustment, and use of the petrographic microscope.

2) To understand the concepts of relief and refractive index.

3) To begin a list of minerals you can identify in thin section.

Your TA will start this lab by assigning you the combination to a microscope locker, and giving you an introductory lecture. The 221 web site contains many helpful modules on petrographic techniques.

1. Below is a diagram of a microscope very similar to yours. You should become familiar with the various parts of the scope. Make a sketch of the optical path of your microscope, locating the main parts (light source, objective, polarizer, analyzer, condensing lens, Bertrand lens, diaphragm, ocular (eye piece), stage, , accessory slot, position of thin section)

Compiled from Nesse (1991), Introduction to Optical Mineralogy and Kerr (1959), Optical Mineralogy.

You have two ways of accessing information about the microscope. The preferred way, if you have Netscape 2.0 or better... click HERE!! for a cool 'FRAMES' version of the microscope. If you do not... click on the blue words below for more information.

All parts of the microscope should function smoothly and easily. NEVER USE FORCE! Ask for help from your TA if your microscope is not operating properly. It is your responsibility to make sure that your assigned microscope is in good working order for all lab exams. Extra bulbs are in the Lab 1 tray.

2. Using a thin section of granitoid 202, center each of the objective lenses. Your TA will demonstrate this. After the objectives are properly centered they will stay centered unless improperly handled. The most common problem is grasping the objectives to rotate the nosepiece of the microscope (the mount which carries the 3-4 objective lenses). Always rotate the objectives only by using the large knurled ring provided for that purpose.

3. What is the magnification of the ocular on your microscope?


Make a table of properties for the objectives of your microscope, listing the following:


total magnification width of field of view (mm) (a micrometer slide may be available, otherwise a metric ruler will do)
free working distance (mm) (IMPORTANT - BE AWARE OF THIS DISTANCE ON HIGH POWER ESPECIALLY!)

4. Using biotite in granitoid 202 thin sections provided, determine the preferred vibration direction of the polarizer. Biotite shows its strongest color absorption when its cleavage direction is parallel to the vibration direction of the polarizer.

5. By means of Becke lines and oblique illumination, check the sign of relief (+ or -) and the degree of relief (low, medium, or high) of the following isotropic minerals:

garnet
fluorite

An isotropic mineral lets light travel through it at the same velocity in all directions. Therefore in crossed nicols (crossed polars) the mineral will be extinct (completely black) no matter what the orientation.

6.

Granitoid 202

Click on the picture to see a granitoid 202 thin section.. Warning!! This is a big file!! (two images of approximately 60 kb each).

Examine this thin section. You should be able to identify at least biotite already. What are some of biotite's diagnostic optical properties? Using your books and knowledge you gain over the next few weeks, by the end of Lab 2 you should be able to identify every mineral in this thin section, and in some cases determine the type or composition of a mineral (e.g.s type of mica, composition of feldspar). Then you can use this knowledge to make some intelligent guesses as to the petrogenesis of the granitoid.