Around the Lab Are Identified Thin Sections

Lab 3c

Extinction Angle and Pleochroism

Name _______________________________

In this lab, we will study Extinction Angle and Pleochroism. These methods are commonly used for mineral identifications and rock classifications.

Around the lab are identified thin sections.

1. Extinction Angle

For a Gabbro or Basalt or Diorite thin section

• 1. Find a Plagioclase crystal showing Albite Twins.

Orient the crystal so that both twins are uniform grey. Note the angle on the stage. If your microscope does not have a goniometer (protractor) on the rotating stage, then note some landmark, and a pencil mark on the stage, as your starting point.

Rotate the stage so that one set of twins goes extinct. Note the angle, then return the stage to center. Now rotate the stage so that the other set of twins goes extinct. Note the angle, and calculate the average. Use the chart below to estimate the composition. For Basalt, Gabbro or Diorite use the right hand side if the angle average is 20 or less.

2. Pleochroism

Additional thin sections have been provided for this part. They are:

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Examine the slides for Biotite and/ or Hornblende in PPL

• If you see Pleochroism, write down the mineral name and the colors you see.

• Look for cleavages or euhedral crystals in some of the Hornblende, and if you find them, measure the extinction angles.

Clean up and put your microscope away.