C. Determination of 2V
The 2V is the acute angle between the two optic axes and can be measured either by the determining the distance between the isogyres at 45 degrees off extinction (Bxa figure) or by estimation of the curvature of the isogyres (centered optic axis figure). When you see a Bxa figure from a mineral with a small 2V (less than about 60 degrees), the isogyres never leave the field of view of the microscope. If the 2V is greater than 60 degrees, the isogyres will leave the field of view but require greater degrees of stage rotation for this to happen as compared to the Bxo figure.

i. Using the oriented muscovite slides, estimate the 2v for each of this mineral. Also, obtain a thin piece of muscovite from your T.A. and determine the 2V for muscovite.
Are your values close to the values in the books? Your estimation should be within ± 5 to ± 10 degrees. If they are not, you should practice a little more on the 2V scale slide.


ii. Using the oriented aragonaite slides BH-250-39, estimate the 2v for each of this mineral Are your values close to the values in the books? Your estimation should be within ± 5 to ± 10 degrees. If they are not, you should practice a little more on the 2V scale slide.


iii. Find a biotite grain that yields a Bxa figure (ask your T. A. to show you this mineral) or use a thin piece of biotite and obtain a figure. What is the 2V of biotite?

Click here to print entire optics lab 8