E. Determination of Optic Sign in biaxial minerals
Optic sign determination in biaxial minerals is more complex than in uniaxial because the sign is determined using the Bxa (a Bxo will give you the opposite sign). In an optically positive mineral the Bxa is about the Z axis and in optically negative minerals, it is about the X axis. What happens to the optic sign when 2V of a positive crystal exceeds 90 degrees?

1. Look at the oriented slide of muscovite and determine the optic sign of the mineral.

2. Look at the oriented slide of topaz BH-250-38a and 38b and determine the optic sign of the mineral.

3.. Determine the optic sign of Biotite. (look for a Biotite that is cut perpendicular to the C-Axis)

4. Determine the optic sign of aragonite

5. Determine the optic sign of olivine.

6. Sketch a Bxa figure for a biaxial positive mineral showing the direction of the X, Y and Z rays. Explain qualitatively the color changes that occur when the mica plate is inserted. Do the same for a biaxial negative mineral.