Optical mineralogy questions.

In which position and why does an anisotropic mineral become extinct when  viewed under a crossed -polar microscope.

What is an extinction angle?

Explain procedures for determining the angle of extinction of a mineral under the microscope.

A student made conoscopic microscopic observation using a length fast accessory plate and observed the features as presented in the image below:

In the area between the two isogyres interference colors change from the 2^nd  order to 3^rd order after inserting the accessory plate. In the areas circled by each of the two isogyres colors change from 2^nd order to the 1^st order after inserting the accessory plate.

1. What is the optic sign of this mineral?
2. Sketch a simple diagram demonstrating the angular relationship between the two optic axes, the Z and X light wave vibration directions of the mineral section under observation.

Discuss the concept of “retardation” in optical mineralogy.

Is it possible to observe wave retardation in Isotropic minerals? Why?

Explain the meaning and cause of “Double Relief” in a mineral gram.

Explain color changes in the four quadrants separated by isogyres of uniaxial positive mineral grain when inserting an accessory plate that is length fast.

What are the main differences in terms of optical properties between Tetragonal and Orthorhombic minerals?

What are the main differences in terms of optical properties between the following  mineral pairs

1. Plagioclase and Quartz
2. Biotite and Hornblende
3. Garnet and Plagioclase
4. Biotite and Muscovite