Optical mineralogy UE Past Papers Questions.

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

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What is an extinction angle?

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

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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.

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Discuss the concept of “retardation” in optical mineralogy.

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