Optical mineralogy UE Past Papers Questions.

Discuss in detail the optical characteristics of the following silica  minerals

1. Opal
2. Chalcedony
3. Quartz
4. Tridymite
5. Cristobalite

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Fieldspars are the most abudant group of minerals in ignesious and metamorphic rocks. Discuss their optical properties detail.

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Arrange the group of plagioclases according to their anorthite content and explain what effect does the anorthite content have on the optical properties. How would you differentiate them under the polarizing 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 at the concave side of the two isogyres interference colors change from 2^nd order to the 1^st order after inserting the accessory plate.

1. Is this mineral uniaxial or biaxial
2. What is the optic sign of this mineral?

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Discuss the concept of ‘Compensation’ when using an accessory plate in optical mineralogy.

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