Which of the following statement(s) is/are correct?
Show Hint
- An isotropic mineral remains dark through 360° rotation of stage under crossed polars
- Pleochroism is the change of colour of a mineral during rotation under crossed polars
- Minerals of the Triclinic system are optically uniaxial
- Melatrope in an interference figure marks the emergence of an optic axis
The Correct Option is A, D
Solution and Explanation
To determine which statements are correct, let's analyze each option based on geological concepts:
- An isotropic mineral remains dark through 360° rotation of stage under crossed polars:
- Explanation: Isotropic minerals have the same optical properties in all directions. When viewed under crossed polarizers in a polarizing microscope, they stay extinct (appear dark) through a 360° rotation because their optical properties do not change with orientation.
- Conclusion: This statement is correct.
- Pleochroism is the change of color of a mineral during rotation under crossed polars:
- Explanation: Pleochroism is the property of some minerals to exhibit different colors when viewed from different angles, especially with polarized light. However, this occurs under plane-polarized light, not crossed polars.
- Conclusion: This statement is incorrect.
- Minerals of the Triclinic system are optically uniaxial:
- Explanation: Triclinic system minerals are usually optically biaxial, meaning they have two optic axes. Uniaxial minerals have only one optic axis, typically found in the hexagonal and tetragonal systems.
- Conclusion: This statement is incorrect.
- Melatrope in an interference figure marks the emergence of an optic axis:
- Explanation: In an interference figure observed with a conoscopic setup in polarized light, a melatope is one of the points where the optic axis of a biaxial mineral emerges. It is a critical point in the interference pattern.
- Conclusion: This statement is correct.
After reviewing all the options, the correct statements are:
- An isotropic mineral remains dark through 360° rotation of stage under crossed polars.
- Melatrope in an interference figure marks the emergence of an optic axis.
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