Question

Which of the following statement(s) is/are correct?

• A. An isotropic mineral remains dark through 360° rotation of stage under crossed polars
• B. Pleochroism is the change of colour of a mineral during rotation under crossed polars
• C. Minerals of the Triclinic system are optically uniaxial
• D. Melatrope in an interference figure marks the emergence of an optic axis

Hint:

Pleochroism occurs when a mineral exhibits different colours when viewed under crossed polars, depending on the crystallographic direction.

Answer 1

The Correct Option is A, D

To determine which statements are correct, let's analyze each option based on geological concepts:

1. 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.
2. 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.
3. 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.
4. 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.