Question

Under crossed polars, a mineral remains dark at all stages of rotation of the microscope stage. Which of the following statement(s) is/are NOT correct?

• A. The mineral is isotropic.
• B. The mineral is biaxial with an optic orientation corresponding to the principal section of the indicatrix.
• C. The mineral is biaxial with an optic orientation containing the Y-vibration direction only.
• D. The mineral is uniaxial with an optic orientation perpendicular to the optic axis.

Hint:

When a mineral remains dark under crossed polars, it is isotropic. If it shows interference colors, it is either uniaxial or biaxial.

Answer 1

The Correct Option is B

Step 1: Understanding the properties of minerals under crossed polars.
- (A) If the mineral remains dark at all stages, it is isotropic because isotropic minerals do not show interference colors under polarized light.
- (B) Biaxial minerals do show interference patterns, but their optic orientation depends on their crystallographic structure.
- (C) Biaxial minerals cannot have an optic orientation that only contains the Y-vibration direction. This statement is incorrect.
- (D) A uniaxial mineral has an optic axis, and its orientation is perpendicular to the optic axis when observed under crossed polars, which is correct.

Step 2: Conclusion.
The correct answer is (C) The mineral is biaxial with an optic orientation containing the Y-vibration direction only.