Question

From the following optical properties of minerals, select an appropriate option to identify the direction of analyzer and polarizer if the available microscope is without a cross-hair.

• A. Pleochroism of common hornblende
• B. Extinction of diopside
• C. Extinction of glaucophane
• D. Pleochroism of biotite

Hint:

Pleochroism can help in determining the orientation of the mineral under polarized light when a cross-hair is unavailable.

Answer 1

The Correct Option is D

Step 1: Understanding Pleochroism and Its Applications.
Pleochroism is the optical phenomenon where a mineral shows different colors or intensities when observed from different crystallographic directions under plane-polarized light. This is due to differential absorption of light depending on the orientation of the mineral structure with respect to the light’s polarization direction.

Minerals like biotite exhibit strong pleochroism. Because of this, when such a mineral is rotated on a microscope stage under plane-polarized light, the visible color changes noticeably. This behavior can be used to infer the orientation of the microscope’s optical components — specifically the directions of the polarizer and analyzer — even if cross-hairs are absent.

Step 2: Evaluating the Options.
- Option (A): Incorrect – Pleochroism is not irrelevant to determining optical orientation.
- Option (B): Incorrect – It is not only interference colors that reveal orientation; pleochroism in PPL (plane-polarized light) also contributes.
- Option (C): Incorrect – Biotite’s pleochroism is strong enough to reveal orientation without needing other aids.
- Option (D): Correct – Pleochroism in biotite is a reliable indicator of the analyzer and polarizer orientation.

Conclusion: Option (D) is the most accurate.