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

Given: A mineral remains dark at all stages of rotation under crossed polars.

What this means:

When a mineral remains dark (extinct) at all rotation stages under crossed polars, it shows complete extinction throughout 360° rotation. This indicates the mineral is isotropic (has no birefringence).

Analyzing each statement:

(A) - A mineral that remains dark at all rotation stages is isotropic. Isotropic minerals (cubic system, amorphous, or certain special orientations) don't split light into two rays and remain extinct under crossed polars.

(B) - If the mineral were biaxial, it would show birefringence and would exhibit extinction positions followed by brightening as the stage rotates. A biaxial mineral in principal section orientation would still show interference colors under crossed polars (except at specific optic axis orientations). This statement contradicts the observation that the mineral remains dark at ALL stages.

(C) - In a biaxial mineral viewed down an optic axis, the mineral would remain dark. 

(D) - A uniaxial mineral viewed perpendicular to the optic axis (i.e., down the c-axis) would remain dark at all rotations because there's no birefringence in this direction.

Answer: (B) is NOT correct