Question

A mineral with a point group symmetry 2/m

• A. has two optic axes
• B. shows inclined extinction in (100) section
• C. shows straight extinction in (010) section
• D. is uniaxial

Hint:

In crystallography, minerals with 2/m symmetry in the monoclinic system are biaxial and exhibit two optic axes, which is a key optical characteristic.

Answer 1

The Correct Option is A

Step 1: Understanding the question.
The question concerns a mineral with point group symmetry 2/m, which is a monoclinic crystal system. This symmetry affects the optical properties of the mineral.
Step 2: Analyzing the options.
(A) has two optic axes: Correct — Minerals with 2/m symmetry in the monoclinic system generally exhibit two optic axes. This is a characteristic feature of biaxial minerals.
(B) shows inclined extinction in (100) section: This is incorrect, as inclined extinction is more typical of minerals that do not have 2/m symmetry.
(C) shows straight extinction in (010) section: Incorrect — Straight extinction is typically observed in orthorhombic crystals, not monoclinic crystals with 2/m symmetry.
(D) is uniaxial: Incorrect — Minerals with 2/m symmetry are biaxial, not uniaxial.
Step 3: Conclusion.
The correct answer is (A) has two optic axes, as minerals with 2/m symmetry typically exhibit two optic axes due to their biaxial nature.