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 show inclined extinction in certain crystal sections due to their unique optical properties.

Answer 1

The Correct Option is B

Step 1: Understanding the question.
The question is related to a mineral with point group symmetry 2/m, which is a monoclinic crystal system. This symmetry affects its optical properties.
Step 2: Analyzing the options.
(A) has two optic axes: Incorrect — Minerals with point group 2/m are monoclinic and typically have only one optic axis.
(B) shows inclined extinction in (100) section: Correct — Monoclinic minerals with 2/m symmetry often show inclined extinction in the (100) section due to their crystal symmetry.
(C) shows straight extinction in (010) section: Incorrect — Straight extinction is typically observed in orthorhombic crystals, not monoclinic ones.
(D) is uniaxial: Incorrect — A mineral with 2/m symmetry is biaxial, not uniaxial.
Step 3: Conclusion.
The correct answer is (B) shows inclined extinction in (100) section, as this is a characteristic optical property of monoclinic minerals with 2/m symmetry.