Question:
Oblique extinction is shown by most prismatic sections of minerals crystallizing in the following crystal system.
Oblique extinction is shown by most prismatic sections of minerals crystallizing in the following crystal system.
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Extinction is the position, seen under crossed polars in a microscope, where a mineral grain appears dark. The angle between this dark position and a crystal direction depends on the symmetry of the crystal system.
Updated On: Jun 16, 2026
- Cubic
- Orthorhombic
- Monoclinic
- Triclinic
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The Correct Option is C
Solution and Explanation
Concept:
Extinction is the position, seen under crossed polars in a microscope, where a mineral grain appears dark. The angle between this dark position and a crystal direction depends on the symmetry of the crystal system.
Step 1:
In low symmetry systems where the crystallographic axes are inclined, the extinction is oblique (inclined). The monoclinic system, having an inclined axis, shows oblique extinction in most prismatic sections.
Step 2:
Cubic minerals are isotropic and stay dark (no extinction angle), while orthorhombic minerals generally show straight (parallel) extinction in prismatic sections. Hence monoclinic best fits oblique extinction.
Answer: Option (3) — Monoclinic.
Extinction is the position, seen under crossed polars in a microscope, where a mineral grain appears dark. The angle between this dark position and a crystal direction depends on the symmetry of the crystal system.
Step 1:
In low symmetry systems where the crystallographic axes are inclined, the extinction is oblique (inclined). The monoclinic system, having an inclined axis, shows oblique extinction in most prismatic sections.
Step 2:
Cubic minerals are isotropic and stay dark (no extinction angle), while orthorhombic minerals generally show straight (parallel) extinction in prismatic sections. Hence monoclinic best fits oblique extinction.
Answer: Option (3) — Monoclinic.
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