Question:
Which one of the following is not the Twins law?
Which one of the following is not the Twins law?
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Twin laws describe the valid symmetry relationship between the individuals of a twinned crystal. A genuine twin law must agree with crystallographic symmetry principles.
Updated On: Jun 16, 2026
- An axis of even fold symmetry can never become a twin axis, only 3-fold symmetry axis can serve as a twin axis
- Twins that develop in classes with a centre of symmetry have both twin axis and twinning plane perpendicular to it
- Twinning plane can coincide with symmetry planes
- Twin axis is always normal to the twin plane but need not be normal to composition plane
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The Correct Option is C
Solution and Explanation
Concept:
Twin laws describe the valid symmetry relationship between the individuals of a twinned crystal. A genuine twin law must agree with crystallographic symmetry principles. The question asks for the statement that is NOT a true twin law.
Step 1:
A correct twin law requires that a twin axis cannot already be a symmetry axis of the untwinned crystal, and the twin plane likewise cannot coincide with an existing symmetry plane. The other statements about even-fold axes, perpendicular relations in centrosymmetric classes, and the twin axis being normal to the twin plane express accepted relationships.
Step 2:
The statement that a twinning plane can coincide with symmetry planes contradicts the basic rule that a twin plane is never a possible symmetry plane of the crystal. Hence this statement is not a valid twin law.
Answer: Option (3) — Twinning plane can coincide with symmetry planes.
Twin laws describe the valid symmetry relationship between the individuals of a twinned crystal. A genuine twin law must agree with crystallographic symmetry principles. The question asks for the statement that is NOT a true twin law.
Step 1:
A correct twin law requires that a twin axis cannot already be a symmetry axis of the untwinned crystal, and the twin plane likewise cannot coincide with an existing symmetry plane. The other statements about even-fold axes, perpendicular relations in centrosymmetric classes, and the twin axis being normal to the twin plane express accepted relationships.
Step 2:
The statement that a twinning plane can coincide with symmetry planes contradicts the basic rule that a twin plane is never a possible symmetry plane of the crystal. Hence this statement is not a valid twin law.
Answer: Option (3) — Twinning plane can coincide with symmetry planes.
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