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Minimum symmetry that a cubic crystal must possess is

  • A.four 3-fold rotation axes.
  • B.three 4-fold rotation axes.
  • C.three orthogonal mirror planes.
  • D.centre of symmetry.

Answer: A

A cubic crystal must have four 3-fold rotation axes.

The defining symmetry of the cubic system is four three-fold axes along the body diagonals of the cube. Every cubic point group, including the lowest one (23, tetrahedral rotations only), has them.

Three 4-fold axes are present only in the higher cubic classes (432 and m3m), not in all, so they are not the minimum. Three orthogonal mirror planes or a centre of symmetry are likewise absent from some cubic classes (for example 23 has neither), and many non-cubic crystals have them.

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