Plane of symmetry: It is an imaginary plane which can divide the crystal into two halves such that one is the mirror image of the other. A cubic crystal, for example, has two types of plane of symmetry.
(i) Rectangular planes of symmetry- These are the planes situated midway and parallel to the opposite faces. Since a cubic crystal has six faces, i.e. three pairs of opposite faces, it has three rectangular plane of symmetry as shown in Fig.
(ii) Diagonal plane of symmetry- These are the plane touching the opposite edges. As they lie on the diagonal of opposite faces, these are also termed as diagonal planes of symmetry. Since there are 12 edges or six pairs of opposite edges in a cubic crystal, as much as six diagonal planes of symmetry are possible in a cubic crystal. One plane passing diagonally through the cubic crystal is shown in Fig.
Thus in a cubic crystal, there are 3 rectangular planes of symmetry and 6 diagonal planes of symmetry, i.e. on the whole there are 3 + 6 = 9 planes of symmetry in a cubic crystal.
