SYMMETRY


Crystals have certain regularity of position of like faces edges etc. this regularity constitute the symmetry of the crystal . It depends on the type of space lattice and unit cell involved in its construction. The symmetry varies in crystals. Different crystals have different degree of symmetry. It is defined with reference to three criteria of symmetry :-

Plane of symmetry - A plane of symmetry divides a crystal into identical or mirror imaged halves. A cube has 9 planes of symmetry.

Axis of symmetry - If a crystal on being rotated comes to occupy the same position in space more than once in a complete rotation, the axis about which rotation has taken place is called an axis of symmetry. Depending upon the degree of symmetry, a crystal may comes to occupy the same position two, three, four or six times in a complete rotation. the terms aplied to these different classes of axes are as folows -

    Two time - two-fold, diad

    Three times - three-fold, triad

    Four times - four fold, tetrad

    Six times - six fold, hexad

Centre of symmetry - A crystal has acentre of symmetry when like faces, edges etc. are arranged in pairs in corresponding position on opposite sides of a central point.