All kinds of crystals have a symmetrical form, which is known as the symmetric characteristic of the crystals. The symmetry elements are used as parameters to describe this symmetry. Three elements of symmetry can be recognized in a crystal. These are (i) plane of symmetry, (ii) axis of symmetry, and (iii) center of symmetry.
(1). Plane of Symmetry:
An imaginary plane, which divides a crystal into two equal halves, is known as the ”Plane of symmetry”. Each of the two equal halves is the mirror image of the other. For example, a cube has nine planes of symmetry. The cube has nine symmetry planes. Three planes lie parallel to the side squares and go through the centre (picture). Six planes go through opposite edges and two body diagonals. They divide the cube into prisms.
(2). Axis of Symmetry:
It is an imaginary line through the crystal about which if the crystal is rotated, it gives the observer exactly the same view more than once in a single rotation. If the same view is repeated 2,3,4, or 6 times, the axis of symmetry is referred to as two-fold, three-fold, four-fold, or sixe-fold respectively. The following figure shows an axis of three-fold symmetry. To understand the axes of rotation better please watch the following video.
(3). Centre of Symmetry:
If each face in a crystal is duplicated by a similar parallel face on the opposite side, it is said to have a centre of symmetry. The cube and octahedron possess a centre of symmetry whereas a tetrhedron does not.
- Introduction to Crystallography
- Unit Cells of the Crystal Lattice
- Crystallographic Axes
- Interfacial Angle of Crystals
- Parameters of Crystal faces
- Crystallographic Notation
- Forms of Crystal faces