Crystals are solid geometric figures which are bounded by well defined more or less plane surfaces called ‘faces’. Crystals show the following general characteristics:
(i) Crystals are polyhedral bodies.
(ii) It possesses a typical internal atomic structure and accordingly the faces of a crystal are arranged in a regular pattern.
(iii) The regular geometry is developed only under suitable physico-chemical condition. Crystals are formed due to slow-cooling.
ADVERTISEMENTS:
(iv) A crystal which possesses both external form as well as internal atomic structure is said to be a ‘perfect crystal’; but if it possesses only the internal atomic structure without the development of corresponding external form, it is said to be ‘crystalline’ when there is neither internal atomic structure nor external form, it will be known as an ‘amorphous substance’.
Elements of Crystal Forms:
(а) Faces:
These are the external expressions of the internal atomic planes of the crystal. Faces are said to be ‘like’ when they have similar properties and ‘unlike’ when they show different characteristics.
ADVERTISEMENTS:
(b) Edge:
The line of intersection of two adjacent faces is known as an edge.
(c) Solid angle:
It is a point where three or more faces meet.
ADVERTISEMENTS:
(d) Form:
It is a group of faces, which have a like position with respect to the crystallographic axes of reference.