Nature of crystals; crystalline and amorphous materials; polycrystalline materials; a brief introduction to Crystal systems.

1. Introduction to crystallography
Nipin K P
kp.nipin@gmail.com

2. Introduction
• Crystallography is the branch of Earth science which deals with all aspects of crystals,
that is, their formation from the melts, their internal structure and their external shape or
morphology.
• It includes the study of their external form, internal atomic structure, mode of their
growth, mathematical relation of their faces with its axis (Miller Indices), and the
measurement of angles between similar faces.

3. • A crystal may be defined as a solid regular, polyhedral form bounded by smooth surface,
and orderly internal arrangement of its atoms. Crystals are natural or synthetic.
• Natural crystals occurring in thousands of varieties are formed in the Earth from natural
fluids by their cooling under different conditions of temperature and pressure. The process
of formation of crystals called crystallization.
• Some substances are non-crystalline, it exhibit both internal molecular structure and
regular external geometric shape are lacking such substances are called amorphous. E.g.:
Natural glass, Opal (composition: SiO2).
• Polycrystalline materials are solids that are composed of many crystallites of varying
size and orientation.

4. Morphological characters of crystal
 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.

5.  Edge. The line of intersection of two adjacent faces is known as an edge.
 Solid angle. It is the point where three or more faces meet.
 Form. In crystallography the term form is applied to an assemblage of one or more
similar faces which may partially or completely constitute a crystal exterior. It is the
group of faces, which have a like position with respect to the crystallographic axes of
reference.

6. Interfacial Angle:
• The angle at which any two adjacent faces are placed on the crystal with respect to each
other is called an interfacial angle.
• Interfacial angles can be measured using a contact goniometer or more precisely using a
reflection goniometer.

7. Crystallographic Axis
 The crystallographic axes are imaginary lines that we can draw within the crystal lattice.
 These will define a coordinate system within the crystal.

8. Crystal Systems
Based on the properties of crystallographic axes crystals are grouped into different crystal
systems
i. Isometric system
ii. Tetragonal system
iii. Hexagonal system
iv. Orthorhombic system
v. Monoclinic system
vi. Triclinic system