**Crystallography**

**A crystal**is a solid object with a geometric shape that reflects a “long-range” regular internal structure.

**Space lattice**: The regular internal structure of a crystal is manifested by the existence of a space lattice, which is "

*an array of points in space that can be repeated indefinitely"*.

**Unit cell: A**unit cell is the smallest number of "points" which completely define the space lattice. The repetition of those points or unit cells in a space lattice is performed by certain

**operations**which build the space lattice.

**The selection of unit cells:**
Ø The
smallest sized unit that retains the characteristics of the space lattice.

Ø Edges
of the cell should coincide with symmetry axes.

Ø Edges
of the cell related to each other by the symmetry of the lattice.

**The Crystal Systems**

The crystal classes are grouped
into seven crystal systems based on the following criteria:

1)
Relative lengths of the crystallographic axes

2)
Number of crystallographic axes

3)
Values of the inter-axial angles

4)
Some essential element of symmetry

The seven crystal systems are:

(1)

__The Cubic system__: Three crystallographic axes, a = b = c, = = = 90. Essential element of symmetry is a three-fold rotary or rotary inversion axis.
(2)

__The Tetragonal system__: Three crystallographic axes, a = b c, = = = 90. Essential element of symmetry is a four-fold rotary or rotary inversion axis.
(3)

__The Rhombohedral system__: Four crystallographic axes, three of which are equal and coplanar and at angles of 120°, fourth axis "c" is perpendicular to the other three axes, and is characterized by commonly being a three fold axis of symmetry.
(4)

__The Hexagonal system__: Four crystallographic axes, three of which are equal and coplanar (a_{1}, a_{2}, a_{3}) and at angles of 120°, fourth axis (c) is perpendicular to the other three axes, and is characterized by being a six fold axis of symmetry.
(5)

__The Orthorhombic system__: a b c, = = = 90. Essential element of symmetry is a two-fold rotary axis.
(6)

__The Monoclinic system:__a b c, = = 90, > 90. Essential element of symmetry is a two fold rotary axis or a plane. The 2-fold rotational axis or the direction perpendicular to the mirror plane is usually taken as the b axis, the a axis is inclined to the front (b > 90°), and c is vertical.
(7)

__The Triclinic system__: a b c, . No essential element of symmetry. Additional criteria: As much as possible, b and a should be both > 90°. The most pronounced zone should be vertical.
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Article By

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**Mr.Ibhrahim**

Assistant Professor

H&S Department

LAQSHYA Institute of Technology & Sciences

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