Crystal Systems

CRYSTAL SYSTEMS

There are '7' types of crystal systems. They are

1. Cubic

2. Tetragonal

3. Orthorhombic

4. Monoclinic

5. Triclinic

6. Rhombohedral

7. Hexagonal

1. Cubic system

In this crystal system, all the three axial lengths of the unit cell are equal and they are perpendicular to each other (fig. 1.9 [a]).

i.e., a = b = c and

α = β = γ = 90°

Example: Iron, Copper, Sodium Chloride (NaCl), Calcium Fluoride (CaF₂).

2. Tetragonal system

In this system, two axial lengths of the unit cell are equal and third axial length is either longer or shorter (fig. 1.9 [b]). All the three axes are perpendicular to each other.

i.e., a = b ≠ c and

α = β = γ = 90°

Example: White tin, Indium.

3. Orthorhombic system

In this system, three axial lengths of the unit cell are not equal but they are perpendicular to each other. (fig. 1.9 [c]).

i.e., a ≠ b ≠ c and

α = β = γ = 90°

Example: Sulphur, Topaz.

4. Monoclinic system

In this system, three axial lengths of unit cell are not equal. Two axes are perpendicular to each other and third axis is obliquely inclined (fig. 1.9 [d]).

i.e., a ≠ b ≠ c and

α = β = 90°, γ ≠ 90°

Example: Sodium sulphite (Na₂ SO₃), Ferrous sulphate (FeSO₄).

5. Triclinic system

In this system, three axial lengths of unit cell are not equal and all the three axes are inclined obliquely to each other (fig. 1.9 [e]).

i.e., a ≠ b ≠ c and

α ≠ β ≠ γ ≠ 90°

Example: Copper sulphate (CuSO₄), Potassium dichromate (K₂Cr₂O₇)

6. Rhombohedral system (Trigonal)

In this system, three axial lengths of the unit cell are equal. They are equally inclined to each other at an angle other than 90° (fig. 1.9 [f]).

i.e., a = b = c and

α = β = γ ≠ 90°

Example: Calcite.

7. Hexagonal system

In of unit cell (say this system, two axial lengths horizontal) are equal and lying in one plane at angle 120° with each other. (fig. 1.9 [g]).

The third axial length (say vertical) is either longer or shorter than other two and it is perpendicular to this plane.

i.e., a = b ≠ c and

α = β = 90°, γ = 120°

Example: Quartz, Tourmaline.