A crystal is a solid in which the component atoms are arranged in a particular, repeating, three-dimensional pattern. When these internal patterns produce a series of external, flat faces arranged in geometric forms, this forms a crystal. These repeating structures are identical structural units of atoms or molecules, and are called unit cells.

The unit cell is reproduced over and over in three dimensions, meaning that the shape of the crystal will resemble that of the individual unit cell. The crystals of different minerals can have unit cells that are the same shape but are made of different chemical elements. Because a crystal is built up of repeating geometric patterns, all crystals exhibit symmetry, depending on the basic geometry of their unit cells.

These fall into seven main groups and are called crystal systems. The final external form a crystal takes is known as its habit, and the shape produced by a mass of numerous identical crystals is a growth habit (see opposite).

**Minerals and crystal systems**

*Mineralogists and crystallographers have a complex set of criteria for determining which mineral belongs in which crystal system, based on symmetry. In practical terms, these systems can be understood as a group of three-dimensional cells starting with the basic cube (below). It should be noted that hexagonal and trigonal systems (right) are considered to be one system by some crystallographers.*

**Cubic a=b=c**

The cubic unit cell has three right angles, and the lengths of its sides are all equal. Thus, relative to the length of its sides, a equals b equals c.

**Tetragonal a=b≠c**

If the cubic cell is stretched vertically, there are still three right angles, but the length of the vertical side is longer than the other two. Now, a equals b does not equal c.

**Orthorhombic a≠b≠c**

If the tetragonal cell is stretched horizontally, the three right angles remain, but now none of the sides are of equal length. Thus, a does not equal b does not equal c.

**Triclinic a≠b≠c**

To create the triclinic system, all of the faces are skewed so that no right angles remain, and none of the edges of the faces are equal.

**Trigonal a=b=c**

The first hexagonal cell is further altered by squeezing two opposing short edges, so that all of the faces are lozenge shaped. The angle between a and b is 120 degrees. In the US, the trigonal is considered a division of the hexagonal.

**Hexagonal a=b≠c**

To create the hexagonal system, two of the opposing long edges of tetragonal cell are squeezed together, leaving rectangular sides but lozenge-shaped ends.

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