What is a Crystal?

We get the word crystal from the ancient Greek word krystallos which roughly translates into “coldness drawn together.” In one aspect, that is completely correct, in scientific terms, crystals represent the “frozen” or cold version of liquids and gasses.

What defines a crystal? It is a body of uniform material with a regular internal structure, an organized internal structure with a regular, repeating arrangement of the small particles of atoms, ions or molecules. This arrangement is called a crystal lattice. The study of crystals is called crystallography.

The shape of a crystal is defined by its crystal lattice. There are seven main categories of crystal lattice shapes called crystal systems based on the shape and orientation of the lattice axes. Five of the systems are defined by three primary axes, these include: cubic, tetragonal, orthorhombic, monoclinic, and triclinic. The other two systems, hexagonal and trigonal include a fourth axis.

• Cubic System (three axes) - All three axis are of equal length and are at right angles to each other.

• Tetragonal System (three axes) - Two axis are of equal length and lie in a horizontal plane at right angles. The third axis, the principal axis, is either longer or shorter than the other two and is oriented vertically, perpendicular to the other axes.

• Orthorhombic System (three axes) - All three axes are of different lengths and are orientated at right angles to each other.

• Monoclinic System (three axes) - Two axis are of equal length and are oriented at right angles to each other. The third axis is either shorter or longer and is situated at an irregular angle.

• Triclinic System (three axes) - All three axes are of different lengths and are oriented at irregular angles to each other.

• Hexagonal System (four axes) - Three of the four axes lie in one plane. Those three axes are of equal length and intersect at angles of 120 degrees. The fourth axis is not the same length and is perpendicular.

• Trigonal System (four axes) - Three of the four axes lie in the same plane and are of equal length and intersect at angles of 120 degrees. The fourth axis is not the same length and is perpendicular. The only difference compared to a hexagonal system is that the corners of the crystal are beveled off.

Typical ice formed at standard pressure has a Hexagonal System