Crystal System
Definition
The classification of crystals based on their symmetry and geometric properties. There are seven crystal systems: cubic, hexagonal, tetragonal, orthorhombic, monoclinic, triclinic, and trigonal.
Example
Halite (table salt) crystallizes in the cubic system, forming perfect cube-shaped crystals.
Symmetry and Atomic Order
Crystals are not random; they are built from atoms arranged in a highly ordered, repeating pattern extending in all three spatial dimensions. This internal order dictates the external shape of the crystal. Crystallographers categorize all minerals into one of seven crystal systems based on the lengths of their axes and the angles between them.
The Seven Systems
- Cubic (Isometric): Highly symmetrical (e.g., Diamond, Garnet, Pyrite).
- Tetragonal: Like a cube stretched along one axis (e.g., Zircon, Rutile).
- Hexagonal: Six-sided symmetry (e.g., Emerald, Aquamarine).
- Trigonal: Three-fold symmetry (e.g., Quartz, Tourmaline, Calcite).
- Orthorhombic: Like a brick shape (e.g., Topaz, Peridot).
- Monoclinic: Like a skewed brick (e.g., Gypsum, Azurite, Jade).
- Triclinic: The least symmetrical (e.g., Turquoise, Kyanite).
Understanding these systems helps geologists predict the angles of crystal faces and cleavage planes.