Crystal Systems

All crystals belong to one of seven crystal systems, each defined by specific symmetry and geometric relationships: 1. Cubic (Isometric): The most symmetric system with three equal axes at 90° angles. Examples include halite (salt), fluorite, pyrite, and diamond. Cubic crystals often form perfect cubes, octahedrons, or dodecahedrons. 2. Tetragonal: Has three axes at 90°, but only two are equal in length. Examples include zircon, rutile, and cassiterite. Common forms include square prisms and pyramids. 3. Orthorhombic: Three unequal axes at 90° angles. Examples include topaz, olivine, and barite. Forms include rectangular prisms and pyramids. 4. Hexagonal: Four axes - three equal horizontal axes at 60° and one vertical axis of different length. Examples include quartz, beryl (emerald, aquamarine), and apatite. Forms include hexagonal prisms and pyramids. 5. Trigonal (Rhombohedral): Similar to hexagonal but with three-fold symmetry. Examples include calcite, tourmaline, and corundum (ruby, sapphire). Forms include rhombohedrons and trigonal prisms. 6. Monoclinic: Three unequal axes with two at 90° and one oblique. Examples include gypsum, orthoclase feldspar, and azurite. Forms are often prismatic or tabular. 7. Triclinic: The least symmetric system with three unequal axes, none at 90°. Examples include plagioclase feldspar, kyanite, and turquoise. Forms are typically irregular.

Symmetry is the key to understanding crystal systems. Each system has characteristic symmetry elements: Rotation Axes: Lines around which a crystal can be rotated to appear identical. Cubic crystals have multiple 4-fold and 3-fold axes, while triclinic crystals have only 1-fold (no symmetry). Mirror Planes: Planes that divide a crystal into mirror-image halves. Higher symmetry systems have more mirror planes. Center of Symmetry: A point from which all faces have corresponding opposite faces. Not all crystals have a center of symmetry. Crystal Classes: Within each system, there are crystal classes (32 total) that represent all possible combinations of symmetry elements. These classes determine the exact shape and properties of crystals. Understanding symmetry helps predict crystal properties, identify minerals, and understand how crystals will interact with light and other materials.

Each crystal system is defined by specific relationships between its unit cell dimensions: Unit Cell: The smallest repeating unit that defines the entire crystal structure. It's like a building block that repeats in three dimensions. Axial Ratios: The relative lengths of the crystal axes. In cubic systems, all ratios are 1:1:1. In other systems, these ratios are unique identifiers. Interfacial Angles: The angles between crystal faces are constant for each mineral, regardless of crystal size. This is the basis for crystal identification using a goniometer. Bravais Lattices: There are 14 possible ways to arrange points in 3D space, called Bravais lattices. These combine with the 7 crystal systems to create the framework for all crystal structures. These parameters are measured using X-ray diffraction and are fundamental to understanding crystal structure and properties.

Crystal system affects many physical properties: Hardness: While not directly determined by system, certain systems tend to produce harder minerals. Cubic and trigonal systems include many of the hardest minerals (diamond, corundum). Cleavage: The way crystals break is related to their system. Cubic crystals often have perfect cubic cleavage (galena, halite), while hexagonal crystals may have basal cleavage (mica). Optical Properties: Crystal system determines whether a mineral is isotropic (same properties in all directions, like cubic) or anisotropic (different properties in different directions, like all other systems). Electrical Properties: Piezoelectricity (generating electricity under pressure) occurs only in crystals without a center of symmetry, excluding certain crystal classes. Thermal Expansion: Different systems expand differently when heated. Cubic crystals expand equally in all directions, while others have directional expansion.

Field identification of crystal systems involves observing crystal forms: Habit: The general shape of a crystal. Cubic crystals form cubes; hexagonal crystals form hexagons. However, many factors can modify the ideal form. Face Angles: Using a contact goniometer, you can measure angles between faces. These angles are constant for each mineral and reveal the crystal system. Symmetry Observations: Look for mirror planes, rotation axes, and other symmetry elements. High symmetry suggests cubic or hexagonal systems. Cleavage Patterns: The number and orientation of cleavage planes can indicate the crystal system. Three perpendicular cleavages suggest cubic or orthorhombic. X-ray Diffraction: The definitive method for determining crystal system. X-ray patterns reveal the unit cell dimensions and symmetry. For rockhounds, visual observation combined with knowledge of common minerals and their typical crystal systems is usually sufficient for identification.