1. Cubic: Three axes of equal length, intersecting at right angles. Examples: Halite (table salt), Diamond, Garnet
2. Tetragonal: Two axes of equal length, one axis of different length, all intersecting at right angles. Examples: Zircon, Rutile
3. Orthorhombic: Three axes of unequal length, all intersecting at right angles. Examples: Topaz, Olivine
4. Hexagonal: Three axes of equal length, intersecting at 120 degrees, and one axis perpendicular to the plane of the other three. Examples: Quartz, Beryl
5. Trigonal: Three axes of equal length, intersecting at 120 degrees, and one axis perpendicular to the plane of the other three (similar to hexagonal, but with different symmetry operations). Examples: Calcite, Tourmaline
6. Monoclinic: Three axes of unequal length, two intersecting at an oblique angle, and the third perpendicular to the plane of the other two. Examples: Gypsum, Orthoclase
7. Triclinic: Three axes of unequal length, all intersecting at oblique angles. Examples: Turquoise, Kyanite
Here's a breakdown of what symmetry means:
* Rotation: A crystal can be rotated by a certain angle and still look the same.
* Reflection: A crystal can be reflected across a plane and still look the same.
* Translation: A crystal can be shifted along a certain axis and still look the same.
The combination of these symmetry elements defines the crystal system. For example, a cube has four threefold rotation axes, six twofold rotation axes, three fourfold rotation axes, and nine reflection planes. This specific combination of symmetry elements defines the cubic crystal system.
Beyond crystal system, other factors are also used for mineral classification:
* Chemical composition: Minerals are classified based on their chemical makeup. For example, quartz is silicon dioxide (SiO2) while calcite is calcium carbonate (CaCO3).
* Physical properties: These can include hardness, color, streak, luster, cleavage, and fracture.
Therefore, classifying crystals of minerals involves a combination of crystal system, chemical composition, and physical properties.
