Crystals are made up of unit cells, which are box-like structures consisting of atoms in a unique and characteristic spatial arrangement. The internal structure of crystals is made up of three-dimensional stacks of these unit cells, which fit together perfectly with no gaps. As a crystal grows, new atoms and molecules attach readily to dangling bonds on the rough, incomplete surfaces. Once the surfaces (faces) become smooth, they are stable and new atoms have a hard time attaching. The facets (faces) of a crystal are oriented at precise angles determined by the underlying arrangement of the atoms and molecules inside.
Euhedral crystals have smooth, flat faces. Anhedral crystals do not show faces, since the crystal is just one grain in a polycrystalline solid.
Crystals grow directly in fluids, or from materials dissolved in fluids. In nature, the crystal's shape is determined by the geological environment under which the crystal grows. These conditions include the chemical makeup of the fluid, the pressure, the temperature, and the speed at which the crystal grows. When these conditions change, the shape of the crystal can change, too. For example, at high temperatures and pressures, fluorite forms octahedral crystals; at lower temperatures and pressures, fluorite forms cubes. Changing conditions can produce cube-octahedral crystals.
There are only 219 possible crystal space groups, or symmetries. These are classified into 7 crystal systems based on their atomic lattices:
1. Cubic (based on the cube). Crystal shapes include:
2. Tetragonal (based on the rectangle). Crystal shapes include:
3. Hexagonal (based on the 6-sided hexagon). Crystal shapes include:
4. Trigonal (based on the 3-sided triangle). Crystal shapes include: calcite, tourmaline
5. Orthorhombic (based on the rhombohedron). Crystal shapes include:
6. Monoclinic (based on the parallelogram). Crystal shapes include:
7. Triclinic (based on 3 inclined angles). Crystal shapes include:
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