1. Internal Structure (Crystal Lattice):
* Arrangement of Atoms: The most fundamental factor is the arrangement of atoms within the crystal. Atoms are held together by chemical bonds, forming a repeating, three-dimensional pattern called a crystal lattice. The specific geometric arrangement of these atoms determines the shape of the crystal.
* Symmetry: Crystal lattices possess specific symmetry elements (like planes, axes, and points of symmetry) that govern the crystal's external shape.
* Unit Cell: The basic repeating unit of the crystal lattice is called the unit cell. The shape and dimensions of the unit cell directly influence the crystal's macroscopic shape.
2. Growth Conditions:
* Temperature: Crystal growth is influenced by temperature. Higher temperatures generally lead to faster growth rates and potentially different crystal habits (shapes).
* Pressure: Pressure during crystal growth can affect the rate and direction of crystal growth.
* Chemical Environment: The presence of impurities, solvents, and other chemicals can influence the crystal's habit by affecting the rate of growth along different crystallographic directions.
* Nucleation Sites: The location where crystal growth begins (nucleation) can affect the overall shape of the crystal.
3. Twinning:
* Intergrowth of Crystals: Twinning occurs when two or more crystals intergrow in a specific orientation. This can create complex and unusual crystal shapes.
4. Habit:
* Dominant Crystal Face: The crystal habit refers to the overall shape of the crystal as it grows. This is influenced by the relative growth rates of different crystal faces, which are determined by the environment and the crystal's internal structure.
Examples:
* Cubic crystals (like salt): The atoms are arranged in a cube-like unit cell, leading to a cubic crystal shape.
* Hexagonal crystals (like quartz): The atoms are arranged in a hexagonal unit cell, resulting in a hexagonal crystal shape.
* Tetragonal crystals (like zircon): The atoms are arranged in a tetragonal unit cell, creating a prism-like crystal shape.
Note: The shape of a crystal can be influenced by a combination of these factors. Understanding these factors is crucial for identifying minerals, analyzing their properties, and controlling crystal growth in industrial applications.
