A crystal is defined by the precise, ordered arrangement of its constituent atoms, ions, or molecules, which repeat in a predictable pattern across a significant distance. This internal structural regularity, known as long-range order, distinguishes crystals from other forms of solid matter. Scientists use systematic classification methods to organize the vast number of specific crystal types. These systems focus on two primary characteristics: the geometry of the repeating structure and the chemical bonds that hold the structure together. Understanding these classification schemes helps determine how many types of crystals exist.
The Seven Geometric Systems
The most fundamental way scientists categorize crystals is based on the geometry of their smallest repeating unit, known as the unit cell. The unit cell is defined by six parameters: the lengths of its three axes (\(a\), \(b\), and \(c\)) and the three angles between those axes (\(\alpha\), \(\beta\), and \(\gamma\)). Variations in these six parameters lead to exactly seven distinct categories, which are called the crystal systems.
The cubic system represents the highest degree of symmetry, where all three axes are equal in length (\(a = b = c\)) and all three angles are 90 degrees (\(\alpha = \beta = \gamma = 90^\circ\)). This highly symmetrical structure is common in minerals like table salt (sodium chloride). Conversely, the triclinic system possesses the lowest symmetry, with all three axes having different lengths (\(a \neq b \neq c\)) and all three angles being unequal and not 90 degrees (\(\alpha \neq \beta \neq \gamma \neq 90^\circ\)).
The remaining five systems lie between these two extremes, each defined by specific geometric constraints. The tetragonal system has two axes of equal length but a third that is different (\(a = b \neq c\)), with all angles remaining at 90 degrees. The orthorhombic system has three unequal axes, but all angles are 90 degrees. The hexagonal system features two equal axes at 120 degrees, with the third axis perpendicular to them. The trigonal (or rhombohedral) system also involves unique symmetries. Finally, the monoclinic system has unequal axes, with two angles at 90 degrees and one angle that is not.
Classification Based on Chemical Bonding
Beyond geometry, crystals are categorized by the type of chemical bond that holds the atoms or molecules together, which significantly influences the material’s physical properties. This classification yields four primary types of crystalline solids.
Ionic solids, such as sodium chloride, are composed of positive and negative ions held together by strong electrostatic forces. These bonds typically result in materials that are hard, brittle, and have high melting points. Covalent network solids are characterized by a vast, continuous network of atoms connected by strong, directional covalent bonds. Diamond is a prime example, where the robust bonding makes the structure exceptionally hard and gives it a very high melting point.
Metallic solids consist of a lattice of positively charged metal ions surrounded by a “sea” of mobile, delocalized valence electrons. This unique metallic bonding allows for properties like high electrical conductivity, malleability, and ductility, seen in materials such as copper or iron. Molecular solids are formed by discrete molecules held together by relatively weak intermolecular forces, such as London dispersion forces or hydrogen bonds. Ice (solid water) and dry ice (solid carbon dioxide) are examples; these weak forces result in low melting points and softer materials.
True Crystals Versus Amorphous Solids
To be classified, a solid must exhibit the defining feature of long-range order. This means the highly regular, repeating pattern of atoms or molecules extends consistently throughout the entire structure. This extensive order allows true crystalline solids to have a sharp, well-defined melting point.
In contrast, amorphous solids, such as glass, rubber, or certain plastics, lack this long-range order. While their atoms may have a predictable arrangement over very short distances, the pattern quickly becomes random over larger scales. These non-crystalline materials do not melt sharply but instead soften gradually over a range of temperatures. Many common materials are polycrystalline, meaning they are composed of a large number of tiny, individual crystals, known as crystallites, which are randomly oriented relative to one another. Although each individual crystallite has long-range order, the material as a whole is an aggregate of many small ordered regions.