A crystal is a solid material where the atoms, molecules, or ions are arranged in a highly organized, repeating pattern that extends in all three spatial dimensions. This internal atomic architecture is known as the crystal structure or lattice. The resulting external appearance, characterized by flat faces and sharp corners, is called its morphology.
How Internal Structure Determines External Shape
The geometric shape of a crystal is dictated by the arrangement of its constituent particles at the atomic level. Scientists use the concept of a unit cell, the smallest repeating block that builds the entire structure. This unit cell is defined by the lengths of its three sides and the angles between them, measured along crystallographic axes. These axes provide a coordinate system that defines the symmetry and the spacing of the atoms within the lattice. The external flat faces of a fully formed crystal grow parallel to planes of highest atomic density. Consequently, the angles between a crystal’s faces are fixed and consistent for a specific mineral, regardless of the crystal’s size. For example, a perfect salt crystal exhibits 90-degree angles between its faces because its unit cell is perfectly cubic.
The Seven Crystal Systems
The internal geometry allows scientists to classify all crystalline solids into seven primary categories, known as the crystal systems. This classification is based on the symmetry and angular relationships of the crystallographic axes, organizing structures from the highest to the lowest degree of symmetry.
The cubic, or isometric, system represents the highest symmetry, defined by three axes of equal length that all intersect at 90-degree angles. Minerals in this system, such as diamond and halite (table salt), often form cubes, octahedrons, or dodecahedrons. The tetragonal system maintains three axes intersecting at right angles, but two axes are equal in length while the third is either longer or shorter. Zircon and rutile are common minerals belonging to this category, typically forming prismatic or pyramidal shapes.
The hexagonal system is characterized by four axes; three equal-length axes lie in a single plane, intersecting at 120-degree angles, with a fourth axis positioned perpendicularly. Beryl is a classic example that often forms six-sided prisms. The trigonal system is closely related, sharing symmetry elements but formally distinct, often defined by a single three-fold rotational axis. Quartz, a ubiquitous mineral, is a prime example of a trigonal crystal.
Crystals in the orthorhombic system possess three axes of unequal length, but all intersect at 90-degree angles. Topaz and olivine are examples of minerals that form within this system. The monoclinic system features three axes of unequal length, with two intersecting at 90 degrees, while the third axis is inclined at an angle other than 90 degrees. Gypsum and orthoclase feldspar commonly crystallize in this lower-symmetry system.
The triclinic system is the least symmetrical, featuring three axes of unequal length that all intersect at three different angles, none of which are 90 degrees. This lack of symmetry results in crystals that often appear irregularly shaped, such as plagioclase feldspar or turquoise.
Environmental Factors That Influence Final Crystal Appearance
While the crystal system establishes the fundamental geometric framework, it only dictates the potential shapes a crystal can take. The final external appearance, referred to as its crystal habit, is heavily influenced by the conditions present during its growth. Crystal habit describes the overall look, proportion, and aggregation of a crystal, which can vary significantly even among minerals that share the same internal crystal system. The presence of chemical impurities often plays a large role, as foreign ions can selectively attach to and slow down the growth rate of certain crystal faces. This differential growth rate causes some faces to enlarge more than others, altering the crystal’s final proportions. For instance, a cubic crystal might grow into a thin, tabular shape if impurities inhibit growth along one axis more than the others.
Temperature and pressure are also significant factors. Crystals that form under high pressure or rapid cooling often have less time to develop well-formed faces, resulting in smaller, less perfect shapes. Conversely, crystals that grow slowly in open cavities tend to form large, nearly perfect, euhedral shapes. The physical space available for growth also determines the habit; a crystal growing in a confined space will be forced to conform to that boundary, producing an anhedral or irregularly shaped specimen. This interplay explains why a mineral like calcite, which belongs to the trigonal system, can be found with habits ranging from flat plates (tabular) to long, slender needles (acicular).