The number of flat surfaces, known as faces, on a crystal lacks a single, simple answer. The count depends entirely on two factors: the material’s specific chemical structure and the environmental conditions during growth. A crystal is defined as a solid material whose atoms, molecules, or ions are arranged in a highly organized, repeating three-dimensional pattern called a crystal lattice. This precise internal arrangement dictates the geometric rules for the external shape, but the actual number of faces visible on any given specimen can vary greatly.
Defining the Crystal Face
The term “face” in crystallography refers to a naturally flat surface that forms as the crystal grows. These surfaces are not random; they are a direct, visible reflection of the underlying internal atomic structure and symmetry. The orientation of a face follows specific planes within the internal lattice where atoms are most densely packed.
The typical external appearance of a crystal is called its “crystal habit,” described by terms like cubic, prismatic, or tabular. For a crystal to develop perfectly flat, distinct faces, it must grow slowly and without obstruction. This allows the atomic structure to fully express its symmetry; if growth is disturbed, the flat face may not fully form.
The Seven Crystal Systems
The maximum possible number of faces a crystal can ideally possess is governed by its internal atomic symmetry, categorized into one of seven distinct crystal systems. These systems—Cubic, Hexagonal, Tetragonal, Trigonal, Orthorhombic, Monoclinic, and Triclinic—classify crystals based on the length of their axes and the angles between them. A mineral belonging to a specific system can only develop faces that conform to that system’s geometric constraints.
The Cubic system, which includes minerals like salt (halite) and garnet, represents the highest degree of symmetry. An idealized crystal in this system can take the form of a cube (six faces), an octahedron (eight faces), or a dodecahedron (twelve faces). These shapes are permissible because they respect the system’s equal-axis, right-angle symmetry.
As internal symmetry decreases, the potential complexity of the external shape increases. The Triclinic system has the lowest symmetry, with unequal axes intersecting at oblique angles. A crystal in this system may theoretically have a very high number of faces, often a multiple of the lowest number required by the symmetry.
How Growth Conditions Affect External Shape
The theoretical, ideal face count is rarely seen in nature because external factors significantly influence the crystal’s final shape. Real-world crystals often have a different number of faces than the mathematically perfect ideal due to the dynamics of their formation environment. Factors such as limited space, temperature fluctuations, pressure changes, and the concentration of the surrounding solution all play a role in determining which faces grow fastest.
When a crystal grows in a confined space, such as within a solid rock matrix, it may not develop any flat faces at all. Such crystals are termed “anhedral,” meaning they possess an irregular, non-faceted shape, effectively having zero visible faces. Conversely, a crystal that grows slowly in an open cavity, like a geode, is more likely to be “euhedral,” displaying its full complement of well-formed faces.
The presence of impurities in the growth medium can also alter the final shape by selectively slowing the growth rate of specific faces. Since the shape of the final crystal is determined by the faces that grow the slowest, this can result in an elongated or flattened crystal. A crystal that begins as a cube might be stretched into a rectangular prism if growth is preferentially accelerated along one axis.
Common Examples of Face Counts
Common minerals provide clear examples of how the crystal system translates into a specific number of faces in an ideal setting. Table salt (sodium chloride) belongs to the Cubic system. When grown perfectly, it forms a simple cube, which has six faces.
Quartz belongs to the Hexagonal system, characterized by six-fold rotational symmetry. An ideally terminated quartz crystal typically forms a six-sided prism, capped at one end by six pyramid faces, resulting in a total of twelve faces.
A simple, well-formed snowflake, which is a single ice crystal, is also governed by the Hexagonal system. The basic shape is a hexagonal plate or prism, fundamentally built upon a structure of six sides. Even complex, branched stellar dendrites maintain this six-fold symmetry, with all branches extending from the center at 60-degree angles.