A crystal is a solid material whose constituent particles are arranged in a highly ordered, repeating pattern, forming a crystal lattice. This precise internal arrangement primarily determines the external shape a crystal can take. It is important to distinguish between the crystal form and the crystal habit. The form is the ideal geometric shape dictated by the internal structure and symmetry. The habit, by contrast, is the actual, observable external shape, which is often modified by environmental conditions during growth. The final shape is a result of the intrinsic atomic blueprint interacting with extrinsic environmental forces.
The Atomic Blueprint: Internal Structure and Symmetry
The internal structure, or atomic blueprint, is defined by a repeating three-dimensional pattern built from the smallest repeating unit of volume, called the unit cell. The dimensions and angles of this unit cell define the geometry of the entire crystal lattice. The constraints on the unit cell’s edges and the angles between them mathematically determine the crystal’s inherent symmetry. This inherent symmetry dictates the angular relationships between the crystal’s faces.
Bravais’ Law and Growth Rate
The external faces of a growing crystal tend to align parallel to internal planes of high atomic density, a principle known as Bravais’ Law. Faces densely packed with atoms are generally the most stable and grow the most slowly, allowing them to define the final external shape. The faces that grow fastest are typically eliminated during the growth process, leaving behind the slower-growing, most stable faces to enclose the crystal. This process ensures that the final external form maintains the symmetry inherent in the underlying unit cell. The unit cell’s geometry is the limiting factor, establishing which geometric shapes are possible for a given material.
The Seven Fundamental Crystal Systems
The geometry of the unit cell provides the basis for classifying all crystals into seven fundamental categories, known as the crystal systems. These systems are defined by the mathematical relationships between the three axes and the three interaxial angles of the unit cell.
The system with the highest symmetry is the Cubic system, where all three axes are equal in length and all three angles are 90 degrees. This results in highly symmetrical shapes like cubes and octahedrons, exemplified by the mineral halite (table salt).
The Tetragonal system is slightly less symmetrical, having two equal axes and one unique axis, but all three angles remain at 90 degrees. Zircon often forms elongated prisms in this system. The Orthorhombic system reduces symmetry further by having three unequal axes while still maintaining right angles between them. Topaz is a common example.
The Hexagonal system has three equal axes lying in a plane at 120-degree angles, with a fourth, unique axis perpendicular to them. Beryl, which includes emerald and aquamarine, typically displays this six-sided prismatic form. The Trigonal system is formally distinct but often grouped with Hexagonal due to shared symmetry elements. Minerals like calcite and quartz crystallize in the Trigonal system.
The Monoclinic system has three unequal axes, with two angles at 90 degrees and one angle oblique. This lower symmetry often results in shapes like tilted prisms, with gypsum being a common example. Finally, the Triclinic system has the lowest symmetry, with all three axes unequal in length and all three angles unequal and not 90 degrees. Minerals like kyanite grow under this least constrained geometric framework.
Environmental Factors Shaping Crystal Habit
While the crystal system dictates the geometric possibilities, the final habit or external appearance is heavily influenced by the conditions under which the crystal grows. The environment determines which of the possible faces grow fastest or slowest, thereby modifying the crystal’s final shape. One major factor is the growth rate, or growth kinetics.
Growth Rate and Kinetics
Rapid growth, often caused by sudden cooling or high supersaturation, frequently results in distorted or less perfect shapes, such as needle-like (acicular) or branching (dendritic) habits. Conversely, slow, steady growth in a stable environment allows the crystal faces to develop fully and equally. This promotes the formation of well-formed, isometric shapes that more closely resemble the ideal geometric form.
Impurities and Habit Modifiers
The presence of impurities or foreign molecules in the growth solution is another highly influential environmental factor. These molecules can preferentially adsorb, or stick, to the surface of specific crystal faces. This adsorption effectively blocks the sites where the solute molecules would normally attach, slowing down the growth rate of that particular face. If a specific impurity slows the growth of one set of faces more than others, those faces will become larger and more prominent in the final crystal habit, even though the internal lattice structure remains unchanged. These impurities act as habit modifiers, subtly altering the external shape by selectively inhibiting the growth kinetics of certain crystallographic planes.