A mineral crystal is a solid material where the constituent atoms, ions, or molecules are arranged in a crystal lattice. This precise internal structure establishes the potential geometric framework for the mineral’s outward appearance. The final, observable external shape is known as the crystal habit or form. The shape a crystal ultimately takes is the result of a delicate interplay between its fixed internal blueprint and the variable external conditions encountered during its growth.
The Internal Blueprint: Atomic Arrangement
The fundamental determinant of a mineral’s shape is the precise, repeating arrangement of its internal components, which establishes the mineral’s inherent symmetry. Chemical composition and the nature of the bonds between atoms dictate the lattice structure. This structure creates a microscopic building block called the unit cell, which is the smallest repeating volume containing all the structural information of the crystal. Stacking these identical unit cells in three dimensions creates the entire crystal structure, determining the overall symmetry of the macroscopic crystal.
The internal symmetry influences how the crystal grows by favoring the development of certain flat surfaces, or faces, over others. Faces parallel to planes of strong chemical bonding tend to grow more slowly than faces parallel to planes of weaker bonding. The faces that grow the slowest are the ones that persist and become the largest, ultimately defining the crystal’s external geometric shape. This internal blueprint pre-programs the potential external symmetry of the mineral.
Crystal Systems: Classifying Geometric Shapes
To bridge the gap between the microscopic atomic structure and the observable external shape, geologists use a classification system based on symmetry. This system groups all possible crystal shapes into seven principal crystal systems. These systems are defined by the mathematical relationships between the crystal’s three imaginary crystallographic axes, specifically their relative lengths and the angles at which they intersect.
The High-Symmetry Systems
The most symmetrical is the Cubic (or Isometric) system, where all three axes are equal in length and intersect at 90-degree angles, leading to shapes like cubes and octahedrons. The Tetragonal system retains three axes at 90 degrees, but one axis is longer or shorter than the two equal horizontal axes. The Hexagonal and Trigonal systems are defined by three equal axes intersecting at 120 degrees in a horizontal plane, with a fourth, vertical axis perpendicular to the others.
The Low-Symmetry Systems
In the Orthorhombic system, all three axes are of unequal length, but they still intersect at 90 degrees, producing rectangular prisms. The Monoclinic system features three unequal axes, with two intersecting at an oblique angle while the third is perpendicular to the plane of the other two, leading to tilted prisms. The least symmetrical is the Triclinic system, where all three axes are of unequal length and all intersect at oblique angles, meaning no axes are perpendicular to one another. Minerals in this system often exhibit distorted or complex shapes due to this low symmetry.
These seven systems mathematically define the boundaries of the unit cell, dictating the possible external geometric forms a mineral can take. The crystal form is the outward expression of the internal symmetry defined by these axial relationships.
Environmental Factors Modifying Crystal Growth
While the internal atomic structure dictates the ideal geometric form, the final, observable shape, or crystal habit, is often modified by environmental conditions. These external factors influence the rate at which new layers of atoms are added to the growing crystal faces. The fundamental crystal system remains unchanged, but the proportions of the crystal can be dramatically altered.
One significant factor is the availability of space during crystallization. If a mineral grows in an open cavity, it develops flat, well-formed faces, resulting in a euhedral crystal. If it grows within a solid rock mass where neighboring grains compete for space, the crystal forms anhedral masses without distinct faces.
Changes in temperature and pressure also affect the growth rates of different crystal faces. Rapid cooling can accelerate crystal growth, potentially leading to smaller, less perfect crystals or skeletal forms. Conversely, slow, steady cooling allows atoms time to migrate and arrange themselves perfectly, fostering the growth of large, well-defined crystals.
The presence of chemical impurities can also modify growth. Foreign ions can adsorb onto specific crystal faces, effectively blocking or slowing down the addition of material to that face. This change in growth rate can cause the crystal to develop a distorted habit, such as a flat, tabular shape instead of the expected equant shape.