A mineral is defined by its specific chemical composition and its highly ordered internal arrangement of atoms, known as a crystal lattice. This internal atomic blueprint dictates the mineral’s preferred, characteristic external shape, which is referred to as its mineral habit. When a mineral is allowed to grow freely and without interruption, it will naturally form a geometric shape with smooth, distinct faces, such as the hexagonal prisms of quartz or the cubic forms of halite. However, in nature, the perfect conditions required for a mineral to achieve its ideal, geometric habit are rarely met. Factors in the surrounding environment, including physical space, the speed of formation, and the presence of foreign chemicals, commonly interfere with the process, resulting in a mineral that looks very different from its theoretical shape.
Physical Confinement and Spatial Restriction
The most direct reason a mineral fails to achieve its ideal form is a simple lack of physical space during its growth. When a crystal develops within a crowded environment, such as a cooling body of magma or a vein within solid rock, its boundaries become constrained by its neighbors. The mineral may have the correct chemistry and enough time to grow, but the physical pressure from surrounding material literally prevents the expansion of its characteristic faces.
This mechanical interference leads to classifications that describe the quality of the crystal’s shape rather than its ideal geometry. A mineral that has perfectly developed faces is called euhedral. If it has partially developed faces and irregular surfaces, it is classified as subhedral. Most commonly, minerals growing in tight spaces are forced into anhedral shapes, meaning they lack any visible, distinct crystal faces and instead fill the irregular voids between other grains.
When many such crystals grow together and interlock, the resulting texture is described by aggregate habits. Massive habits appear as a solid, compact block without visible crystal boundaries, while granular habits show an aggregate of many small, intergrown grains. Other physical constraints can promote growth along a single direction, leading to fibrous or columnar habits. In these cases, the crystal’s boundaries are dictated by the physical stress and direction of the confining space, not its internal structure.
Rapid Crystallization Rates
A second major factor inhibiting ideal habit formation is the speed at which the mineral forms, a concept known as the kinetic control of crystallization. The formation of a perfectly ordered lattice requires sufficient time for atoms or ions to migrate through the surrounding medium and precisely align themselves at the growing crystal face. When cooling or precipitation occurs rapidly, the atoms do not have this necessary time for optimal arrangement, resulting in a disordered or imperfect structure.
Rapid cooling, such as that experienced by lava extruded onto the Earth’s surface, causes a high degree of supersaturation or supercooling. This condition favors the rapid formation of a large number of tiny crystal nuclei rather than the slow, deliberate growth of a few large crystals. The resulting small crystals, often too small to exhibit distinct faces, are described as microcrystalline.
In extreme cases of rapid cooling, the atoms are essentially frozen in place before they can organize into any repeating, long-range structure. This process leads to the formation of an amorphous solid, such as volcanic glass, which lacks the internal atomic order that defines a mineral. A fast rate of crystallization often results in a dense mass of small, intergrown crystals or a completely non-crystalline material, making the achievement of a distinct, geometric habit impossible.
Chemical Interference and Impurities
The presence of foreign ions or molecules in the growth environment is a major barrier to achieving ideal mineral habit. Even trace amounts of chemical impurities can disrupt the orderly addition of atoms to the growing crystal face. These foreign substances often adsorb, or stick, to specific crystal faces, effectively blocking the attachment sites for the intended mineral components.
When certain faces are blocked, the growth rate along that direction slows down, while unblocked faces continue to grow quickly. Since the final shape of a crystal is determined by the relative growth rates of its faces, this selective blocking by impurities can alter the mineral’s habit, leading to shapes that deviate significantly from its ideal form. For instance, the presence of certain ions in solution can change the habit of calcite from a typical rhombohedron to a more complex shape.
Impurities can also be trapped within the crystal’s structure, causing internal stress and lattice defects. If the foreign ion is similar in size to the host ion, it may substitute directly into the lattice, but this substitution introduces strain that can result in distorted or curved faces. These chemical interferences often lead to complex habits like skeletal growth (where edges grow faster than face centers) or twinning (where two or more crystals intergrow in a symmetrical but non-ideal manner).