How Crystals Grow: From Atoms to Specimens

The growth of a crystal — from a disordered collection of ions in solution or melt to a geometrically perfect solid bounded by flat faces — is one of the most elegant processes in nature. Understanding crystal growth requires thinking at multiple scales simultaneously: from the behavior of individual atoms at the crystal surface, to the flow of fluids through a hydrothermal vein, to the geological conditions of an entire mineral deposit.

Crystal growth begins with nucleation — the formation of the first tiny cluster of atoms or ions in the correct arrangement to act as a seed for further growth. Nucleation is fundamentally a competition between two opposing tendencies: the energy gained by atoms joining an ordered crystal lattice (favorable), and the energy cost of creating a new interface between crystal and surrounding medium (unfavorable). When a solution or melt is only slightly supersaturated, the energy barrier to forming new crystal nuclei is high. Very small clusters of atoms are energetically unstable and dissolve back into solution before they can grow. Only clusters above a critical size — the critical nucleus — are stable enough to persist and grow.

Supersaturation is the driving force of crystallization. A solution is supersaturated when it contains more dissolved material than it could hold at equilibrium. Supersaturation arises when a solution cools (reducing the solubility of most dissolved minerals), when water evaporates (concentrating the dissolved minerals), when the pressure drops (particularly important in hydrothermal systems), or when chemical reactions produce new compounds that are sparingly soluble. The degree of supersaturation controls both the rate of nucleation and the rate of crystal growth. At very high supersaturation, many nuclei form quickly, producing numerous small crystals. At low supersaturation, few nuclei form but grow slowly, producing fewer, larger, better-formed crystals.

Once a nucleus exceeds the critical size, it grows by the attachment of ions or molecules from the surrounding medium to the crystal surface. Growth does not occur uniformly across all faces. Different crystal faces have different surface energies and different rates of ion attachment. Faces that grow quickly eventually eliminate themselves — they shrink and may disappear entirely as slower-growing faces dominate. This is why the fastest-growing faces are often not the ones visible on a finished crystal.

Crystal habit — the overall shape of a crystal — is determined by the relative growth rates of different faces, which in turn are controlled by the crystal structure and the growth environment. The same mineral can develop radically different habits depending on temperature, pressure, concentration, pH, and the presence of impurities. Quartz grows as long prismatic crystals in hydrothermal veins but as short, stubby crystals in some other environments. Calcite can adopt over 300 documented habits.

Habit modifiers are substances present in the growth environment that selectively adsorb onto certain crystal faces, slowing their growth and changing the overall habit. Iron in solution can modify the habit of calcite dramatically. Organic molecules at biomineralization sites direct the growth of biological crystals (shells, bones, teeth) with extraordinary precision. In industrial crystallization, habit modifiers are intentionally added to control crystal size and shape for pharmaceutical and other applications.

Twinning is a common phenomenon in which two or more individual crystals grow together in a specific geometric relationship dictated by the crystal structure. In a twin, the two individuals share a common crystallographic plane (the composition plane) but are oriented as mirror images or rotated versions of each other. Twins can form during growth (growth twins), during phase transformations (transformation twins), or under mechanical stress (deformation twins). The characteristic interpenetrant cross shape of staurolite crystals, the heart-shaped twins of gypsum, and the characteristic herringbone pattern of chrysoberyl are all well-known twinning examples. Polysynthetic twinning in plagioclase feldspar — fine repeated lamellae of twins visible as striations on cleavage faces — is diagnostic of this mineral group.

Epitaxy occurs when one mineral grows on the surface of another in a specific crystallographic orientation, guided by the fit between the two crystal lattices. Hematite on magnetite, rutile on hematite, and muscovite on topaz are examples. Epitaxy can produce remarkable specimens where small, perfectly oriented crystals of one species decorate the surface of another like a crystallographic mosaic.