How Crystals Form: The Science of Crystal Growth

Crystal Systems and Lattice Structures

All crystals belong to one of seven crystal systems defined by the symmetry of their unit cell, the smallest repeating unit of the lattice. The cubic (isometric) system has the highest symmetry, with three equal axes at right angles, and includes minerals like halite, pyrite, garnet, and diamond. The hexagonal system, with four axes (three equal horizontal axes at 120 degrees and one vertical axis of different length), includes quartz and beryl. The tetragonal system has two equal horizontal axes and one different vertical axis. The orthorhombic system has three unequal axes at right angles and includes olivine and topaz. The monoclinic system, with three unequal axes and one oblique angle, is the most common crystal system and includes gypsum, orthoclase feldspar, and augite. The triclinic system has the lowest symmetry, with three unequal axes at oblique angles, and includes plagioclase feldspar and kyanite. The trigonal system is sometimes grouped with hexagonal and includes calcite, dolomite, and tourmaline.

The crystal system determines the possible external shapes (crystal habits) a mineral can display. Cubic minerals may form cubes, octahedra, or dodecahedra. Hexagonal minerals often form six-sided prisms. However, the ideal crystal shape is rarely achieved in nature because growth conditions are seldom perfectly uniform. Crystal faces that have access to open space and a steady supply of chemical components grow freely and develop well-formed faces, while faces crowded against neighboring crystals are prevented from developing. The study of crystal symmetry and structure, called crystallography, is foundational to both geology and materials science.

Crystallization from Magma

When magma cools, the atoms within the melt lose kinetic energy and begin to bond together in orderly crystal lattices. Crystallization begins when the temperature drops below the liquidus, the temperature at which the first crystals begin to form. Different minerals crystallize at different temperatures, a principle formalized in Bowen reaction series, developed by Canadian petrologist Norman Bowen in the early 20th century. In a cooling basaltic magma, olivine crystallizes first at around 1200 degrees Celsius, followed by pyroxene, amphibole, biotite mica, and finally quartz at around 700 degrees Celsius. As high-temperature minerals crystallize and settle or are removed from the melt, the remaining liquid changes in composition, potentially producing a range of different igneous rocks from a single parent magma through a process called fractional crystallization.

The cooling rate determines crystal size. Slow cooling deep underground allows atoms to migrate to growing crystal faces over long periods, producing the large, well-formed crystals seen in granite and pegmatite. Rapid cooling at the surface freezes atoms in place before large crystals can develop, producing the fine-grained textures of basalt and rhyolite. Extremely rapid cooling produces volcanic glass (obsidian) in which no crystalline structure forms at all. Pegmatites, which crystallize from the volatile-rich residual fluids of a nearly solidified magma body, can produce crystals of extraordinary size because the dissolved water and gases lower the viscosity of the remaining melt and allow atoms to move freely to growing crystal surfaces. Pegmatite crystals of spodumene (a lithium mineral) exceeding 15 meters in length have been documented.

Crystallization from Solution

Minerals also crystallize from aqueous solutions when the concentration of dissolved ions exceeds the saturation point. Evaporation is the most straightforward mechanism: as water evaporates from a lake, sea, or groundwater system, the remaining solution becomes increasingly concentrated until minerals begin to precipitate. Evaporite minerals crystallize in a predictable sequence based on their solubility. Calcite and gypsum, which are least soluble, precipitate first. Halite (rock salt) precipitates at higher concentrations. The most soluble minerals, including potash salts and borate minerals, precipitate last and only from extremely concentrated brines.

Hydrothermal crystallization occurs when hot, mineral-rich fluids circulate through fractures and pore spaces in rock and deposit crystals as the fluid cools or reacts with the surrounding rock. Hydrothermal veins, filled with quartz, calcite, fluorite, and metallic ore minerals, are produced this way. Geodes, hollow rock cavities lined with inward-growing crystals (often quartz amethyst or calcite), form when mineral-bearing groundwater slowly precipitates crystals on the inner walls of voids in rock over thousands to millions of years. Cave formations like stalactites and stalagmites are crystallized calcite deposited from dripping, calcium-rich water in limestone caves.

Recrystallization During Metamorphism

Metamorphism can cause existing minerals to dissolve and recrystallize into new minerals that are stable at the elevated temperatures and pressures. This process does not involve melting. Instead, atoms migrate through the solid state or through thin films of fluid along grain boundaries, reorganizing into new crystal structures. Metamorphic recrystallization can produce large, well-formed crystals called porphyroblasts that grow within the metamorphic rock. Garnet porphyroblasts in schist, staurolite crosses in pelitic rocks, and andalusite prisms in hornfels are examples of metamorphic crystals prized by mineral collectors for their geometric perfection.