When admiring the many types of crystals in the Dynamic Earth Collection, it's hard not to ponder: how did they form? Why are the shapes and growth patterns so diverse?
Much of this can be explained through understanding how crystals actually grow.
There are three main ways crystals grow:
- In magma chambers or lava flows
- In water or a chemical solution
- As a vapor
The growth of crystals is a fascinating process that occurs in nature and can also be controlled in various scientific and industrial applications. Crystals are solid materials with a highly ordered, repeating atomic or molecular structure, and their growth typically involves the following key steps:
Nucleation:
- Crystals begin to grow through a process called nucleation, which involves the formation of tiny, stable clusters of atoms, ions, or molecules. Nucleation can occur spontaneously or be induced by introducing foreign particles (seed crystals) into a supersaturated solution or a supercooled melt. These clusters serve as the initial building blocks for the crystal.
Growth from Solution (Crystallization):
- In many cases, crystals grow from a solution or melt. When a solute (the substance that will form the crystal) is dissolved in a solvent (the liquid medium), it can become supersaturated when the solution becomes saturated due to cooling, evaporation, or chemical reactions. Supersaturation is a driving force for crystallization.
- Cooling: A solvent that is hot may support a greater quantity of solute than the same volume of solute that is cold.
- Evaporation: A solvent has a certain proportion of solute, which becomes become more concentrated when evaporation reduces the volume of solvent, but leaves behind the same volume of solute.
- As the solute concentration in the solution exceeds its saturation point, it becomes thermodynamically unstable, and molecules or ions start to aggregate and form solid nuclei.
- These nuclei then continue to attract additional solute particles, leading to the growth of the crystal lattice. The molecules or ions in the solution collide with the growing crystal surface and stick to it, allowing the crystal to expand layer by layer.
Solid-State Growth (Sublimation or Deposition):
- Some crystals grow directly from the vapor phase in processes like sublimation or deposition. Sublimation occurs when a substance transitions from a solid to a vapor without passing through the liquid phase, while deposition involves the direct transformation of a vapor into a solid.
- In sublimation, molecules or atoms in the vapor phase condense onto a substrate, forming a solid layer. In deposition, this process takes place on a cold surface, such as in the case of frost forming on a cold window during winter.
Crystal Structure:
- Crystals have a highly ordered and repeating three-dimensional arrangement of atoms, ions, or molecules. This regular structure is defined by the crystal lattice, which consists of unit cells that repeat in all directions. The specific arrangement of atoms within these unit cells is unique to each crystal and determines its properties.
Growth Rate:
- The rate at which a crystal grows depends on various factors, including temperature, pressure, concentration, and the presence of impurities. Higher temperatures, for instance, generally promote faster crystal growth, while the presence of impurities can slow down or disrupt the growth process.
Crystal Morphology:
- The shape and size of a crystal can vary significantly based on growth conditions. This variation in crystal morphology is due to differences in growth rates along different crystallographic directions. It can result in various crystal forms, including cubic, hexagonal, or prismatic shapes.
Overall, the growth of crystals is a complex and highly structured process that can be influenced by a range of factors. Scientists and engineers use this understanding to control and optimize crystal growth for various purposes, such as in the production of semiconductors, pharmaceuticals, gemstones, and more.