Crystallization (geology)

Introduction

Crystallization in geology refers to the process by which a solid forms, where the atoms or molecules are highly organized into a structure known as a crystal. Some of the ways by which crystals form in nature is through precipitating from a solution, freezing, or more rarely deposition directly from a gas. Attributes of the resulting crystal depend largely on factors such as temperature, air pressure, and in the case of liquid crystals, time of fluid evaporation.

Process of Crystallization

Crystallization occurs in two major steps. The first is nucleation, the appearance of a crystalline phase from either a supercooled liquid or a supersaturated solvent. The second step is known as crystal growth, which is the increase in the size of particles and leads to a crystal state. Nucleation and growth continue to occur simultaneously while the supersaturation exists. Supersaturation is the driving force of the crystallization, hence the rate of nucleation and growth is driven by the existing supersaturation in the solution.

Nucleation

Nucleation is the step where the solute molecules or atoms dispersed in the solvent start to gather into clusters, on the microscopic scale (elevating solute concentration in a small region), that become stable under the current operating conditions. These stable clusters constitute the nuclei. Therefore, the clusters need to reach a critical size in order to become stable nuclei. Such critical size is dictated by the operating conditions (temperature, supersaturation, etc.). It is at the stage of nucleation that the atoms arrange in a defined and periodic manner that defines the crystal structure — note that "crystal structure" is a special term that refers to relative positions of atoms, not the macroscopic properties of the crystal (size and shape), although those are a result of the internal crystal structure.

Crystal Growth

Once the first small crystal, the nucleus, forms it acts as a pattern on which further crystal growth can occur. The addition of new molecules to the crystal lattice is a random process, and the growth of a crystal is similar to the increase of a pile of sand grain by grain. Once the crystal nucleus reaches the critical size, addition of new molecules to the crystal becomes easier and the growth of the crystal becomes faster as the size of the crystal increases. Hence, the crystal grows at a rate that is proportional to the surface area, not the volume.

Types of Crystallization

Crystallization can occur in various forms based on the conditions under which it occurs. Some of the common types of crystallization are:

Evaporative Crystallization

Evaporative crystallization occurs when evaporation of the solvent leads to an increase in the concentration of the solute in the solution, leading to crystallization. This is a common method used in the manufacturing and purification processes of many substances.

Cooling Crystallization

Cooling crystallization occurs when the temperature of the solution is lowered, usually by removing heat through a cooling jacket around the container of the solution. This lowers the solubility of the solute in the solvent, leading to crystallization.

Reactive Crystallization

Reactive crystallization is a process that involves a chemical reaction that leads to the formation of a crystalline product. This can occur in solution or from the melt.

Crystallization in Nature

Crystallization in nature occurs in a variety of geological environments including in igneous, metamorphic, and sedimentary rocks. It is also a common process in the formation of mineral deposits.

Igneous Rocks

In igneous rocks, crystallization occurs as the magma cools and begins to harden. The type of crystals that form depend on the chemical composition of the magma. For example, quartz and feldspar crystals will form in magma that is rich in silica.

Metamorphic Rocks

In metamorphic rocks, crystallization occurs under conditions of high temperature and pressure. The process results in a reorganization of the atoms in the rock's minerals, leading to the formation of new minerals with a crystalline structure.

Sedimentary Rocks

In sedimentary rocks, crystallization often occurs during diagenesis – the process by which sediments compact under pressure, expel pore fluids, and gradually become solid rock. Minerals such as calcite, aragonite, and silica are often involved in the crystallization process.

Crystallization and Mineral Formation

The process of crystallization is integral to the formation of many types of mineral deposits. Crystallization in a geological setting often involves the precipitation of minerals from water solution, or magma, as it cools and starts to harden. The size and shape of the crystals that form depend on the cooling rate and the chemical composition of the magma or fluid.