Crystalline and Amorphous Materials: Formation and Properties
Crystalline and Amorphous Materials
The most abundant elements on Earth are Fe, O, Si, and in the crust: O, Si, Al, Fe, Mg, Ca, Na, and K. These elements form mineral materials. There are two main types of mineral materials:
- Amorphous material: The elements that constitute it are arranged randomly, without any regular order. Examples include volcanic glass (obsidian), opal, and limonite.
- Crystalline Material: Its constituents are periodically ordered in space, driven by chemical bonds, forming a crystalline internal structure. In the crystalline state, the atoms are in thermal equilibrium.
Understanding Crystals
A crystal is a solid, organic or inorganic, with an orderly internal structure and a defined chemical composition. It can be natural or synthetic. A crystal is presented with polyhedral shapes when it has the appropriate space, time, and chemical composition during its formation.
Minerals are composed of crystalline material. All minerals are crystals, but not all crystals are minerals.
Crystal Formation
Crystals can form through several processes:
- Solidification from a melt: This occurs when materials in a liquid state experience a drop in temperature and change to a solid state. Cooling must be slow; if it is done quickly, amorphous material forms.
- Crystallization from a solution: Due to chemical or biochemical precipitation.
- Sublimation from a gas: Crystals form directly from a gas as a consequence of a drop in temperature. For example, sulfur crystals form from sulfide gas expelled by some volcanoes.
- Formation of crystals in a solid-state:
- Recrystallization: The size of a crystal increases, adding more chemicals.
- Mineralogical readjustment: The formation of a new crystal through the internal reorganization of existing components of a crystal.
The Process of Crystal Formation
Crystallogeny is the formation of crystals. Nucleation is the formation of an initial nucleus or particle with the properties of the crystal, which is the germ or seed of this crystal. After nucleation, growth begins.
Crystalline Properties of Matter
Atoms are regularly repeated at equal intervals in space. The sites where the atoms or atomic groups are repeated are called nodes. The intervals at which the nodes are repeated are called translations. A monocrystal is a single crystal formed from a nucleus without any interruptions, maintaining the same direction of growth.
Crystalline Material Properties
- Homogeneous: Because the material is a crystalline periodic medium, it is equivalent to a sequence of nodes; all nodes have identical physical and chemical properties.
- Anisotropy: Most of their physical properties depend on the direction and are constant in coinciding directions. Crystalline material exhibits isotropy for a property when its value is constant in all directions.
- Symmetric: The material possesses elements of symmetry, such as the ideal translation vector, planes of symmetry, and axes of symmetry.
Understanding Minerals
A mineral is a solid substance, element, or chemical compound with a composition that varies within certain defined limits. It is of inorganic, natural origin and has a definite crystalline structure.
- Polymorphs: Minerals with the same composition but different crystalline internal structures (e.g., diamond and graphite).
- Isomorphs: Minerals with the same crystal structure but different chemical compositions.
Petrogenetic Environments
Petrogenetic environments are areas on Earth where geologic processes lead to the formation of minerals and rocks. Each environment is characterized by specific pressure and temperature conditions.
- Igneous or Magmatic: Characterized by high pressures and temperatures that can melt rocks, giving rise to magmas. The solidification of magma results in igneous or magmatic rocks. The main minerals formed are silicates.
- Metamorphic: Characterized by intermediate pressures and temperatures that do not cause melting but cause transformations in the solid state. This results in the formation of minerals and metamorphic rocks.
- Sedimentary: Characterized by low pressures and temperatures, where processes like weathering, erosion, transport, sedimentation, and diagenesis occur. These are called sedimentary processes and result in sedimentary rocks.
Understanding Rocks
Rocks are mineral aggregates. The term “aggregate” means that the minerals are found together so that each retains its properties; that is, rocks are heterogeneous mixtures of substances.