The Structure of Crystals and Their Importance in Nature
I. Crystal Structure
Sodium chloride (table salt) serves as an excellent example of a common crystal. Found in rocks or produced through seawater evaporation, it consists of a regular stacking of ions, representing the crystalline state.
Definition: A crystal can be defined by its elementary crystal lattice, which is the smallest repeating unit of the crystal structure.
The crystal lattice is the repetition of these elementary units. At the macroscopic level, salt crystals form in a cubic geometry, mirroring their microscopic crystal structure. This demonstrates the link between the microscopic (atomic) structure and the macroscopic appearance of matter.
Highlight: A crystalline structure is defined by an elementary unit cell repeated periodically. A crystal type is determined by the geometric shape of the unit cell, as well as the nature and position of its constituent entities within this cell.
Solids have varying properties such as density, optical characteristics, etc. These differences are often explained by chemical composition. However, sometimes materials with identical chemical compositions can exhibit different properties due to variations in their crystal structure.
Example: Graphite and diamond are both composed solely of carbon atoms, yet they have vastly different properties due to their distinct crystal structures.
Some solids are characterized by a lack of organization in their chemical entities. These are called amorphous solids.
Example: The chemical entity SiO2 can crystallize as quartz or form an amorphous solid like opal.
II. Rocks and Minerals
Rocks are composed of one or more mineral species. For instance, granite consists of minerals such as quartz, micas (biotite), and feldspars. While a mineral species can be amorphous (like volcanic glass in obsidian), it typically has a crystalline structure.
Highlight: The study of minerals allows for the characterization of solid matter in nature (non-synthetic solids).
Rocks are characterized by their mineral composition (diversity and proportions), while minerals are defined by their chemical composition and crystalline organization. It's possible to have two solids with identical chemistry but different properties (called polymorphs).
Vocabulary: Crystallization fractionnée (fractional crystallization) is the process of crystal formation influenced by factors such as temperature and pressure.
These factors influence the geometry of the elementary unit cells and thus the properties of the formed solid, potentially producing two distinct mineral species.
Highlight: Some volcanic rocks contain glass, resulting from the very rapid solidification of lava. Volcanic glass is an amorphous solid (a "non-crystallization").
III. Crystals in Living Organisms
Living beings can possess crystalline structures called biominerals. These play various roles:
- Protection (mollusk shells)
- Structure (vertebrate skeletons)
- Defense (plant oxalate raphides)
- Food intake (teeth)
Definition: Biominerals are crystalline structures found in living organisms, resulting from complex biochemical processes.
Biominerals are of interest to numerous scientific disciplines, including biology, ecology, evolution, medicine, and geology. Crystalline structures also exist in biological organisms, such as shells, skeletons, and kidney stones.
