Chapter 5 Crystals

5.1 Crystalline minerals

Crystalline minerals are naturally occurring substances with a defined crystal structure. Here are some examples of crystalline minerals that have been historically significant or are commonly known:

Quartz: A very common mineral composed of silicon dioxide, known for its hardness and clarity.
Galena: A lead sulfide mineral, historically used in crystal radios as a detector due to its semiconducting properties.
Pyrite: Often called “fool’s gold,” this iron sulfide mineral has a metallic luster and pale brass-yellow hue.
Calcite: A calcium carbonate mineral, known for its variety of crystal shapes and its ability to refract light.
Halite: Also known as rock salt, this mineral is composed of sodium chloride and forms cubic crystals.
Fluorite: A colorful mineral composed of calcium fluoride, often used in industrial applications.
Mica: A group of silicate minerals known for their sheet-like structure and ability to split into thin, flexible layers.
Beryl: A mineral that comes in various colors, including emerald (green) and aquamarine (blue).
Corundum: Known for its hardness, this mineral forms gemstones like sapphires and rubies.
Gypsum: A soft sulfate mineral used in plaster and drywall.

These minerals are characterized by their specific chemical compositions and crystal structures, which give them unique physical properties and appearances.

5.2 Classification of Crystalline Minerals

Crystalline minerals are primarily classified based on their chemical composition and crystal structure. Here’s a breakdown of how they are classified and the role of electron valence:

Chemical Composition: Minerals are grouped into classes based on their dominant anion or anionic group. The major classes include:
- Silicates: Contain silicon and oxygen, and are the largest mineral group.
- Oxides: Consist of metal cations bonded to oxygen.
- Sulfides: Composed of metal cations bonded to sulfur.
- Carbonates: Contain carbonate groups (CO₃²⁻).
- Halides: Include minerals with halogen elements like chlorine or fluorine.
- Sulfates: Contain sulfate groups (SO₄²⁻).
- Phosphates: Include phosphate groups (PO₄³⁻).
Crystal Structure: Minerals are also classified by their crystal lattice structure, which determines their physical properties. Common structures include cubic, hexagonal, tetragonal, and orthorhombic.

5.3 Electron Valence and the Periodic Table

Valence Electrons: The valence electrons of the elements in a mineral influence how atoms bond together, affecting the mineral’s structure and properties. For example, the sharing or transfer of valence electrons leads to the formation of ionic or covalent bonds.
Periodic Table Patterns: Elements in the same group of the periodic table have similar valence electron configurations, which means they often form similar types of bonds and compounds. This can influence the types of minerals they form. For instance:
- Alkali Metals (Group 1): Typically form ionic bonds with nonmetals, resulting in minerals like halides.
- Transition Metals: Often form complex structures with variable oxidation states, leading to diverse mineral types like oxides and sulfides.

In summary, crystalline minerals are classified based on their chemical composition and crystal structure, with electron valence playing a crucial role in determining the types of bonds and structures that form. The periodic table helps predict these patterns based on the elements’ valence electron configurations.

5.4 Seven Crystal Lattice Systems

There are exactly seven primary crystal systems. This classification is based on the symmetry and geometric constraints of three-dimensional space. Here’s why there are seven and not more:

Symmetry and Axes: The classification of crystal systems is based on the symmetry of the crystal lattice and the lengths and angles between the axes of the unit cell. The seven systems encompass all possible combinations of these parameters that result in distinct symmetry groups.
Geometric Constraints: In three-dimensional space, there are only a limited number of ways to arrange points (atoms) such that they repeat periodically and maintain a consistent symmetry. These arrangements lead to the seven crystal systems.
Mathematical Derivation: The seven crystal systems are derived from the 14 Bravais lattices, which are the distinct lattice types that can fill space without gaps. These lattices are grouped into the seven crystal systems based on their symmetry properties.
Completeness: The seven systems cover all possible symmetries for crystalline materials. Each system represents a unique combination of axial lengths and angles, and no additional system is needed to describe any other symmetry.

The seven crystal systems are a complete and exhaustive classification of the possible symmetries in three-dimensional space, based on the constraints of geometry and symmetry.

Cubic (Isometric):
- All sides are equal, and all angles are 90 degrees.
- Examples: Diamond, salt (halite).
Tetragonal:
- Two sides are equal, one is different; all angles are 90 degrees.
- Examples: Zircon, rutile.
Orthorhombic:
- All sides are different, but all angles are 90 degrees.
- Examples: Olivine, aragonite.
Hexagonal:
- Two sides are equal, one is different; angles are 90 degrees and 120 degrees.
- Examples: Quartz, beryl.
Trigonal (Rhombohedral):
- All sides are equal, but angles are not 90 degrees.
- Examples: Calcite, cinnabar.
Monoclinic:
- All sides are different; two angles are 90 degrees, one is not.
- Examples: Gypsum, orthoclase.
Triclinic:
- All sides and angles are different.
- Examples: Kyanite, plagioclase feldspar.

These crystal systems describe the basic geometric arrangement of atoms in a crystal, influencing the mineral’s physical properties and appearance. Each system can have different lattice types, such as primitive, body-centered, face-centered, and base-centered, which further define the specific arrangement of atoms within the unit cell.