How many crystal systems exist?

How many crystal systems exist?

There are seven crystal systems that currently exist, each with unique qualities and attributes. Crystal systems are used to classify crystals based on their atomic lattices and physical properties. These crystal systems are further divided into crystal groups. Here are the seven crystal groups and their corresponding crystal systems:

  • Cubic: This crystal group has a high degree of symmetry and is commonly known as the isometric system. Examples include diamond, pyrite, and fluorite.
  • Tetragonal: This crystal group is similar to the cubic system but is elongated in one direction. Examples include zircon, apatite, and chalcopyrite.
  • Orthorhombic: Crystals in this group have three axes of unequal lengths intersecting at right angles. Examples include topaz, aragonite, and barite.
  • Monoclinic: Crystals in this group have three axes of unequal lengths, with one axis at a right angle to the other two. Examples include azurite, gypsum, and celestine.
  • Triclinic: In this crystal group, the axes are all of different lengths and are oblique to each other. Examples include labradorite, turquoise, and microcline.
  • Hexagonal: These crystals have three axes of equal length forming a horizontal hexagonal plane, with a fourth vertical axis. Examples include quartz, calcite, and beryl.
  • Trigonal: Crystals in this group have three axes of equal length forming an equilateral triangle, with one axis perpendicular to the other two. Examples include tourmaline, rhodochrosite, and cinnabar.

Each crystal system and group has its unique properties and attributes, making each crystal special in its way. Crystals can be utilized for a variety of purposes, such as healing, meditation, and energy work. Knowing which crystal system and group a crystal belongs to can provide insight into its capabilities and aid in selecting the appropriate crystal for any specific need.

The Basics of Crystal Systems

Crystals are a semi-perfectly ordered arrangement of atoms, ions, or molecules. A crystal system is defined as a category of symmetric arrangements of these components from which different minerals are formed. There are primarily seven crystal systems, which are based on the distinctive optical and physical properties of each system. The fundamental crystal shapes in each system are assigned to one of seven crystal groups based on inter-axial angles of the unit cell.

Understanding Crystal Lattices

In crystallography, a lattice is a three-dimensional mathematical model that illustrates a crystal’s long-range ordering. Although it doesn’t reflect the atomic arrangements precisely, it gives the basis for studying the crystal structure. Crystal lattices are classified into seven crystal systems, and each system has its unique properties. The crystal lattice of each system is the most straightforward arrangement of space in the crystal unit cell.

The Seven Crystal Groups: An Overview

In crystallography, the seven crystal groups define unique ways that a crystal’s symmetry can be expressed. All the crystal systems have a unique number of symmetry axes and angles between them. The seven crystal groups are cubic, hexagonal, tetragonal, orthorhombic, monoclinic, triclinic, and rhombohedral. The symmetry of each group is expressed in three axes, defined as a, b, and c parameters. The systems can have equal or different properties in all three dimensions.

Cubic Crystal Group and its Characteristics

The cubic crystal system has a cube shape with a geometric shape of angle 90° between each axis edge. It falls under the highest symmetry class, having four three-fold rotational axes, four-fold rotational axis, six two-fold axes as well as one four-fold inversion center. The arrangement of atoms in cubic crystals is regular and repeating, and they are cubic. Common examples of cubic minerals include diamond, halite, and pyrite.

Hexagonal Crystal Group and its Characteristics

The hexagonal crystal system has a hexagonal shape with angles of 120° and 90°. It has four axes of symmetry and is commonly found in minerals with a hexagonal pattern. Six points intersect at angles 60° with each other within a hexagonal pattern. This crystal system is commonly found in quartz, calcite, and beryl.

Tetragonal Crystal Group and its Characteristics

The tetragonal crystal system has four axes. The shape of the crystal is parallelepiped with a rectangular base and four square faces. The angles between the axes are 90°, but only two crystallography parameters are equal. The most common minerals in this system are zircon, cassiterite, and rutile.

Orthorhombic Crystal Group and its Characteristics

The orthorhombic crystal system has three axes of unequal length that intersect at right angles to each other, forming a parallelogram-shaped prism. There are two or more faces perpendicular to each axis. This crystal system is present in minerals like kyanite, topaz, and gypsum.

Monoclinic Crystal Group and its Characteristics

The monoclinic crystal system has a set of three unequal axes, two of which are not at right angles to each other, creating a parallelogram prism shape with a skewed angle. The crystals have a single mirror plane, and two types of twinning are possible, depending on the direction of the angle. Common minerals in this group include gypsum, malachite, and epidote.

Triclinic Crystal Group and its Characteristics

The triclinic crystal system has three axes of unequal lengths which intersect at oblique angles to form a parallelepiped. The triclinic system has the least symmetry and is usually more complicated with its free-form angles. It is present in minerals like labradorite, albite, and microcline.

In summary, understanding the crystal systems is essential since it helps in identifying and classifying minerals. The seven crystal systems have unique properties that make them distinct from each other. The shape, symmetry, and physical properties of minerals depend on the crystal system they belong to. Knowing which crystal group a mineral belongs to, one can determine its physical properties, optical properties, and chemical composition.