How Are Crystals of Minerals Classified into the Seven Crystal Systems?

Crystals of minerals are classified into seven crystal systems based on their symmetry and the arrangement of their atoms in three-dimensional space. Understanding these systems helps in understanding the physical properties of minerals and their crystallographic behavior.

The Seven Crystal Systems

The seven crystal systems differ based on the lengths of their crystal axes and the angles between them.

Cubic (Isometric) System

The Cubic or Isometric system features three axes of equal length that intersect at right angles (90°). This fundamental symmetry gives these crystals their cubic appearance and uniformity.

Examples: Halite (NaCl), Diamond, Pyrite

Tetragonal System

The Tetragonal system has two axes of equal length and one axis of a different length, all intersecting at right angles. This system possesses higher symmetry compared to the orthorhombic system but not as high as the cubic system.

Examples: Zircon, Rutile

Orthorhombic System

In the Orthorhombic system, the axes are all of unequal lengths but intersect at right angles. This system is less symmetrical than the cubic or tetragonal systems, which leads to a diverse range of crystal shapes.

Examples: Olivine, Barite

Hexagonal System

The Hexagonal system comprises four axes: three axes of equal length lying in a plane at 120° to each other, with the fourth axis perpendicular to this plane and of a different length. This system exhibits a hexagonal cross-sectional plane.

Examples: Quartz, Beryl

Rhombohedral (Trigonal) System

The Rhombohedral or Trigonal system features three axes of equal length intersecting at angles that are not 90° (typically 60° or 120°). This system is characterized by strong trigonal symmetry.

Examples: Calcite, Tourmaline

Monoclinic System

The Monoclinic system features three axes with two at right angles to each other, and the third axis is inclined. The axes are all of different lengths. The monoclinic system often exhibits a basal pinacoid structure and prisms with inclined end faces.

Examples: Gypsum, Mica, Diopside, Petalite, Kunzite, Hiddenite, Howlite, Vivianite

Triclinic System

The Triclinic system is the least symmetrical of all the systems, featuring three axes that are of unequal lengths intersecting at angles that are all different from 90°. The crystal shapes in the triclinic system can be varied and unique.

Examples: Labradorite, Amazonite, Kyanite, Rhodonite, Aventurine, Feldspar

Summary of Classification Criteria

The classification of minerals into these systems is based on several criteria:

Axis Lengths: Can be equal or unequal.

Angles: Can be right angles (90°) or oblique angles.

Symmetry: Different arrangements and symmetries determine the classification into these systems.

Understanding these systems provides insight into the physical properties of minerals and their crystallographic behavior, aiding in the identification and analysis of mineral samples.