CRYSTALLINE SYSTEMS: CONCEPT AND CHARACTERIZATION, TYPES, EXAMPLES - CHEMISTRY - 2025

The crystal systems are a set of geometrical characteristics and symmetry elements that allow different sorting crystal clusters. Thus, depending on the relative lengths of its sides, the angle between its faces, its internal axes, and other geometric aspects, the shape of one crystal ends up differentiating itself from another.

Although crystalline systems are directly linked to the crystalline structure of minerals, metals, inorganic or organic compounds, these refer more to the qualities of their external form, and not to the internal arrangement of their atoms, ions or molecules.

The rich diversity of mineralogical crystals and their symmetries are supported by six crystal systems. Source: Pexels.

The six crystal systems are cubic, tetragonal, hexagonal, orthorhombic, monoclinic, and triclinic. From the hexagonal system derives the trigonal or rhombohedral. Any crystal in its pure state, after being characterized, becomes one of these six systems.

In nature, sometimes it is enough to look at the crystals to know which system they belong to; provided you have a clear command of crystallography. On many occasions, however, this is an arduous task, because the crystals are "scrambled" or "deformed" as a result of the conditions of their environment during their growth.

Concept and characterization

Crystalline systems can at first seem like an abstract and difficult to understand subject. In nature you are not looking for crystals that have the exact shape of a cube; but share with it all its geometric and isometric qualities. Even with this in mind, it may still be visually impossible to figure out which crystal system a specimen belongs to.

For this, there are instrumental characterization techniques, which among their results show the values ​​for certain parameters that reveal which crystalline system is under study; and furthermore, it points out the chemical properties of the crystal.

The preferred technique for characterizing crystals is thus X-ray crystallography; specifically, X-ray powder diffraction.

In short: an X-ray beam interacts with the crystal and a diffraction pattern is obtained: a series of concentric points, whose shape depends on the internal arrangement of the particles. Processing the data, it ends by calculating the parameters of the unit cell; and with this, the crystalline system is determined.

However, each crystalline system is in turn composed of crystalline classes, which add up to a total of 32. Likewise, other different additional forms derive from these. That is why the crystals are very diverse.

Types of crystal systems

Cubic or isometric

The cube is just one of the crystalline classes that the cubic system encloses. Source: Smiddle

The cubic or isometric system corresponds to highly symmetric crystals. The cube, for example, presents a series of symmetry operations that characterize it. In the center of the cube, imagine that a cross is drawn that touches the faces above, below, and the one on the sides. The distances are equal and intersect at right angles.

If a crystal complies with the symmetry of the cube, even if it does not have exactly that shape, it will belong to this crystalline system.

This is where the five crystalline classes that make up the cubic system come to light: the cube, the octahedron, the rhombic dodecahedron, the icositetrahedron and the hexacisohedron. Each class has its own variants, which may or may not be truncated (with flat vertices).

Tetragonal

Tetragonal unit. Source: Stannered via Wikipedia.

The tetragonal system can be visualized as if it were a rectangle that has been given volume. Unlike the cube, its c-axis is longer or shorter than its a-axes. It could also look like a cube stretched up or compressed.

The crystal classes that make up the tetragonal system are the prime and four-sided pyramids, the double eight-sided pyramids, the trapezohedrons, and again the icositetrahedron and the hexacisohedron. Unless you have paper shapes on hand, it will be difficult to recognize these shapes without the help of years of experience.

Hexagonal

Hex drive. Source: Stannered via Wikipedia.

Any crystalline form whose base corresponds to that of a hexagon will belong to the hexagonal crystal system. Some of its crystalline classes are: twelve-sided pyramids and double pyramids.

Trigonal

The base a crystal that belongs to the trigonal system is also hexagonal; but instead of having six sides, they have three. Its crystalline classes come to be: prisms or three-sided pyramids, the rhombohedron and the scalenohedron.

Orthorhombic

In the orthorhombic system, its crystals have a rhombohedral base, giving rise to shapes whose three axes have different lengths. Its crystalline classes are: bipyramidal, bisphenoidal and pinacoid.

Monoclinic

This time, in the monoclinic system the base is a parallelogram and not a rhombus. Its crystalline classes are: sphenoid and three-sided prisms.

Triclinic

Triclinic unit. Source: Stannered via Wikipedia.

The crystals that belong to the triclinic system are the most asymmetric. To begin with, all its axes have different lengths, as well as the angles of its faces, which are inclined.

This is where its name comes from: three inclined, triclinic angles. These crystals are often confused with orthorhombic, hexagonal, and also adopt pseudocubic shapes.

Among its crystalline classes are the pinacoids, pedions, and shapes with even numbers of faces.

Examples of crystal systems

Some corresponding examples for each of the crystal systems will be cited below.

Cubic or isometric

Halite has exceptional cubic crystals. Source: Parent Géry

Halite, also known as common salt or sodium chloride, is the most representative example of the cubic or isometric system. Among other minerals or elements that belong to this system are:

-Fluorite

-Magnetite

-Diamond

-Espinela

-Galena

-Bismuth

-Silver

-Gold

-Pyrite

-Garnet

Tetragonal

Wulfenite is the most representative example of the tetragonal crystal system. Source: Rob Lavinsky, iRocks.com - CC-BY-SA-3.0

In the case of the tetragonal system, the mineral wulfenite is the most representative example. Among other minerals in this system we have:

-Casiterite

-Zircon

-Chalcopyrite

-Rutile

-Anatase

-Scheelita

-Apophyllite

Orthorhombic

The mineral tanzanite belongs to the orthorhombic system. Source: Rob Lavinsky, iRocks.com - CC-BY-SA-3.0

Among the minerals that crystallize in the orthorhombic system we have:

-Tanzanite

-Baryta

-Olivine

-Sulfur

-Topaz

-Alexandrite

-Anhydrite

-Potassium permanganate

-Ammonium perchlorate

-Chrisoberyl

-Zoisite

-Andalusita

Monoclinic

Gypsum crystals belong to the monoclinic system. Source: Lysippos

Among the minerals of the monoclinic system we have:

-Azurite

-Cast

-Pyroxene

-Mica

-Spodumene

-Serpentine

-Moon stone

-Vivianita

-Petalite

-Crisocolla

-Lazulite

Triclinic

Chalcanthite crystals belong to the triclinic system. Source: Ra'ike

Among the minerals of the triclinic system we have:

-Amazonite

-Feldspar

-Calcantite

-Rhodonite

-Turquoise

Hexagonal

Perfectly hexagonal aquamarine crystals. Source: Robert M. Lavinsky via Wikipedia.

In the image above we have an example of when natural forms instantly reveal the mineral's crystalline system. Among some minerals that crystallize in the hexagonal system we have:

-Emerald

-Calcite

-Dolomite

-Tourmaline

-Quartz

-Apatite

-Zincite

-Morganite

Trigonal

The mineral axinite belongs to the trigonal system. Source: Rob Lavinsky, iRocks.com - CC-BY-SA-3.0

And finally, among some minerals belonging to the trigonal system we have:

-Axinite

-Pyrargyrite

-Nitratin

-Jarosita

-Agate

-Ruby

-Tiger's Eye

-Amethyst

-Jasper

-Sapphire

-Smoky quartz

-Hematite

References

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