Last Updated on October 27, 2022 by Francis

There are seven crystal systems, and each has its own distinct shape. They are: cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic and triclinic. But how can you remember them all?

## Trick to remember 7 crystal systems / Solid State/ Class 12 Chemistry

The seven crystal systems are a classification scheme for crystals. Each system is defined by the symmetry of its unit cell. The first three systems (cubic, tetragonal, orthorhombic) are based on the symmetry of a cube, while the remaining four (hexagonal, trigonal, monoclinic, triclinic) are based on other symmetries.

There are many ways to remember the seven crystal systems. One way is to memorize their names and symmetries:
Cubic: all three axes are equal in length and have 90° angles between them; four 3-fold axes of rotation; examples include NaCl and CsCl Tetragonal: one axis is shorter or longer than the other two; examples include ZnS and PbI2 Orthorhombic: all three axes are different in length; no axis has a 4-fold symmetry; examples include BeO and Fe2O3 Hexagonal: one axis is shorter or longer than the other two; six 2-fold axes of rotation parallel to the shortest/longest axis; examples include graphite and beryl Trigonal: one axis is shorter or longer than the other two; three 2-fold axes perpendicular to the shortest/longest axis at 120° angles from each other Monoclinic: all three axes are different in length; one axis has a 2-fold symmetry while the others do not have any rotational symmetry elements; examples include HgS and CaCO3 Triclinic: all three axes are different in length with no rotational symmetry about any of them; example minerals include turquoise and plagioclase feldspar

## Orthorhombic Crystal System

The orthorhombic crystal system is characterized by three mutually perpendicular axes of unequal length. All three axes intersect at 90° angles. Orthorhombic crystals are relatively rare.

Some well-known minerals in the orthorhombic system include tourmaline, topaz, garnet, and beryl.
The word “orthorhombic” comes from the Greek words for “right” and “angle.” This refers to the fact that the sides of an orthorhombic crystal are perpendicular to each other (unlike, for example, a cubic crystal, where all sides are parallel to each other).

Orthorhombic crystals can be distinguished from other types of crystals by their symmetry. There are two main types of symmetry in crystals: translational and rotational. Translational symmetry means that the crystal can be shifted or translated in any direction without changing its overall shape.

Rotational symmetry means that the crystal can be rotated around any axis without changing its overall shape.
Orthorhombic crystals have neither translational nor rotational symmetry; instead, they have a type of symmetry called glide plane symmetry. This means that if you were to take an orthorhombic crystal and slide it along one of its axes, it would look exactly the same as before (although it would be upside down).

One way to visualize this is to imagine taking a sheet of paper and folding it along one axis so that the two edges touch each other; this creates what is called a fold line or glide plane. You can then slide the paper along this fold line without changing its overall shape; this is analogous to what happens with an orthorhombic crystal when it is slid along one of its axes.

## Trigonal Crystal System

The trigonal crystal system is one of the 7 crystal systems. A crystal system is a method used to describe the shape of a crystal. The trigonal system is defined by 3 equal axes intersecting at 120° angles.

There are 4 point groups in the trigonal system: 1, 2, 3, and 4. These point groups are further divided into 3 categories: those with 1 axis of symmetry (1 and 2), those with 2 axes of symmetry (3), and those with 3 axes of symmetry (4).
The most common minerals in the trigonal system are quartz, tourmaline, calcite, rhodonite, realgar, and wulfenite.

## Rhombohedral Crystal System

In crystallography, the rhombohedral crystal system is one of the 7 crystal systems. A crystal system is a set of point groups in which the symmetry elements are related by spatial rotations. The rhombohedral crystal system consists of the triclinic, monoclinic, and cubic point groups.

It is named after the rhombus, which is a two-dimensional figure with sides of equal length and angles of 60°.

Rhombohedral crystals occur naturally as calcite (CaCO3), rutile (TiO2), aragonite (CaCO3), cerussite (PbCO3), kyanite (Al2SiO5), scheelite (CaWO4), vesuvianite ((Ca,Na)16(Al,Mg)8(Fe3+,Mn)4(Si2O7)8(OH)8·H2O)), axinite-(Fe), uvarovite ((Ca3Cr2)(SiO4)3), wulfenite ((PbMo)(AsO4)), brucite (Mg(OH)2), Mosesita ((Na0.5K0.25Li0.25)(Zr1-xNbx/2+yTey/6)[Si7Be12B5Cl]Cl1-zFz · 4H2O where x = 0 to 0.33 and y = 0 to 0.17 and z = 0 to 1).

## Monoclinic Crystal

Monoclinic crystals are characterized by having two unequal axes, with the angle between them being less than 90 degrees. They can be either right-handed or left-handed, and are further classified into six different subclasses depending on the specific angles between their axes.
Monoclinic crystals are relatively uncommon in nature, but can be found in minerals such as beryl, tourmaline, andgraphite.

They also have a wide range of applications in industry, from electronics to optics.
While monoclinic crystals may not be as popular as other crystal classes, they still offer a unique and interesting structure that is worth learning more about!

## Cubic Crystal System

One of the most popular crystal systems is the cubic system. This system includes a variety of minerals, such as salt, diamond, and pyrite. Each mineral in this system has a unique cubic shape with four equal sides and angles.

The cubic system is also known for its high symmetry, which means that each face of the crystal can be rotated to create an identical structure. This type of crystal is found in many different environments, including igneous, sedimentary, and metamorphic rocks.

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## What are the 7 Different Crystal Systems?

There are seven different crystal systems: triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal, and cubic. Each system is defined by the symmetry of its unit cell. The triclinic system has the least symmetry, with a unit cell that is skewed and asymmetric.

The monoclinic system has two axes of unequal length and one axis of unique direction; the orthorhombic system has all three axes of different lengths; the tetragonal system has two equal axes; the trigonal system has one three-fold axis and two mirror planes; the hexagonal system has one six-fold axis and four mirror planes; and the cubic system has three equal axes and six mirror planes.

## How Do You Remember a Solid State Example?

There are a few ways to remember a solid state example. One way is to think about common objects that are solid states. For example, a chair is a solid state.

A table is also a solid state. Another way to remember a solid state example is to think about the definition of a solid state. A solid state is defined as matter that has a definite shape and volume and does not change its size or shape when it is subjected to an external force.

With this in mind, examples of solids include things like rocks, metal, and glass.

## Are There 6 Or 7 Crystal Systems?

There are six crystal systems: triclinic, monoclinic, orthorhombic, tetragonal, trigonal and cubic. Each system is defined by the symmetry of its basis (the smallest repeating unit of the crystal). The number of axes and angles between these axes determine the type of symmetry for each system.

The triclinic system has no axis of four-fold or higher symmetry. The basis vectors are not constrained to be perpendicular to one another. This results in a wide variety of shapes for triclinic crystals.

The monoclinic system has one axis of two-fold symmetry and two unequal axes that intersect at 90 degrees. The angle between the other two axes is not 90 degrees. Monoclinic crystals can be tabular, prismatic or acicular in shape.

In the orthorhombic system, all three axes are unequal and intersect at 90 degrees. Orthorhombic crystals can be blocky, platy or columnar in shape. The tetragonal system has three equal axes that intersect at 90 degrees; two horizontal basal planes parallel to each other containing one axis; and a vertical body diagonal containing the third axis crossing through the center of the basal planes perpendicularly.

Tetragonal crystals can be cuboid, rectangular or needle-like in shape with square cross sections perpendicular to the c-axis (body diagonal). Tricohral crystals have three equal axes that intersect at 60 degree angles; four 3 fold rotation axes parallel to each other contained within plane faces; and four 2 fold rotation contained within vertices where 3 faces meet..

Trigonal crystals can be pyramidal, rhombohedral or bipyramidal in shape depending on how acute or obtuse their angles are where faces meet at vertices.. Finally, Cubics have three equal axes that intersect at right angles.

. They also have 4 fold , 8 fold ,and 6 fold rotation around any face diagonals passing through centers & body diagonals.. Cubes are naturally occurring but many other shapes exist including octahedrons(8 faced), dodecahedrons (12 faced) & icosahedrons(20 faced) .

## How is the Classification of Crystals Made into the 7 Crystal System?

In crystallography, the seven crystal systems are the set of all possible arrangements that crystals can adopt. The symmetry of a crystal is described by its space group. Each point in the space group is associated with a symmetry operation, such as a rotation or reflection.

The classifications are based on these symmetry operations and not on the actual shape of the crystal.
The seven systems are triclinic, monoclinic, orthorhombic, tetragonal, trigonal (or hexagonal), cubic, and rhombohedral (or hexagonal). There are 92 space groups associated with these systems; however, there are only 14 Bravais lattices.

Each system has its own characteristic patterns of atomic stacking; however, some patterns may appear in more than one system.

## Conclusion

There are seven crystal systems: triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal, and cubic. Each system is defined by the symmetry of its axes and the angles between them. The triclinic system has three unequal axes that intersect at oblique angles; the monoclinic system has two unequal axes that intersect at an oblique angle; the orthorhombic system has three equal axes that intersect at right angles; the tetragonal system has two equal axes that intersect at a right angle; the trigonal system has three equal axes that intersect at an acute angle; and the cubic system has three equal axes that intersect at right angles.

To remember which crystal system is which, it can be helpful to think of them in terms of their shapes. Trigonal crystals are triangular in shape, hexagonal crystals are six-sided (like a cube), and cubic crystals are square or box-shaped. Monoclinic crystals can look like a capital M (for “mono”), tetragonal crystals can look like a capital T (for “tetra”), and orthorhombic crystals can look like a capital O (for “ortho”).