## Crystallography and Mineral Crystal Systems

*by* Ritika

The six crystal systems could be well understood by getting a fair knowledge about the topic of solid geometry. For this one must have a base knowledge about crystallographic axes. In broad classification, there are six crystal systems in general. The crystallographic axes play a vital role to segregate and place all well known minerals into these six crystal systems.

A Crystal is a regular polyhedral form which is surrounded by faces which are smooth. A Crystal is also termed as a chemical compound in solid state due to inter atomic forces action. Crystallography refers to the complete study of mineralogy. The flat planes which bounds the solid are referred to as crystal faces.

### Crystal Systems

The six crystal systems could be well understood by getting a fair knowledge about the topic of solid geometry. For this one must have a base knowledge about crystallographic axes. In broad classification, there are six crystal systems in general. The crystallographic axes play a vital role to segregate and place all well known minerals into these six crystal systems.

### Types of Crystal Systems:

The six general types of crystal systems are:

#### Cubic also referred as isometric

In this type of crystal system, the three crystallographic axes have equal length. Also, in this the three crystallographic axes intersect at 90 degrees with each other. All fifteen crystal forms are closed in the isometric crystal system. The isometric or cubic crystal form has the highest degree of symmetry among all the six crystal systems.

The seven forms of crystal which have the same symmetry are namely:

#### CUBE:

Some examples to name in this are galena, pyrite, fluorite, perovskite.

#### OCTAHEDRON:

Minerals like magnetite, chromite, franklinite, spinel, pyrochlore, cuprite, gold, and diamond fall in this group.

#### DODECAHEDRON:

Magnetite and sodalite are placed in this category

#### TETRAHEXAHEDRON:

Fluorite, magnetite or copper and garnet are examples of this form.

#### TRAPEZOHEDRON:

Example minerals of this form are analcime and leucite.

#### TRISOCTAHEDRON

#### HEXOCTAHEDRON

The other eight forms are namely:

#### TETRAHEDRON:

This is a four faced form and some minerals to name in this form are diamond, helvite, and sphalerite.

#### TRISTETRAHEDRON:

This form generally has twelve triangular faces and mineral instances of this type are sphalerite and boracite.

#### HEXTETRAHEDRON:

This is similar to hexoctahedron and the mineral diamond is generally available in this form.

#### DELTOID DODECAHEDRON:

The resulting face in this is rhombic with 12-faced form.

#### GYROID:

Generally there is no natural mineral crystallizing in this form and this type is classified to have in it a pyritohedron and the diploid.

#### PYRITOHEDRON:

This has twelve pentagonal faces in it and the mineral Pyrite is available in this form.

#### DIPLOID:

This has a look similar to the pyritohedron with resulting face as trapezia. The only mineral in this form is Pyrite.

#### TETARTOID:

The mineral named Cobaltite usually crystallizes in this form.

#### Tetragonal:

This crystal system has three crystallographic axes intersecting at 90 degrees as cubic crystal system but only two of the crystallographic axes are of equal length. The third crystallographic axis has different length from the other two. In this, there are open forms consisting of order types namely of first order, second order and third order. The third prism form is referred to as the ditetragonal prism.

#### Orthorhombic:

The orthorhombic crystal system like the previous two crystal systems have three crystallographic axes intersecting at 90 degrees. But in contrast, in this crystal system all the three crystallographic axes are of different length. In this system, the highest available symmetry is 2-fold.

The orthorhombic system has generally three symmetry classes. They are:

#### The pinacoid also referred to as the parallelohedron:

This consists of 2 parallel faces and is available in three different crystallographic orientations.

#### The rhombic prism:

This open form class has four faces with two of them intersecting each other.

#### The rhombic dipyramid:

This class has in it eight triangular faces and all these intersect the three crystallographic axes.

There is yet other symmetry class in the orthorhombic crystal system which has the lowest symmetry named as rhombic disphenoid, also termed as rhombic tetrahedron.

#### HEXAGONAL:

This crystal system is defined with four crystallographic axes of which three of the axes lie in the same plane and intersect among themselves at 120 degrees. The fourth crystallographic axes are of either shorter or longer length as compared to the third axes. This system is thus referred by the axial cross relationships. This has in it seven possible classes each of these having 6-fold symmetry placed in the division named as hexagonal division and five possible classes each of these having 3-fold symmetry placed in the division named trigonal division.

The possible classes of the hexagonal division are:

#### Normal which is also referred to as the Dihexagonal dipyramidal class:

This has seven forms under it named as:

#### Base or basal pinacoid:

This has 2 faces in it and is denoted by Miller indices as (0001) and (000-1).

#### First order prism

#### Second order prism

#### Dihexagonal prism:

This has twelve bounding faces and is therefore a 12-sided prism and beryl crystal is instance to name in this category.

#### First order pyramid

#### Second order pyramid

#### Dihexagonal dipyramid:

Minerals like zincite, wurtzite, and greenockite fall in this class category.

#### Hexagonal Trapezohedral:

The minerals to name in this category are quartz and kalsilite.

#### Hexagonal Dipyramid :

The apatite group minerals fall in this category.

#### Trigonal Dipyramid

#### Hemimorphic:

Minerals like incite, wurtzite, and greenockite are present in this class.

The possible classes of the trigonal division are:

#### Hexagonal-scalenohedral:

This has two principal forms under the class namely:

- rhombohedron and
- the hexagonal scalenohedron

The mineral to name generally in this class is Calcite.

#### Trigonal-trapezohedral:

The forms to name in this class are nacoid, trigonal prisms, hexagonal prism, ditrigonal prisms, and rhombohedrons. Quartz is the common mineral to name in this class.

#### Rhombohedral:

Dolomite and ilmenite are the two most common minerals of this class.

#### Monoclinic:

Monoclicnic crystal systems like orthorhobmic crystal system have all the three crystallographic axes are of different length. But here the crystallographic axes intersect at an oblique angle. Generally the dome referred as {hkl} is best form present in this class. There are two sides of orientations possible in dome referred as {hkl} and {-hkl).

#### Triclinic:

In triclicnic crystal system, both the length and angle of intersection all the three crystallographic axes are of different length and different angles respectively. The triclinic pinacoid is also termed as parallelohedron which has in it two identical and parallel faces.