Crystal Groups Forms and Classes

by Ritika

The crystal groups are broadly classified into thirty two classes of symmetry. These broad classifications of crystal classes are got based on the exterior form of crystal. These broad classifications of thirty two crystal classes further have 230 sub space groups. These vary among themselves and are classified based on the analysis and observation of x-rays. It is also an interesting fact that even crystals made from the same mineral possess different crystal forms. This is attributed to each mineral’s crystal conditions of growth.

The crystal groups are broadly classified into thirty two classes of symmetry. These broad classifications of crystal classes are got based on the exterior form of crystal. These broad classifications of thirty two crystal classes further have 230 sub space groups. These vary among themselves and are classified based on the analysis and observation of x-rays. It is also an interesting fact that even crystals made from the same mineral possess different crystal forms. This is attributed to each mineral’s crystal conditions of growth.

Parameters for Crystal Groups and Classes

The two basic parameters used for the formation of crystal groups and classes are:

• Crystallographic axes
• Forms

In the above, if the axes are represented as a, b, c then the parameter to be maintained by the crystallographic axes for the formations of crystal groups and classes are

a=1, b=1, c=1

The method of drawing the images and construction of the crystal groups and classes use the above morphology. Each crystal class is a member of one of the six crystal systems, namely

• Isometric
• Hexagonal
• Tetragonal
• Orthorhombic
• Monoclinic and
• Triclinic Crystal systems

In the above, the hexagonal crystal system is further sub divided into hexagonal and rhombohedral systems.

About Crystal Forms and Thirty Two Crystal Classes

The technical term used to refer the outward appearance of crystal is ‘habit’. The term ‘form’ is a more casual usage by people to denote the outward appearance of crystal. The outward appearance of crystals includes in it the attributes like drusy, tabular, massive, encrusting, equidimensional, reniform and acicular.

A simple crystal may consist of only a single crystal form. A more complicated crystal may be a combination of several different forms. Fifteen different forms are possible within the isometric or cubic system. These include the hexoctahedron, gyroid, hextetrahedron, diploid, and tetartoid, among others. The crystal forms of the remaining five crystal systems are the monohedron or pedion, parallelohedron or pinacoid, dihedron, or dome and sphenoid, disphenoid, prism, pyramid, dipyramid, trapezohedron, scalenohedron, rhombohedron, and tetrahedron.

Crystal form is nothing but a term used to denote a group of crystal faces and each of the elements present in this group have the same relationship to the elements of symmetry of a given crystal system. It is also a mandatory requirement that each of the crystal faces present in this group has the internal geometrical relationships as other crystal faces present in the same group. That is, the physical and chemical properties are same for all crystal faces present in the same group. The reason for this attribute is the atomic arrangement of all atoms used for composing each of the crystal face present in the group is same.

There are two classifications with respect to crystal forms. They are:

• General form
• Special forms

General form also denoted as {hkl} denotes the name for each of the 32 classes. In each of these 32 classes, there is a form associated with each crystal class in which the faces intersect each crystallographic axes at different lengths. The rest of the forms are called special forms.

Types of Crystal Forms

The crystal forums are broadly classified as two types namely

• Open  Forms
• Closed Forms

Open  Forms:

Open forms are those groups of faces that are all related by symmetry but that do not completely enclose a volume of space. These types of open form crystal require additional faces also. There are about 18 open forms.

Some examples of open forms are namely

• Pedion: This refers to flat faces that are not parallel and are geometrically linked to any other faces.
• Pinacoid: Pinacoids have only two parallel faces that forms tabular crystal. An example of such type of crystal form is ruby.
• Dome: This type of crystal form is available in monoclinic and orthorhombic minerals. For instance, the intersection of two faces would result in mirroring effect which result in this form namely dome.
• Sphenoid: This type of crystal form has two-fold rotational axes. For example, monoclinic and orthorhombic minerals have such crystal forms.
• Pyramid :In this type of crystal form multiple facets converge on a single crystallographic axis. However isometric, monoclinic or triclinic systems do not have this crystal form.
• Prism :In case of prisms, the facets run parallel to an axis of a crystal, however they do not converge with the same. The isometric or triclinic minerals do not possess this type of crystal form. Quartz possesses this type of crystal form with two sets of three sided prisms. There are also sub types of prism, namely:
• Hexagonal Prisms
• Triangular Prisms

Hexagonal prisms have two hexagonal bases which are connected by a set of six rectangular faces. In addition an important point is these six faces run parallel to each other but do not converge with axes in the crystal. In case of triangular prisms it has two triangular bases which are well connected by a set of three rectangular faces. These also posses the basis attribute namely all faces run parallel to each other but do not converge with an axes in the crystal.

Closed Forms

The groups of faces which are all related by symmetry that are completely enclosed in a volume of space are called Closed forms. There are about 30 closed forms.These closed forms are broadly subdivided into two categories, namely

• Closed Isometric Forms
• Closed Non-Isometric Forms

Closed Isometric Forms

The main crystal forms in crystal system are:

• Hexahedron :This form has eight points, six faces, and twelve edges. They are all perpendicular to each other, forming 90 degree angles.
• Octahedron :This form type has two four sided pyramids lying base to base which are totally symmetrical with no top, or bottom and has eight faces.
• Tetrahedron :This form type has four equilateral triangular faces.
• Dodecahedron :There are twelve faces in this crystal form types and there are four sub types of dodecahedron namely:
• Symmetrical pentagonal dodecahedrons
• Asymmetrical pentagonal dodecahedrons
• Delta dodecahedrons
• Rhombic dodecahedrons
• Hexoctahedron :This is a multi-faceted dodecahedron. This has 48 triangular faces in it. The above include the important gemstones namely diamond, garnet, spinel and various other symmetrical gemstones.

Closed Non-Isometric Forms

The closed non-isometric forms have six sub-categories:

• Hexagonal (Trigonal) Closed Forms
• Tetragonal Closed Forms
• Rhombohedral Closed Forms
• Orthorhombic Closed Forms
• Monoclinic Closed Forms
• Triclinic Closed Forms