September 2021 - Global Epidemic Observational Studies Organization

# Month: September 2021

## Tetragonal Crystal System

In our previous post, we discussed in detail the classes and forms of Cubic/ Isometric Crystal System, which is a very common type of crystal and includes hundreds of minerals in this system. This post is about the ”tetragonal crystal system”. The tetragonal crystal system is another important system. Zircon, Cassiterite, and Rutile are the

## Cubic/ Isometric Crystal System

Some common minerals like galena, garnet, fluorite, and magnetite are found in cubic or isometric crystal systems. The crystals of this group have three mutually perpendicular axes of equal lengths. These axes are interchangeable and are designated as a1, a2, and a3. The axes of cubic/isometric crystal systems are as follows. Classes of Isometric System

## Six types of Crystal Systems

There are hundreds of minerals on the earth, which are found in different crystal forms. These crystal forms are grouped into six major crystal forms. These crystal forms are as follows. (1). Cubic or Isometric Crystal System: The crystals belonging to this system have three mutually perpendicular axes of equal lengths. These axes are designated

## Symmetry elements of crystals

All kinds of crystals have a symmetrical form, which is known as the symmetric characteristic of the crystals. The symmetry elements are used as parameters to describe this symmetry. Three elements of symmetry can be recognized in a crystal. These are (i) plane of symmetry, (ii) axis of symmetry, and (iii) center of symmetry. (1).

## Forms of Crystal Faces

Crystal faces are found in different forms. A ”form” consists of a group of crystal faces all of which have the same relation to the elements of symmetry. The number of faces in a form is determined by the symmetry of the crystal class. Miller indices may be used to represent forms and they are

## Crystallographic Notations

We can describe the slope of crystal faces with reference to the intercepts they make on the crystallographic axes. The methods that are commonly used for expressing the intercepts of crystal faces are : (a) Weiss Parameter System, (b) Index System of Miller. These two are the most common parameters used to determine the slopes

## Parameters of crystal faces

To study and understand the crystal faces, different types of parameters are used. The parameters of crystal faces may be defined as the intercepts made by the crystal face on the crystallographic axes. Parameters are expressed in terms of the unit lengths. Consider the axes shown in the following figure, a,b and c are the

## Interfacial Angle of Crystals

Interfacial angle is the angle between adjacent faces of the crystal lattice. Because crystal faces have a direct relationship to the internal structure, it follows that the faces have a definite relationship to each other. This relationship is expressed ”law of constancy of interfacial angles”, which states that ”the interfacial angles between corresponding faces are

## Unit Cells in a Crystal Lattice

Unit Cell: A unit cell is the smallest repeated portion of a crystal lattice that shows the three-dimensional pattern of the entire crystal. The unit cells are used to define the lengths of various crystallographic axes. When arrays of atoms or molecules are laid out in a space lattice we define a group of such

## Crystallographic axes

Crystallographic axes are the imaginary lines, which we can draw within the crystal lattice. In order to describe the faces and symmetry of crystals, a set of three or four reference axes are established.  These imaginary reference axes are established. These imaginary reference lines are called ”crystallographic axes” and are generally taken parallel to the