What is Crystallography ?

by Manisha G

The science of crystals is called CRYSTALLOGRAPHY. Crystallography is important as it is the foundation upon which gemological knowledge is based. The properties by which gemstones are identified are so bound up with their crystal structure that knowledge of crystals is very essential. In addition, crystals themselves are often very beautiful and exceedingly interesting. Careful scrutiny of well formed crystals enables them to be recognized not only by their crystal form but by other characteristic features, such as cleavage, surface striations etc.

During the slow formation of minerals, which ultimately form rocks, atoms of different elements come together and arrange themselves in a pattern depending upon the forces acting on these atoms. This regular arrangement of atoms in a definite orderly manner ultimately results in a framework or lattice of atoms. When this framework develops in a regular manner, then it gives rise to a structure, which has smooth surfaces and geometrical form – this structure is called CRYSTAL.

The chief characteristics of CRYSTALS are:

1. Definite geometrical shape.

Geometrical structure and appearance of crystals: Minerals by definition usually have a characteristic INTERNAL STRUCTURE, which may or may not be expressed in typical outward forms called CRYSTALS. The crystal form is the general equilibrium state of all solid substances. Most minerals crystallise into distinct, geometrically regular, homogeneous bodies bounded by, FACES, EDGES and ANGLES and with an ordered internal structure. This internal homogeniety is retained even if the growing crystals come into contact with each other in the solidifying melt and their growth thus becomes irregular.

2. An orderly and symmetrical internal arrangement of atoms.

Internal structure: AMOLECULEis the smallest possible unit cell of a compound that can exist independently and has the same properties as the compound. A molecule is composed of a definite number of elements. Each atom of an element is the smallest possible unit for that element and has a definite arrangement of sub-atomic particles like protons, neutrons, electrons, etc. The atoms of individual elements as well as atoms of different elements exercise very strong forces of mutual attraction and repulsion very similar to the gravitational and magnetic forces. Thus in every molecule there is a definite arrangement of atoms called the Atomic Lattice, since the atoms and ‘bonds’, or imaginary lines linking them look very much like the lattice or frame-work. This structure is also known as CRYSTAL LATTICE or a SPACE LATTICE.

 Crystal Lattice

The arrangement of atoms in the structure may vary in substances with the same chemical composition; for example: Diamond and Graphite; the two crystalline forms of carbon (C), have different internal structures. In diamond the atoms are closely packed, while in graphite the lattice has a much looser arrangement. As a result, diamond and graphite have different crystal forms and different physical properties. This is known as “Polymorphism” and diamond and graphite are said to be polymorphs of each other.

Internal structure of Graphite

 

Internal structure of a Diamond

3. Properties which with direction such as: hardness, cleavage, velocity of light, etc.

 Directional Properties: Depending upon variations in the forces between the atoms in a crystal, the physical properties of the crystal  will vary in different directions. When the forces of attraction are uniform in all three directions, then the physical and optical properties may remain isotropic. Diamond shows different hardness in different directions. Iolite shows two distinct colors in two different directions.

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