What is Crystallography

by Ritika

Crystals are known for their beautiful external appearance. However, it is their internal structure, which is too small to be seen by the naked eye, that makes them interesting to scientists. The study of the growth, shape, and geometric character of these beautiful forms of minerals is called crystallography.

Crystals are known for their beautiful external appearance. However, it is their internal structure, which is too small to be seen by the naked eye, that makes them interesting to scientists. The study of the growth, shape, and geometric character of these beautiful forms of minerals is called crystallography.

A crystal is matter which is homogeneous and has a specific and orderly atomic structure. The outward appearance of the crystal has plane and smooth surfaces which are arranged symmetrically. Whenever a solid is formed from a fluid, a crystal is formed. Crystals could be formed as of result of either a liquid being frozen, or dissolved matter being deposited or even a gas being directly converted into a solid state.

The angles that are formed between the corresponding sides of any 2 crystals of the similar matter are identical even if there are differences in size and external appearance. Almost all solid matter has an organized atomic arrangement and has a crystalline structure. Amorphous solids, like glass, are the solids that do not have a crystalline structure. Amorphous solids are more like liquids in structure.

Under the earths surface there are liquids that slowly freeze to form granite. These liquids sometimes flow out of volcanoes and cool down quickly. They thus form a rock that looks glassy and is known as obsidian. If this cooling is even a little slower, it forms a rock – felsite. Felsite is crystalline in nature but the crystals cannot be seen by the naked eye. It is also called cryptocrystalline or aphanitic. When the lava cools down even slower than this, it forms a porphyritic rock. The crystals are however, larger and can be seen easily. This rock called rhyolite may be identical in composition to obsidian rocks, felsite rocks or even granite.

Under favorable conditions some chemical elements and compounds also form crystals which are of a distinct and characteristic form. Salt, as an example of this, forms cubic crystals, while garnet which forms cubes too, sometimes is in the form of dodecahedrons (which has 12 faces) or trisoctahedrons (which has 24 faces). Though there may be differences in shape the crystallization of both salt and garnet is found to be of the same class and in the same system.

In theory, there are 32 classes under which crystals can be formed. Most minerals fall into the first twelve classes. Some of these classes are yet to be observed by scientists. These thirty-two classes can be classified into 6 different crystal systems. These systems are based on the length and position of the crystal axes, on the imaginary lines which are believed to be passing through the center of the crystal, on the intersection of each face, and having clear relations with the crystal symmetry. In each of these systems, the minerals share some details where the crystal forms are symmetric and several significant optical properties are common too.

The 6 crystal systems that are extremely focal to the study of mineralogists and gemologists are named and explained below. The specifications of the system are necessary in the explanation of any mineral.

Isometric

In this system all the crystals have three axes which are all perpendicular to one another and are all equal in length. An example of an Isometric crystal is pyrite which has three perpendicular axes of equal length. Of all the crystals this structure is the most symmetrical. Pyrite crystal system forms rocks that are hard and yet brittle. Pyrite is yellow in colour and has a metallic lusture which results in its being called ‘fool’s gold’.

Tetragonal

In this system all the crystals have three axes which are all perpendicular to each other and only two of these are equal in length. A fitting example of this is the Siberian idocrase which has three axes that are all perpendicular to one another and two are equal in length. Other rocks which Idocrase is grouped with are zircon, rutile, and wulfenite, which are not very hard rocks and at times possess a fire like a diamond.

Orthorhombic

In this system all the crystals have three axes which are mutually perpendicular and are all of different lengths. An example of this is Barite, from which barium is obtained. Barite has three axes that are mutually perpendicular and are of different lengths. Barite also exhibits a perfect cleavage, which means that it can split easily along specific planes that intersect.

Monoclinic

In this system all the crystals have three axes of which are not of equal lengths and two of them are not perpendicular to one another, but are both perpendiculars to the third axes. Gypsum is an example of this system. Gypsum is a soft, sedimentary rock from which plaster of Paris is obtained. It is also used in agriculture and construction.

Triclinic

In this system comprises all the crystals are with three axes which are not equal in length and are oblique to one another. Of all the crystal systems, crystals of this system are the least symmetrical. A good example is the Brazilian Axinite.

Hexagonal

In this system all the crystals have four axes. Of these, three axes are in a single plane; they are symmetrically spaced, and are of equal length. The fourth axes is perpendicular to the other three. According to some crystallographers this system can be split into two, thus forming a seventh system calling it the Trigonal or rhombohedral system.

The technique used to investigate the structure of matter in the crystalline state is called Crystallography. This technique studies the tri-dimensional arrangement of all matter; whether they are atoms, molecules or ions of minerals or molecules of life.

By using x-rays, in which crystals are subjected to an extremely energetic radiation, we can get information which allows a crystallographer to locate the specific entities that the crystals are made up of. There has been tremendous progress, in this field of science thanks to the introduction of automatization of the methods used and with computer development.

The results of these experiments and methods often explain the chemical, physical, biological and pharmaceutical properties of substance being analyzed. The most stimulating steps utilized in the study of crystallography today are aiding scientists in understanding the workings of life at the molecular level, which is leading medicinal practitioners in their discovery of new drugs to treat various diseases.