What Is Crystallography?

Crystallography is the branch of science that deals with the study of crystals. It includes the classification of crystals into different systems and classes, their descriptions, study of mathematical relationships existing between different faces in addition to imperfections of crystal growth and crystalline aggregates.

The term crystal has been derived from the Greek word krustallos, which means clear ice. The name was given to transparent quartz crystal with welldeveloped flat surfaces.

CRYSTALLINE AND AMORPHOUS SUBSTANCES

A crystal is a regular polyhedral form characterised by flat surfaces known as faces. The development of faces is the manifestation of the internal ordered arrangement of the atoms and depends on the physicochemical conditions prevailing at the time of formation of the mineral by solidification of gas or liquid or precipitation from solution. The process of formation of crystals is known as crystallisation.

Thus, a crystal is a special form of a mineral. Common salt (NaCl) is prepared from seawater in coastal parts of Odisha, mostly in Ganjam district. The seawater is allowed to stand in rectangular fields and allowed to evaporate by sunrays. The end product of evaporation is solid common salt, which appears in different shapes, predominantly in rectangular forms.

Close examination confirms the cubic shape of most of the solids. In fact, one NaCl molecule is formed by combination of one Nat and one Cl. A number of NaCl molecules join together to form a NaCl crystal. One such crystal is shown in Fig.3.1. The solid circles are Na and Cl ions, which occur at the corners of small cubes. Nine such small cubes

a cube. The crystal grows by successive addition of ions in all directions. It is to be noted that the seawater is collected during high tide time and is allowed to stand for about 15 days, during which the water

evaporates; reactions and stacking of NaCl Fig. 3.1: Atomic structure of NaCl cubes are completed. Crystallisation of CuSO4 from solution is a common experiment that is carried out in chemical laboratories. The experiment is to be performed within a stipulated time of about two hours. CuSO4 crystals are formed by cooling of the solution. It is seen that big crystals are formed if the rate of cooling is slow. On the other hand, small sugar-grain like crystals are formed if the rate of cooling is rapid.

Examination of these small crystals by high magnification or by X-ray methods confirms the existence of miniature crystals. No crystal is formed if instantaneous cooling is achieved by addition of ice-cold water. In this case, crystals are not formed, rather a noncrystalline mass of CuSO4 remains.

If any foreign chemical substance is added during cooling, crystal formation is affected and distorted crystals are formed. From the above observations it can be concluded that suitable physicochemical conditions are necessary for formation of good crystals. The crystals have two characteristics i.e. internal atomic structure and outer form.

Since the natural conditions in which the minerals are formed vary appreciably, the minerals show different degree of crystal growth. In ideal conditions, both the internal atomic structure and the corresponding outer form develop. In this case the mineral is said to be crystallized.

Common examples are rock crystal (transparent variety of quartz with well developed faces), calcite, staurolite, beryl etc. In case of some minerals, development of well-defined faces may be absent even though internal

atomic structure are present. In these cases, the minerals are said to be crystalline. Crystalline substances may also occur in such a fine-grained state that their crystalline character can be ascertained only with the aid of a microscope. In such case, the material is said to be microcrystalline. If the crystal formation is so imperfect that crystallinity cannot be detected with microscope, but can be determined by X-ray analysis, the material is referred to as cryptocrystalline. Statistical study has shown that more than 98% of the minerals show definite crystalline structure.

and the outer form are absent, the substance is said to be amorphous. The fundamental difference between a crystalline body and an amorphous substance is the presence and absence of definite internal ordered arrangement of the atoms in space respectively. In amorphous substance, the atoms exhibit a haphazard and random disposition.

The difference between arrangement of atoms or ions in crystalline and amorphous substances is analogous to the difference between disciplined soldiers and chaotic mob. Opal and lechatelierite, two varieties of silica and limonite, a variety of hydrated iron oxide are examples of amorphous minerals. Many people use dazzling stones in their rings, most of which are amorphous glasses artificially cut with flat surfaces. These colourful glasses should not be confused as natural crystals.

The natural glasses are formed when superheated lava cools instantaneously coming in contact with water. The cooling is so rapid that the atoms do not get time to arrange themselves in any ordered manner. The disordered state of atoms results in the formation of natural glass.

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