A crystal is regular polyhedral forms characterized by flat surfaces know as faces.
The development of faces is the manifestation of the internal order arrangement of the atoms and depends on the physicochemical conditions to prevail at the time of formation of the mineral by solidification of gas or liquid precipitation from solution - a process known as crystallization.
Thus, a crystal a special form of a mineral. Common salt (NaCl) is prepared from seawater coastal parts of Orissa, particularly in Ganjam district. The seawater is allowed stand in rectangular fields and allowed to evaporate by sunrays.
The end product evaporation is solid common salt, which appears in different shapes, particularly rectangular forms. Close examination confirms the cubic shape of most of t' solids. In fact, one NaCl molecule is formed by combination of one Na+ and one CI. A number of NaCl molecules join together to form a NaCl crystal.
One such crystal is shown in.The solid circles are Na+ and CI" ions, which occur at the corners of small cubes. Nine such small cubes make the large. Atomic structure of NaCl three-dimensional form, which is also 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 cubes are completed.
Crystallization of CuS04 from solution is a common experiment that is carried out in college laboratories. The experiment is to be performed within a stipulated time of about two hours. CuS04 crystals are formed by cooling of the solution. It is seen those big crystals are formed if the rate of cooling is slow.
On the other hand, small sugars 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 achieved by addition of ice-cold water.
In this case, crystals are not formed, rather a no crystalline mass of CuS04 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 physic-chemical 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 is 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 determined 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 crystalline cannot be detected with microscope, but can determined by x-ray diffraction method, the material is referred to cryptocrystalline. Statistical study has shown that more than 98% of the min show definite crystalline structure.
When both the internal atomic structure and the outer form are absent substance is said to be amorphous. The fundamental difference between crystalline body and an amorphous substance lies in the fact that while in crystalline body, there exists a regular arrangement of the atoms in space, amorphous substance is characteristically devoid of such regular arrangement, amorphous substance the atoms exhibit a haphazard and random disposition, difference between arrangement of atoms or ions in crystalline and morph substances is analogous to the difference between disciplined soldiers and chaos mob.
Opal and lechatelierite, two varieties of silica and limonite, a variety hydrated iron oxide are examples of amorphous minerals. Many people dazzling stones in their rings, most of which are amorphous glasses artificially c with flat surfaces.
These colorful glasses should not be confused as name crystals. The natural glasses are formed when superheated lava coo 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 sat of atoms results in the formation of natural glass.