We have already explained the way in which rocks are made up of minerals, and that minerals take the form of regular solids called crystals that may come in a great many different shapes, hardness’s, and colors. Let’s know how many shapes of crystals are there?
Naturally, you would expect that if you wanted to know more about rocks, you would need to understand more about the way in which crystals develop. The scientific study of crystals in known as crystallography.
The shapes of crystals are very important to anyone wishing to identify a mineral because these regular forms exactly express the way in which the atoms of that mineral are arranged.
This means that if you are able to recognize the shape of the crystal of a mineral, then, with the help of a number of other properties, you could work out what the mineral was by a process of elimination. Other important properties are weight, hardness (which in this case means the resistance to scratching), and so on.
The most important property, then, in recognizing crystalline materials is symmetry, that is, the regularity of the shape. You can try an experiment in symmetry for yourself. Take a fairly large potato and peel it. Be very careful not to cut yourself with the knife.
Then cut the potato into the shape of a cube; that is, a dice shape with six square faces and sharp corners. Some minerals such as iron pyrite (fool’s gold) often develop as cubic crystals. Now cut off the corners of the potato cube and you will have a shave with eight more faces.
If you continue to make these eight faces larger and larger, you will eventually cut away the six original faces leaving a shape with eight faces only called an octahedron.
You have now shown for yourself that three seemingly different shapes are related to each other. These relationships occur throughout the world of crystals, so that of all the many crystal shapes there are only seven crystal systems into which they can all be classified. You have already seen one of these systems, the cubic system.
The other six are given the names
If you can place any crystal that you find into one of these systems then you have gone a long way towards identifying the mineral.
With some crystals, it is quite easy, but with others, it does require considerable practice. We shall explain how to recognize some of the simpler crystals in the next question.
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