In the previous question, we mentioned that there are seven crystal systems. Knowing their names is of little value unless you know how to place a given crystal in its system, and then some of the crystal in its class is really quite simple and depends upon what are known as elements of symmetry. In fact, each crystal system is actually defined by certain elements of symmetry. Let’s know more how do you recognize crystals.
Look at your potato again the one that you have cut into an octahedron. Hold it between your fingers and thumb so that your finger is at one of its points and your thumb at the one exactly opposite.
Now turn the potato slowly about these two points, and you will find that each edge or face is repeated by an exactly similar edge or face four times as you turn it through one complete turn. This means that this shape has s fourfold axis of symmetry.
You will find that you can hold the potato octahedron by two other pairs of points and repeat the process of that there are three of these axes. Now hold the potato by two opposite faces instead.
When you turn it this time you will find that there are three repeats in one full turn and that there are three other pairs of faces that you can hold. You can see, then that the octahedron has four of these threefold axes of symmetry.
You already know that this shape was cut from a cube. You have now seen how to recognize any cubic crystal. To belong to the cubic system, a crystal must have four threefold axes of symmetry, and you can look for them n exactly the same way as you did with the potato -octahedron.
You can soon recognize the other six systems of crystals by working out their axes of symmetry. Each system is defined by a different set of rules. Crystals belonging to the tetragonal system must have one fourfold axis of symmetry and one only.
It might also have four twofold axes if it were a very regular example of the system, but there are some that do not. An example of this system is the mineral zircon which is sometimes used as a stone is jewelry.
Orthorhombic crystals must have three twofold axes, hexagonal crystals one sixfold axis, trigonal one threefold axis, monoclinic one twofold axis, and finally triclinic with no taxes at all.
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