We have already explained the way in which the form of a crystal is directly related to its internal atomic structure. You have also seen how the many possible crystal shapes can be grouped into just seven crystal systems. Now let’s know what are the main mineral groups?
To simplify their study, the earth’s minerals can also be grouped together. The way in which they are grouped depends upon the purpose of the grouping. For example, you might want to group minerals on the basis of the elements which compose them; this would be a chemical classification.
On the other hand, you could group together all the minerals of the same color or all the minerals which have crystals of the same system, but these groupings would not tell you anything about the ways in which the different minerals were formed.
A simple grouping which is often used by mineralogists (those who study minerals) is into rock-forming minerals and on rock-forming minerals. Naturally, every mineral occurs in a rock, but the rock-forming minerals actually make up the bulk of the igneous rocks.
You will remember that igneous rocks are formed by the cooling of magma and they provide the materials for sedimentary and metamorphic rocks.
Another important grouping is into silicates and non-silicates, but this is rather more complicated and the details are beyond the scope of this book. A very great simplification of this grouping would be to say that silicates contain silica in combination with other elements and non-silicates do not contain silica.
The Silicates are by far the most important of the rock-forming minerals and are worth more detailed consideration. These silicate minerals may be further divided into groups which have similar internal, or atomic, make-up.
To be more scientific, it has been shown by a researcher called Bowen, how members of a group are formed from molten rock at different temperatures. As you know, heat is just one form of energy. Minerals that are formed at very high temperatures, such as olivine, are formed by a great deal of energy which is reflected by the way in which its atoms are arranged.
The other main silicate minerals are given the names pyroxenes, amphiboles, micas, feldspars, and quartz. The order in which they have been mentioned exactly corresponds to the temperature at which you would expect them to form from the melted rock material.
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