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Opal Ethiopia set M3241
Chemistry and minerals
Every mineral is a chemical compound with its own unique chemical formula. If a chemical compound occurs in nature, and if it has a Crystal structure' target='_parent'>crystal structure, then it gets a mineral name. Which minerals occur in a rock, depends primarily on the chemical composition of the particular rock. Only minerals consisting of elements that are present in the rock, can form in the rock.
Every single mineral has a specific chemical composition. This composition is shown in the most simple representative way in the chemical formula. The formula shows which elements form the mineral and the ratio of those elements in the mineral.
Minerals with the simplest chemical formulas are the elements. They contain only one chemical element. The numbers in subscript on the right of the abbreviation of the chemical element show the amount of the particular element present in the mineral. The numbers in superscript on the right site of the formula, with a + or - sign, indicating the charge of the ions.
The specific shape of minerals is caused by the fact that the atoms of which the mineral consists are all arranged in the same manner in a three dimensional pattern. That is called the Crystal structure' target='_parent'>crystal structure. Matter with such a repeating Crystal structure' target='_parent'>crystal structure are called crystals. Apart from a couple of exceptions (like obsidian, which is completely amorphic), most minerals have a Crystal structure' target='_parent'>crystal structure. Some of those structures are very simple, e.g. kitchen salt. Other Crystal structure' target='_parent'>crystal structures are very complex. These can only be unravelled using advanced techniques such as X-ray imagery.
Each mineral has its own unique Crystal structure' target='_parent'>crystal structure and internal structure. Crystal structure' target='_parent'>Crystal structures are classified into seven types. The Crystal structure' target='_parent'>crystal structure determines the shape and cleavage of the crystals that form the mineral. The angle between the different sides of a crystal is directly caused by the structure of the crystal. Some minerals can have multiple Crystal structure' target='_parent'>crystal structures, each with a different specific weight. Those minerals are called polymorphic. In the heaviest variety of a polymorphic mineral, the atoms are arranged tightly and close to each other in the Crystal structure' target='_parent'>crystal structure. A nice example of a polymorphic mineral is graphite that consists of the pure element Carbon. A denser variety of this mineral is diamond.
Types of crystal structure
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