Introduction

A mineral is defined as a natural inorganic solid that has an atomic-scale arrangement and a well-defined chemical composition.

The fact of being solid excludes gases and liquids from this definition, so, for example, ice can be defined as a mineral, but water cannot.

The natural attribute, on the other hand, excludes all those substances produced by man in laboratories, such as synthetic minerals.

Another characteristic of minerals is that they have an order on an atomic scale that can be traced back to a defined geometric model, which makes them crystalline solids. The solids in which a geometric ordering is lacking are defined as amorphous solids, of which glass is an example.

Having a well-defined chemical composition means that this can be expressed through a chemical formula. Although each mineral has its own well-defined chemical formula, in nature most minerals have impurities within it that locally modify their chemical composition making it "non-fixed".



Over time, different criteria have been proposed to group minerals, today the most used method to classify minerals is the one developed by the German scholar H. Strunz, who divides the various minerals into nine classes:

1) NATIVE ELEMENTS

They are minerals formed by atoms of the same species. They are mostly low oxidizable metals, such as gold and silver, but inside there are also semimetals, such as arsenic and selenium, and non-metals, such as sulfur and carbon.

Sulfur

2) SULFURS

They are minerals composed of a metallic and a non-metallic element. The non-metallic element in sulphides is sulfur, however there are other minerals belonging to this group in which the non-metallic element is not sulfur, among these subgroups we find selenides, in which the non-metallic element is selenium, or also arsenides, in which the non-metallic element is arsenic.

Typical sulphides are pyrite, galena, sphalerite and chalcopyrite.

Chalcopyrite

3) HALIDE

They are a small group of minerals that includes substances similar to common table salt, produced by evaporation of closed waters in basins, but also species that have been produced by alteration of other minerals or deposited by the fumaroles of volcanoes.

Typical halides are rock salt, sylvite and fluorite.

Fluorite

4) OXIDES

They are a group of minerals formed by metals chemically united with oxygen that do not have distinctly saline characteristics. Some of them have considerable industrial interest. Typical oxides are spinel, magnetite, corundum and hematite.

Hematite

5) BORATES, NITRATES, CARBONATES

They are a relatively small group of minerals; the BO3 group is present in the borates, the NO3 group in the nitrates and the CO3 group in the carbonates. Among the three most frequent are carbonates, including calcite, aragonite and malachite.

Aragonite

6) SULPHATES

They are a large group of minerals, with various species formed mainly by alteration of metal ores, united by having the SO4 group in the crystal lattice. There are two subclasses: anhydrous sulphates, or those minerals that do not contain water in their crystal lattice, such as anhydrite or celestine, and hydrated sulphates, or minerals that contain water in their crystalline lattice, such as gypsum.

Celestina

7) PHOSPHATES, ARSENIATES, VANADATES

They are a group of minerals among the most beautiful in nature; the PO4 group is present in the phosphates, the As4 group in the arsenates and the VO4 group in the vanadates. Generally they are formed by alteration of various metal ores, hence their very beautiful color; they form magnificent crystals, mostly small in size. Typical examples are apatite and turquoise.

Turquoise

8) SILICATES

They are a very large group of minerals and have a very varied chemical composition. The basic unit of all silicates is the SiO4 group. Silicates are divided into six groups: nesosilicates, sorosilicates, cyclosilicates, inosilicates, phyllosilicates, tectosilicates. Typical silicates are quartz, garnet, plagioclase.

Garnet

9) ORGANIC COMPOUNDS

They are a small group of minerals, an example is given by amber.

Fossil Amber