Pyrite is one of the most common sulphide minerals. It is yellow or gold in colour and has a distinct metallic lustre. It is able to form over a very wide range of temperature and pressure, which means it can be found in igneous, sedimentary or metamorphic rocks. It is a typical representative of the hydrothermal crystal synthesis, in which case it occurs together with galena, sphalerite and quartz. It also frequently forms in sediments, where reduction reactions prevail.
Pyrite consists of iron (46.55 %) and sulphur (53.45 %). It often contains no other elements, which makes it particularly useful for the production of sulphur or sulphuric acid.
Pyrite often exists in flawless crystal structure form. The most characteristic pyrites have the shape of a cube (A) or a rhombic dodecahedron (B). Crystals with developed octahedron crystal surfaces (C) are rarer. Cuboid pyrites often display characteristic streaks. In addition to single pyrite crystals, twined crystals are also known. Pyrites can also be massive, in the form of dendrites, fine grains, etc.
Weathering or oxidation can turn a pyrite into a limonite. Limonite consists mainly of the minerals goethite and lepidocrocite, which often form pseudomorphs after pyrite. Pseudomorphism means that the original crystal form is preserved, but the chemical composition is changed.
The most beautiful pyrite crystals were found in Spain, Peru and Elba. It is also very common in Slovenia, where it was found in single, twinned and massive form, or sometimes replacing plant and animal remains. Due to its colour and distinct metallic lustre, pyrite used to be regularly mistaken for gold. Ironically, pyrite can contain impurities of gold and in such cases, when limonite is formed from it, gold is precipitated and can sometimes even become concentrated in economically significant quantities.