Pyrite

Formula: FeS2

Colour: Pale brass-yellow

Lustre: Metallic

Hardness: 6 – 6½

Specific Gravity: 4.8 – 5

Crystal System: Isometric. Pyrite commonly forms in cubes, but octaedra and pyritohedra ( pentagonal dodecahedra) are also common. Crystals are often deeply striated. Pyrite can also be massive, granular, nodular or botryoidal.

Member of: Pyrite Group

Name: Named in antiquity from the Greek “pyr” for “fire”, because sparks flew from it when struck with another mineral or metal. Known to Dioscorides (~50 CE) under the name “περι υληζ ιατρικηζ” which included both pyrite and chalcopyrite.

The isometric (cubic) polymorph of orthorhombic marcasite.
Pyrite is a very common mineral (also one of the most common natural sulfides, and the most common disulfide), found in a wide variety of geological formations from sedimentary deposits to hydrothermal veins and as a constituent of metamorphic rocks. The brassy-yellow metallic colour of pyrite has in many cases lead to people mistaking it for Gold, hence the common nickname ‘Fool’s gold’. Pyrite is quite easy to distinguish from gold: pyrite is much lighter, but harder than gold and cannot be scratched with a fingernail or pocket knife.

Pyrite forms under varied conditions, occurring in hydrothermal veins, by segregation from magmas, in contact metamorphic rocks, and in sedimentary rocks, such as shale, coal and limestone. Pyrite is often found in large deposits, and would be a source of iron were there not other minerals better suited for its extraction. It has been used as a source of sulfur for the manufacture of sulfuric acid, although today the acid is mostly made from the hydrogen-sulfide gas recovered from natural gas. Pyrite is widespread worldwide though the Rio Tinto region of Spain has been a source of an enormous amount of pyrite, which has been mined since Roman times.

Nodules of pyrite have been found in prehistoric burial mounds, although the suggestion that they were used as a mean of producing fire is questionable, since iron, other than scarce meteoritic iron, was unknown in antiquity. Native people of the American southwest set polished pyrite slices into a wooden base to construct mirrors.
Pyrite is commonly found to contain minor nickel, and forms a series with Vaesite, NiS2; Bravoite is a Ni-bearing variety of pyrite. It usually contains minor cobalt too and forms a series with Cattierite, CoS2. Many pyrites contain minor As and Pb-bearing pyrite has been described. It can also contain traces of other metals, including gold. Most of the foreign metal contents in pyrite can be traced back to metal nanoparticles.