What is Pyrite?
Pyrite, often called "Fools Gold", has a silvery-yellow to golden metallic colour. It is very common and may occur in large crystals. It has been used by ancient civilisations as jewellery, but is hardly used nowadays. Pyrite is sometimes incorrectly known as Marcasite in the gemstone trade. Marcasite is mineral that is a polymorph of Pyrite, and can be fragile and unstable, and is not fit for gemstone use.
The mineral pyrite, or iron pyrite, also known as fool's gold, is an iron sulphide with the chemical formula FeS2. This mineral's metallic luster and pale brass-yellow hue give it a superficial resemblance to gold, hence the well-known nickname of fool's gold. The colour has also led to the nicknames brass, brazzle, and Brazil, primarily used to refer to pyrite found in coal.
Pyrite is the most common of the sulphide minerals. The name pyrite is derived from the Greek (pyritēs), "of fire" or "in fire", in turn from (pyr), "fire". In ancient Roman times, this name was applied to several types of stone that would create sparks when struck against steel; Pliny the Elder described one of them as being brassy, almost certainly a reference to what we now call pyrite. By Georgius Agricola's time, c. 1550, the term had become a generic term for all of the sulphide minerals.
Pyrite is usually found associated with other sulphides or oxides in quartz veins, sedimentary rock, and metamorphic rock, as well as in coal beds and as a replacement mineral in fossils. Despite being nicknamed fool's gold, pyrite is sometimes found in association with small quantities of gold. Gold and arsenic occur as a coupled substitution in the pyrite structure. In the Carlin–type gold deposits, arsenian pyrite contains up to 0.37 wt% gold.
Occurrence of Marcasite
Marcasite can be formed as both a primary or a secondary mineral. It typically forms under low-temperature highly acidic conditions. It occurs in sedimentary rocks (shales, limestones and low grade coals) as well as in low temperature hydrothermal veins. Commonly associated minerals include pyrite, pyrrhotite, galena, sphalerite, fluorite, dolomite and calcite.
As a primary mineral it forms nodules, concretions and crystals in a variety of sedimentary rock, such as in the chalk layers found on both sides of the English Channel at Dover, Kent, England and at Cap Blanc Nez, Pas De Calais, France, where it forms as sharp individual crystals and crystal groups, and nodules.
As a secondary mineral it forms by chemical alteration of a primary mineral such as pyrrhotite or chalcopyrite.
Marcasite and Pyrite
A mineral is defined both by its chemical composition and its crystal structure. In some cases two different minerals have the same chemical composition, but different crystal structures. Known as polymorphs, these intriguing cases illustrate how the different crystal structures can result in quite different physical properties.
Perhaps the most famous case of a polymorph pair is diamond and graphite. Though both are composed entirely of pure carbon, diamond has a cubic structure with strong bonds in 3 dimensions. Graphite, by contrast, forms in layers with only weak bonds between layers. As a result of their structural differences, diamond has a hardness of 10 on the Mohs scale, while graphite rates only 1. Diamond is the ultimate abrasive, while graphite is a superb lubricant.
Another interesting polymorph pair is marcasite and pyrite. Both minerals are composed of iron sulphide. But where no one could ever confuse diamond and graphite, it can be difficult to tell pyrite and marcasite apart. In fact pyrite is often sold under the name marcasite in the gemstone trade. But despite their apparent similarities, they have some important differences, such that one can be used as a gem material while the other cannot.
Pyrite has a cubic structure, metallic luster and a yellow-gold colour that has earned it the nickname "fool's gold". With a hardness of 6 to 6.5 on the Mohs scale, pyrite is hard enough to be used in jewellery. Pyrite is also exceptionally dense, with a specific gravity of 5.0 to 5.2. Only hematite has a higher density.
Marcasite tends to be lighter in colour, and is sometimes referred to as "white iron pyrite". Sometime marcasite has a greenish tint, or a multi-coloured tarnish that is the result of oxidation. But marcasite has an unstable orthorhombic crystal structure and is liable to crumble and break apart. In some cases marcasite will react with moisture in the air to produce sulphuric acid. For these reasons marcasite is never used in jewellery. When a gemstone is sold as marcasite you can be quite sure that it is actually pyrite.
Varieties and blends
Blueite (S.H.Emmons): Nickel variety of marcasite, found in Denison Drury and Townships, Sudbury Dist., Ontario, Canada.
Lonchidite (August Breithaupt): Arsenic variety of marcasite, found at Churprinz Friedrich August Erbstolln Mine (Kurprinz Mine), Großschirma Freiberg, Erzgebirge, Saxony, Germany; ideal formula Fe(S, As)2.
Synonyms for this variety:
- kausimkies,
- kyrosite,
- lonchandite,
- metalonchidite (Sandberger) described at Bernhard Mine near Hausach (Baden), Germany.
Sperkise designates a marcasite having twin spearhead crystal. Sperkise derives from the German Speerkies (Speer meaning spear and Kies gravel or stone). This twin is very common in the marcasite of a chalky origin, particularly those from the Cap Blanc Nez.
Properties of Pyrite (Marcasite)
| Colour | Metallic, Yellow, Gray |
| Hardness | 6 - 6.5 |
| Crystal System | Isometric |
| SG | 4.9 - 5.2 |
| Transparency | Opaque |
| Double Refraction | None |
| Luster | Metallic |
| Cleavage | None |
| Mineral Class | Pyrite |
