Pyrrhotite

In short

Pyrrhotite is a sulfide mineral made of iron and sulfur in a slightly iron-poor composition (Fe₁₋ₓS), bronze-yellow to bronze-brown with a metallic lustre. It is shiny and yellowish like pyrite but magnetic: a magnet attracts it, which is why it is also called “magnetic pyrite”. It occurs in mafic igneous rocks, particularly in nickel and copper ores together with pentlandite and chalcopyrite. In the crushed stone used for concrete, it oxidises and makes the concrete crack.

How to recognise it

1. 1

   Its colour is bronze-yellow to bronze-brown and it tarnishes quickly; its streak (powder colour) is dark grayish black.

2. 2

   It reacts to a magnet: a small magnet attracts pyrrhotite, whereas pyrite is not attracted. The strength varies from specimen to specimen; the greater the iron deficiency, the stronger the magnetism.

3. 3

   It is soft (hardness 3.5–4.5): a steel knife scratches it, whereas pyrite is too hard for a knife to scratch (6–6.5).

4. 4

   Crystals are mostly hexagon-like plates (up to 40 cm), sometimes forming rosettes; it is often massive or granular.

5. 5

   It has no cleavage but shows a distinct parting along the {0001} plane.

Myths and misconceptions

• MythPyrite is magnetic; a magnet-attracting brass-yellow stone is pyrite.

  FactPyrite does not react to a magnet. If a yellowish stone is attracted by a magnet it is often pyrrhotite, which is why it is called “magnetic pyrite”.

• MythPyrrhotite is only a collector’s mineral; it does no harm to buildings.

  FactIf pyrrhotite gets into the crushed stone of concrete, it reacts with air and water and swells, cracking the concrete. In Connecticut a pyrrhotite-bearing quarry has been identified in the cracking of some houses’ concrete foundations.

Did you know?

Where is it found?

• Sudbury Basin, Ontario, Canada · Major depositAccording to Wikipedia, the magma in the 1.85-billion-year-old impact structure at Sudbury formed pyrrhotite, chalcopyrite and pentlandite rocks; the region is known for nickel and copper mining
• Herja (Kisbánya), Baia Mare district, Romania · Known localityThe Handbook of Mineralogy mentions well-crystallised pyrrhotite from Herja in the Baia Mare district (coordinate is the city of Baia Mare)
• Dalnegorsk, Primorsky Krai, Russia · Known localityThe Handbook of Mineralogy mentions very large pyrrhotite crystals from Dalnegorsk (coordinate is the town centre)
• Morro Velho gold mine, Nova Lima, Minas Gerais, Brazil · Known localityThe Handbook of Mineralogy mentions large pyrrhotite crystals from the Morro Velho gold mine (coordinate is the town of Nova Lima)
• Santa Eulalia, Chihuahua, Mexico · Known localityThe mining district from which the Handbook of Mineralogy took its X-ray pattern for pyrrhotite (coordinate is the town of Santa Eulalia)
• Falun, Dalarna, Sweden · Historic sourceA town famous for its copper mine that the Handbook of Mineralogy names for pyrrhotite in Sweden (coordinate is the town centre)
• Riondel, British Columbia, Canada · Known localityThe place of the Bluebell mine, which the Handbook of Mineralogy names for pyrrhotite (coordinate is the settlement centre)
• Maden (Ergani copper deposit, Anayatak), Elazığ, Türkiye · Known localityAt Anayatak, pyrrhotite is reported in the magnetite-bearing chloritites and the ore-bearing black mudstones; it locally contains pentlandite exsolutions (coordinate is the district centre)
• Karabaşvengi, Divriği, Sivas, Türkiye · Known localityA little pyrrhotite is reported beside magnetite in the Karabaşvengi iron mineralisation, about 5 km north-east of the main Divriği iron deposit (coordinate is the district centre)
• Dağküplü ophiolite, Eskişehir, Türkiye · Known localityA small pyrrhotite-pentlandite±chalcopyrite association is reported in the ophiolite’s ultramafic and mafic rocks; the source gives the place only as “Eskişehir” (coordinate is the provincial centre)

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