Pyrrhotite
Iron sulfide (Fe(1-x)S)
A bronze-colored iron sulfide notable for being the most magnetic of the common sulfide minerals and an important nickel host.
- Mohs hardness
- 3.5-4.5
- Color
- bronze-yellow to bronze-brown, tarnishing dark brown
- Type
- mineral
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Overview
Pyrrhotite is an iron sulfide mineral with the unusual formula Fe(1-x)S, where iron deficiencies in the crystal structure give it both its variable composition and its distinctive magnetism. Its name derives from the Greek for reddish, reflecting its bronze color.
It is typically bronze-yellow to bronze-brown, tarnishing to a darker brown, and usually occurs massive or as tabular crystals. It is the most strongly magnetic of the common sulfides.
Pyrrhotite is economically important because it commonly hosts pentlandite, making it a key associate in nickel ore deposits.
Formation & geology
Pyrrhotite forms in a wide range of high-temperature environments. It is a common accessory mineral in mafic and ultramafic igneous rocks, where it crystallizes from sulfur-bearing magmas, frequently together with pentlandite and chalcopyrite in magmatic nickel-copper sulfide deposits.
It also occurs in contact metamorphic deposits, hydrothermal veins, and some metamorphic rocks and pegmatites.
Major occurrences include the Sudbury Basin in Ontario, Canada, Norilsk in Russia, Kisbanya in Romania, and numerous nickel and sulfide districts worldwide.
How to identify it
Look for a bronze-yellow to bronze-brown metallic mineral, often massive, that tarnishes to dull brown. The key diagnostic test is magnetism: pyrrhotite is attracted to a magnet, unlike most other sulfides.
It is moderately soft (3.5-4.5) and has a dark gray-black streak. Luster is metallic.
Look-alikes include pyrite and chalcopyrite (both more brassy and non- to weakly magnetic) and pentlandite (which it is often intergrown with). The bronze color combined with clear magnetic response is the most reliable way to identify pyrrhotite.
Uses & significance
Pyrrhotite itself is a minor iron ore but is economically significant mainly as a host and associate of pentlandite, the chief nickel ore, in magmatic sulfide deposits. It is also a source of sulfur and has been used in producing sulfuric acid.
Notably, pyrrhotite is a problem mineral in construction: when present in concrete aggregate, it can oxidize and expand, cracking foundations, which has caused major structural damage in some regions.
It has no jewelry use and limited metaphysical following. For collectors, magnetic crystalline specimens are of interest, particularly from classic nickel districts.
Frequently asked questions
Is pyrrhotite magnetic?
Yes. Pyrrhotite is the most magnetic of the common sulfide minerals, which is its most useful identifying feature.
How is pyrrhotite different from pyrite?
Pyrrhotite is bronze-brown and magnetic, while pyrite is brassy-yellow, harder, and essentially non-magnetic.
Why is pyrrhotite a problem in concrete?
When pyrrhotite-bearing aggregate oxidizes, it expands and forms new minerals that crack and crumble concrete foundations.
What is pyrrhotite used for?
It is mainly important as an associate of nickel ore (pentlandite) and as a source of iron and sulfur.
Where is pyrrhotite found?
Famous localities include the Sudbury Basin in Canada and Norilsk in Russia, both major nickel-copper sulfide districts.
Pyrrhotite guides
In-depth guides for identifying, valuing, and understanding Pyrrhotite.
