How do I identify Pyrrhotite?

Pyrrhotite can be identified by its hardness of 3.5 - 4 on the Mohs scale, Bronze brown color, Metallic luster, Monoclinic crystal system. Look for these key characteristics when examining specimens.

What color is Pyrrhotite?

Pyrrhotite typically appears in Bronze brown, bronze red, dark brown. Color can vary depending on impurities and formation conditions.

How hard is Pyrrhotite?

Pyrrhotite has a hardness of 3.5 - 4 on the Mohs scale. This gives it moderate hardness.

Pyrrhotite

A variety of Pyrrhotite Group

Rare

Pyrrhotite specimen - rock identification

What is Pyrrhotite?

Although pyrrhotite is a naturally occurring mineral used in concrete production, it can actually reverse the strength of concrete and cause it to crumble. Cement is mixed with pyrrhotite and solidifies into concrete; however, it starts to decompose when exposed to water and air. This process can crumble foundations and devastate homeowners. This mineral is common and is quite magnetic.

Etymology & Origins

The name pyrrhotite is derived from Greek pyrrhos, flame-colored.

Uses & Applications

Pyrrhotite is currently used as a source of nickel, and has also historically been used as a source of iron and sulfur. Well-formed specimens are popular among collectors. It has been included in construction aggregates, but is implicated in the cracking of concrete and other such materials in more recent studies.

Pyrrhotite Market Value Calculator

Estimate the market value of Pyrrhotite using size, quality, and finish. This preview calculator is for quick context and is not a formal appraisal.

Pyrrhotite Localities Map

See where Pyrrhotite is found with a localities map, collecting zones, and geology context. Generate a sample map preview below.

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North ZoneCentral RidgeSouth Basin

Key Characteristics

Characteristics of Pyrrhotite

The ideal FeS lattice, such as that of troilite, is non-magnetic. Magnetic properties vary with Fe content. More Fe-rich, hexagonal pyrrhotites are antiferromagnetic. However, the Fe-deficient, monoclinic Fe7S8 is ferrimagnetic. The ferromagnetism which is widely observed in pyrrhotite is therefore attributed to the presence of relatively large concentrations of iron vacancies (up to 20%) in the crystal structure. Vacancies lower the crystal symmetry. Therefore, monoclinic forms of pyrrhotite are in general more defect-rich than the more symmetrical hexagonal forms, and thus are more magnetic. Monoclinic pyrrhotite undergoes a magnetic transition known as the Besnus transition at 30 K that leads to a loss of magnetic remanence. The saturation magnetization of pyrrhotite is 0.12 tesla.

Formation of Pyrrhotite

Pyrrhotite is a rather common trace constituent of mafic igneous rocks especially norites. It occurs as segregation deposits in layered intrusions associated with pentlandite, chalcopyrite and other sulfides. It is an important constituent of the Sudbury intrusion where it occurs in masses associated with copper and nickel mineralisation. It also occurs in pegmatites and in contact metamorphic zones. Pyrrhotite is often accompanied by pyrite, marcasite and magnetite. Pyrrhotite does not have specific applications. It is mined primarily because it is associated with pentlandite, sulfide mineral that can contain significant amounts of nickel and cobalt.

Composition of Pyrrhotite

Pyrrhotite exists as a number of polytypes of hexagonal or monoclinic crystal symmetry; several polytypes often occur within the same specimen. Their structure is based on the NiAs unit cell. As such, Fe occupies an octahedral site and the sulfide centers occupy trigonal prismatic sites. Materials with the NiAs structure often are non-stoichiometric because they lack up to 1/8th fraction of the metal ions, creating vacancies. One of such structures is pyrrhotite-4C (Fe7S8). Here "4" indicates that iron vacancies define a superlattice that is 4 times larger than the unit cell in the "C" direction. The C direction is conventionally chosen parallel to the main symmetry axis of the crystal; this direction usually corresponds to the largest lattice spacing. Other polytypes include: pyrrhotite-5C (Fe9S10), 6C (Fe11S12), 7C (Fe9S10) and 11C (Fe10S11). Every polytype can have monoclinic (M) or hexagonal (H) symmetry, and therefore some sources label them, for example, not as 6C, but 6H or 6M depending on the symmetry. The monoclinic forms are stable at temperatures below 254 °C, whereas the hexagonal forms are stable above that temperature. The exception is for those with high iron content, close to the troilite composition (47 to 50% atomic percent iron) which exhibit hexagonal symmetry.

Health & Safety Information

⚠️Pyrrhotite has been linked to crumbling concrete basements in Quebec, Massachusetts and Connecticut when local quarries included it in their concrete mixtures. The iron sulfide it contains can react with oxygen and water over time to cause swelling and cracking.