What Is Pyrrhotite? 

Pyrrhotite is a type of iron sulfide. Pyrrhotite can be found with pentlandite in basic igneous rocks, veins, and metamorphic rocks. It is a a nonstoichiometric variant of FeS, the mineral known as troilite.


What Happens When Pyrrhotite Is Exposed to Moisture?

When the sulfide is exposed to moisture, several chemical reactions take place and the sulfide changes to sulfate. When this reaction happens, the pyrrhotite oxidizes and expands when exposed to water or humidity.


How Does Pyrrhotite Get into Concrete?

Pyrrhotite gets into concrete by its presence in the aggregate that is used in the mixing of the concrete. When the aggregate is mined, the pyrrhotite is usually found in very small amounts in the rock and material that is used to create the aggregate. Once the aggregate is broken down to size to be mixed with the other concrete materials the pyrrhotite is also broken down and gets spread throughout the final concrete structure. The pyrrhotite is usually spread throughout the concrete in very tiny pieces.


What Does Pyrrhotite Do in Concrete?

When pyrrhotite is present in concrete, it can cause a lot of problems. The first problem is when moisture, damp air, or water are exposed to the poured concrete that contains pyrrhotite. When pyrrhotite is exposed to this moisture, it expands within the concrete. Even though the pyrrhotite is only in very small pieces within the concrete, the additional stress that is caused by the expansion of the pyrrhotite from corroding is usually enough to crack or spall the concrete.


Why Was Pyrrhotite Used in Construction? 

Pyrrhotite was not purposely included in the concrete used in construction. Most of the construction projects that contain pyrrhotite are because the aggregate that was used in the project came from a quarry that contained pyrrhotite deposits.  


Where can Pyrrhotite be found?

Pyrrhotite can be found all around the continental United States. Pyrrhotite in concrete has caused enormous problems for homeowners in parts of Connecticut and Massachusetts specifically. Above is conterminous United States showing the location of rock units that may contain pyrrhotite (Mauk and Horton, 2020); locations of pyrrhotite from the USGS Mineral Resources Data System database (U.S. Geological Survey, 2019); and locations of pyrrhotite from the Mindat.org database (Mindat.org, 2019).

How can pyrrhotite affect me?

Pyrrhotite related issues threaten the livelihood of homeowners. These issues continue to be looked past and fail to be addressed until dangers are observed in full effect. According to Great American Insurance Group, In Connecticut alone, there are several hundred cases, and thousands are suspected of having foundations with material sourced from a quarry with this mineral. 

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