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April 29, 2020

The USGS has released its first-ever map of where the mineral pyrrhotite may occur in the contiguous United States. This research was mandated by Congress in the FY2019 appropriations bill for the USGS and was supported by the USGS Mineral Resources Program.

Pyrrhotite, a mineral of concern for the construction industry, consists of iron and sulfur, and when exposed to water and air, it can break down to form secondary minerals that expand and crack concrete, causing concrete structures, like home foundations, to fail.

“This project was a bit unusual for us, because typically we’re trying to help people find mineral deposits that they want, not minerals that they don’t want,” said USGS scientist Jeff Mauk, who led the project. “Pyrrhotite in concrete has caused enormous problems for homeowners in parts of Connecticut and Massachusetts.”

Image shows the contiguous United States with potential occurrences of the mineral pyrrhotite
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 database (, 2019).

Pyrrhotite becomes an issue in concrete manufacturing if pyrrhotite-bearing stone is crushed up and used as filler for the concrete. Thus, identifying where it may occur can help identify where there may be a risk of pyrrhotite being included in crushed stone production. The new national map shows that pyrrhotite may be distributed widely in metamorphic rock along the Appalachian Mountains and in smaller pockets in the western United States.

This map uses data from three sources: the USGS State Geologic Map Compilation, the USGS Mineral Resources Data System database and the database. In addition, the map was refined and improved with feedback from 35 state geological surveys.

“We wanted to give Congress a guidemap so they’d know where pyrrhotite may occur in the United States,” said Mauk. “This map is a starting point, and we are so grateful for the fantastic support and feedback that the state geologists provided to help us improve it.”

Image shows a sample of pyrrhotite against a black background
A collector sample of pyrrhotite. Pyrrhotite, although not one of the Fool's Gold minerals, is sometimes called magnetic pyrite as it is weakly magnetic. It is most valued as a collectors mineral, and this sample would not have been in stone mined for aggregate.Sample provided by Carlin Green, USGS. Sample originated from Eagle Mine, Michigan, and is 4.5cm in size.

Pyrrhotite is related to the more common and well-known mineral pyrite, also known as Fool’s Gold. Pyrrhotite differs from pyrite because it has less sulfur and is far more reactive to water than most pyrite.

There are some limitations to this map. Pyrrhotite is not a particularly common mineral, so there are no maps that show where it does occur throughout the United States. USGS scientists instead used their geologic knowledge of where and how pyrrhotite forms to infer which rock formations may have it. The map, therefore, shows rock formations that may have pyrrhotite, but they are not guaranteed to have it.

The map and accompanying fact sheet can be found here. To find out more about USGS mineral resource information, please visit the USGS Mineral Resources Program webpage or follow us on Twitter.

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