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.”
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).
(Public domain.)
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 Mindat.org 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.”