Pyrrhotite: Mapping the Mineral That Crumbles Concrete Foundations

U.S. Geological Survey Fact Sheet 2020–3017, Mineral Resources Program. By Jeffrey L. Mauk, Thomas C. Crafford, John D. Horton, Carma A. San Juan and Gilpin R. Robinson Jr.

Crumbling foundations

In parts of Connecticut and Massachusetts, the concrete foundations of some homes are cracking and crumbling. A failing foundation cuts a home's value, and lifting a house to pour a new one is expensive; some owners have defaulted on their mortgages and walked away. The culprit is pyrrhotite, an iron sulfide mineral in the crushed stone used as filler in the concrete. Exposed to water and oxygen, pyrrhotite breaks down into sulfuric acid and new minerals, such as gypsum, that take up more space than the pyrrhotite they replace — and the swelling cracks the concrete.

Pyrrhotite occurs in rocks in many parts of the country. Congress's fiscal year 2019 appropriation for the USGS Mineral Resources Program funded a national map of where it may occur. The fact sheet presents that map, explains what controls pyrrhotite in rocks, points to the underlying datasets, and describes aggregate standards meant to prevent failing concrete. The map is no substitute for testing and quality control of the aggregate actually used.

How pyrrhotite forms

• Iron is plentiful, sulfur is not. Iron is the fourth most abundant element in Earth's crust (5.6 percent on average), so most rocks have enough to form iron sulfides. Sulfur averages only 0.025 to 0.050 percent, so sulfur usually limits how much forms.
• Pyrite first. Pyrite, the commonest sulfide, forms where natural processes add sulfur to rock, in ore deposits, or where bacteria in oxygen-free mud — the black mud of a swamp — add sulfur that combines with iron. Seawater holds more sulfur than fresh water, so pyrite is more abundant in marine and estuary sediments.
• Heat and pressure make pyrrhotite. Deeply buried sedimentary rocks turn metamorphic, driving off water and sulfur. Pyrrhotite holds less sulfur than pyrite, so losing sulfur turns pyrite into pyrrhotite — common in metamorphosed marine sediments. Sea-floor black smoker vents and volcanic rocks, rich in sulfides, can also yield abundant pyrrhotite when metamorphosed. Tectonic forces later bring these rocks to the surface.
• Why pyrrhotite matters most. Both pyrite and pyrrhotite react in concrete, but pyrrhotite is far more reactive.

Making the map

No map of pyrrhotite existed, but three sources together give a good approximation:

1. The State Geologic Map Compilation geodatabase — from which the team chose units where pyrrhotite has been reported (such as the Brimfield Schist in Connecticut), units described as sulfidic, and moderately to highly metamorphosed rocks.
2. The USGS Mineral Resources Data System — every recorded pyrrhotite occurrence in the conterminous United States.
3. Mindat.org — every recorded occurrence except those in meteorites; many duplicate the USGS records.

What it shows. In the East, possible pyrrhotite-bearing rocks form a belt along the core of the Appalachians. Most of the central United States sits on unmetamorphosed sedimentary rock and is unlikely to hold any. In the West, such rocks occur in belts broken up by the region's complex geology. The recorded occurrences mostly fall on the selected rock units, with extra isolated points at mineral deposits and elsewhere — together, a reasonable landscape-scale picture.

Limitations

• Geologic map databases describe rocks' main minerals, so rare pyrrhotite is seldom mentioned: in more than 2,000 records, pyrrhotite is named only 13 times and "sulfide" or "sulfidic" 106 times. Most selected units are simply metamorphic rocks — places where pyrrhotite may occur, not where it does. That recorded occurrences mostly follow them suggests the approach is broadly sound.
• Pyrrhotite may occur where mapping or descriptions are lacking.
• The map shows possible presence, not amount; some units may hold only harmless traces.
• Other iron sulfides — pyrite and marcasite — can also damage concrete, but pyrite is so common that a national map of it would be of little use, and it is less reactive.

Standards for aggregate

Sources

• Mauk, J.L., Crafford, T.C., Horton, J.D., San Juan, C.A., and Robinson, G.R., Jr., 2020, Pyrrhotite distribution in the conterminous United States, 2020: U.S. Geological Survey Fact Sheet 2020–3017. https://pubs.usgs.gov/publication/fs20203017
• The fact sheet's photograph is by a photographer outside the USGS, and its map is drawn over a commercially licensed shaded-relief base, so neither is reproduced here.
• Rewritten in hubnx's own words.