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The 2020 U.S. Geological Survey map highlights geologic units across the conterminous United States where pyrrhotite may occur. It can help screen for possible risk, but it cannot tell you whether a particular home’s foundation or a particular aggregate source contains harmful pyrrhotite. That requires site-specific characterization and quality control.
Why pyrrhotite can damage concrete foundations
Pyrrhotite is an iron sulfide mineral. When pyrrhotite-bearing crushed stone is used as concrete aggregate, the mineral may be exposed to water and oxygen inside the concrete. It can weather and form sulfuric acid and secondary minerals, including gypsum. Those products occupy more volume than the original mineral, contributing to cracking and deterioration.
Chemistry World reported in 2020 that thousands of homes in Massachusetts and Connecticut had been damaged, attributing the estimate to the National Institute of Standards and Technology. The cited passage does not identify the underlying NIST publication, so the figure is best understood as the report’s attributed estimate rather than a verified count from a named NIST study.
What the USGS map shows
The map identifies geologic units that may host pyrrhotite across the conterminous United States. It is a national screening resource: a highlighted area indicates possible host geology, not confirmed pyrrhotite at every location or a harmful concentration in every rock source.
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The USGS published the fact sheet on March 26, 2020, and accompanying data on June 2, 2020. These are 2020 resources; the dates alone do not establish whether a later map or update has been released.
How USGS selected candidate geologic units
USGS combined national geologic mapping data with mineral-occurrence records, then sought input from state geologists to refine candidate units. The approach considered rock descriptions and geological characteristics associated with potential host rocks, including sulfide attribution and moderate-to-high metamorphic grade. The team also compared candidate units with known occurrence locations in USGS and Mindat databases.
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The underlying records have important limits. In the relevant geologic map database, pyrrhotite appeared by name 13 times, while “sulfide” or “sulfidic” appeared 106 times among more than 2,000 records. These are counts of descriptions in the database—not estimates of pyrrhotite prevalence in rock, affected homes, or harmful mineral abundance. Incomplete mapping, limited rock descriptions, and inconsistent reporting of rare minerals constrain what the map can establish.
What a map location means for a home or quarry
| Evidence | Geographic scope | What it can support |
|---|---|---|
| USGS map | National-scale geologic units | Identifies areas where host geology may occur; does not confirm mineral presence or abundance at a property or aggregate source. |
| Site-specific characterization and quality control | A particular property or aggregate source | Can assess the material or site directly; the USGS fact sheet says this work is needed to judge suitability for a particular purpose. |
A mapped unit near a home does not diagnose a foundation, and a location outside a highlighted unit is not a guarantee about the material used in that home. Likewise, the map cannot determine whether stone from a specific quarry is suitable for concrete. USGS cautions that its map and fact sheet “are no substitute for site-specific characterization and quality control programs designed to ensure that aggregate used in concrete is of appropriate quality for its intended purpose.”
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What homeowners should do with the information
- If you are assessing a particular foundation: Treat the map as regional context, not a diagnosis. Seek qualified, site-specific evaluation of the foundation and relevant materials.
- If you are evaluating an aggregate source: Ask for appropriate characterization and quality-control evidence for that source and intended use. The map does not supply a testing protocol or contaminant threshold.
- If you are interpreting a mapped area: Read it as a signal of possible host geology, not proof that pyrrhotite is present or harmful locally.
Chemistry World quoted civil engineer Sergio Cavalaro of Loughborough University: “this valuable information alone is not enough to judge the suitability of a particular aggregate, which requires appropriate testing and verification against clear limits for contaminant contents”. The map can direct attention to a possible geological concern; it does not replace the direct evidence needed for decisions about a home or aggregate source.
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