The U.S. Geological Survey has developed the first map of rock deposits exhibiting the potential to contain or form pyrrhotite, whose presence in otherwise ASTM C33 aggregate can prove extraordinarily deleterious in concrete. The map is part of a new fact sheet, “Pyrrhotite Distribution in the Conterminous United States,” publication of which abides a Congressional directive addressing widespread residential foundation failure cases in Connecticut and Massachusetts.
|U.S. Geological Survey’s Carlin Green provided and photographed a mineral collector’s grade sample of pyrrhotite, related to the more common pyrite, aka fool’s gold. The sample originated in Michigan’s Eagle Mine, but is not characteristic of the pyrrhotite that would form in an aggregate deposit such as Becker’s Quarry, Connecticut.|
“When naturally exposed to water and oxygen, pyrrhotite breaks down to produce sulfuric acid and secondary minerals, including gypsum, which have larger volumes than the pyrrhotite they replace,” the fact sheet notes. “The expanded volume of the secondary minerals cracks and degrades concrete.” In Connecticut and, more recently Massachusetts, owners of homes with foundations cast from pyrrhotite-prone aggregate have observed serious cracking and crumbling. The extent of cases across more than 35 communities gave rise to the Connecticut Foundations Solutions Indemnity Company, a state entity assisting homeowners with repair or replacement work typically reaching the six-figure range. Officials have traced the pyrrhotite aggregate to the defunct Becker’s Quarry, whose sister operation, J.J. Mottes Co., supplied ready mixed for homes throughout northeast Connecticut. The aggregate was apparently used in ready mixed produced in Massachusetts, where some homeowners have observed foundation problems matching those of their Connecticut neighbors.
The USGS map shows about one-quarter of Connecticut having pyrrhotite permissive geology. Two of the five “Pyrrhotite Distribution” authors, Research Scientist Jeffrey Mauk and Physical Scientist John Horton, prepared it from the USGS State Geologic Map Compilation and Mineral Resources Data System; Mindat.org database; and, 35 recent state geological surveys. Their mapping shows how pyrrhotite may be distributed widely in metamorphic rock along the Appalachian Mountains and smaller pockets in the western United States.