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Data to accompany U.S. Geological Survey Fact Sheet 2020-3017: Pyrrhotite distribution in the conterminous United States

June 2, 2020

The mineral pyrrhotite, which is an unstable sulfide mineral, was present in crushed stone aggregate used in concrete foundations of homes in Connecticut and Massachusetts, many of those foundations are failing as the mineral weathers. Because of the substantial costs of lifting homes and replacing their foundations, in 2019 the U.S. Congress directed the U.S. Geological Survey?s (USGS) Mineral Resources Program to help assess the national risk of pyrrhotite in aggregate. The data presented here help form a map that shows where pyrrhotite may occur across the conterminous United States, which can help inform risk assessments. Pyrrhotite occurs in some mineral deposits, and it also forms from pyrite during metamorphism, so it occurs in rocks in many areas across the U.S. The data herein represent selected geologic units from the State Geologic Map Compilation (SGMC) (Horton and others, 2017) that may contain pyrrhotite. Candidate units were initially selected from the national-scale, geospatial data using report descriptions, such as the Brimfield Schist in Connecticut, and by selecting rock units that were attributed as sulfidic, or of moderate to high metamorphic grade. Candidate units were compared to known point locations of pyrrhotite reported in the USGS Mineral Resources Deposit System (https://mrdata.usgs.gov/mrds) and the Mindat.org database. Candidate units were further winnowed by USGS and state geologist on a regional and state-by-state basis. The resulting data show a subset of rock units that might be permissive for pyrrhotite. However, confirming the presence or absence of pyrrhotite would require extensive new mapping, sampling, and laboratory study, and is therefore beyond the scope of this study. The national representation of rock units that may be permissive for occurrences of pyrrhotite has additional limitations. For example, pyrrhotite may occur in areas where data are incomplete or not available due to a lack of detailed geologic mapping and (or) rock descriptions. Where the presence of pyrrhotite is noted, there usually is not enough information to distinguish the abundance. Therefore, the amount of pyrrhotite in rock units may vary substantially, and some rock units in the database may only contain trace quantities of pyrrhotite that would not impair concrete durability. Some rock units selected on the basis of their attribution as sulfidic may lack pyrrhotite because other sulfide minerals, such as pyrite or marcasite, are present, giving the unit its sulfidic nature. Finally, some rock units were selected because they appear to be of moderate to high metamorphic grade, which is one geological environment where pyrrhotite commonly forms. Horton, J.D., San Juan, C.A., and Stoeser, D.B., 2017, The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States (ver. 1.1, August 2017): U.S. Geological Survey Data Series 1052, 46 p., https://doi.org/10.3133/ds1052. U.S. Geological Survey Fact Sheet 2020-3017: Mauk, J.L., Crawford, 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, 4 p., https://doi.org/10.3133/fs20203017.