The US Geological Survey (USGS) has released its first map of where pyrrhotite – a mineral with the potential to compromise the integrity of concrete – is most likely to found across the continental US. It is hoped that the map will help the US construction industry avoid pyrrhotite-bearing rocks.

‘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,’ says author and geologist Jeff Mauk, who led the USGS mapping project. ‘Pyrrhotite in concrete has caused enormous problems for homeowners in parts of Connecticut and Massachusetts.’

An iron sulfide mineral, pyrrhotite is related to pyrite – which is commonly known as ‘fool’s gold’ – and is formed when pyrite loses its sulfur during recrystallisation. Unlike its more well-known cousin, however, pyrrhotite is both rarer and more reactive. When exposed to water and oxygen, it breaks down to form sulfuric acid and various secondary minerals, including gypsum, which have larger volumes that the pyrrhotite they replace. This has led to thousands of homes being damaged in Massachusetts and Connecticut alone, according to the National Institute of Standards and Technology.

This becomes a serious issue if pyrrhotite-bearing rocks are accidentally crushed up and used as filler in the production of concrete, as the expansion can cause the concrete to crack and fail. Unwitting incorporation of the mineral in concrete that later went on to be used in building foundations has caused damage to properties in the US, Canada and Europe.

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