Sulfide minerals can spontaneously combust when exposed to air, USGS study finds

Deep-sea sulfide minerals can spontaneously heat and combust when exposed to air. A USGS-led study found that some metal sulfide-rich rocks collected from Escanaba Trough underwent spontaneous combustion during laboratory processing. The finding identifies a potential hazard that could be important to understand if seafloor massive sulfide deposits are ever mined.

Seafloor massive sulfide deposits form around hydrothermal vents, where hot fluids circulating through Earth's crust release metals that precipitate when they encounter colder seawater. These deposits can contain copper, zinc, iron and other economically important elements.

The combusting rocks were composed primarily of nanocrystalline marcasite. The researchers found that marcasite may be especially unstable. Oxidation of sulfide minerals can be exothermic, meaning it releases heat. Under the right conditions, that heat can contribute to further self-heating of the material.

The study was published in Scientific Reports. The researchers emphasize that the study does not establish that all seafloor massive sulfide deposits will spontaneously combust.

The mineralogy and environmental conditions associated with the Escanaba Trough samples are sufficiently different that safety approaches developed for terrestrial deposits may not directly apply to deep-sea materials. Rocks brought to the surface would experience conditions dramatically different from those on the seafloor, including exposure to oxygen and changes in temperature, pressure and moisture.

The agency said the study points to the value of conducting dedicated risk assessments for seafloor massive sulfide deposits and developing engineering controls specifically designed to reduce the possibility of self-heating and spontaneous combustion.