Pyrrhotite is a mineral composed primarily of iron and sulfur, often found in igneous and metamorphic rocks. It typically appears as metallic, bronze-colored crystals and can vary in composition, leading to different magnetic properties. Its chemical formula is often represented as Fe1-xS, where x indicates the deficiency of iron. This mineral is significant in geology and mining, as it can be associated with valuable ores like copper and nickel. However, pyrrhotite can also cause problems in construction, as it may lead to the deterioration of concrete when it reacts with moisture and oxygen, producing sulfuric acid.