Jarosite is a hydrated potassium iron sulfate mineral, yellow to orange-brown in color, that forms from the alteration of iron sulfides in acidic, water-rich environments. It was discovered in the Jaroso ravine (Almería, Spain) in 1852, from which it takes its name, and is commonly known as alum stone. It has also been found in large quantities on Mars, suggesting the presence of water and acidic conditions on the planet.