Tarapacáite is the mineral form of potassium chromate with the chemical formula O 4. It forms bright yellow crystals and was discovered in 1878. It is named for the former Tarapacá Province, Peru; nowadays belonging to Chile. The boundaries between Peru, Bolivia and Chile were vague in the Atacama Desert (1879–1883). Its type locality is Oficina Maria Elena, Maria Elena, Tocopilla Province , Antofagasta Region, Chile. It is unlikely to occur anywhere except in highly arid conditions as it is easily soluble in water.