Pyrolusite is a soft, gray-black oxide mineral composed of manganese (IV) oxide (MnO2). Of widespread occurrence, pyrolusite (pyrolucite) is the chief ore of manganese. It is a secondary mineral formed as a precipitate in lakes and bogs at low temperatures, often in veins of quartz, but also occurs as manganese nodules on the floor of the Pacific Ocean. It usually occurs in masses that have an earthy luster, although it may form prismatic crystals (in the tetragonal system) and fern-like aggregates in the joints of sedimentary rocks. Hardness: massive form 2–6, crystals 6–7; relative density 4.5–5. See also manganite.