Mineral, MnO₂Mn(OH)₂; sp. gr. 4.2–4.4; hardness 3–4; monoclinic; dark steel-grey to black; reddish-brown to black streak; sub-metallic lustre; crystals prismatic with striated faces, often as bundles or radiating aggregates; cleavage perfect, pinacoidal; occurs in deposits precipitated in sedimentary conditions in which there is an oxygen deficiency, and in low-temperature hydrothermal veins in association with barite and goethite; soluble in hydrochloric acid with the evolution of chlorine. It is often used in steel-making.