Manganite is a mineral. Its composition is manganese oxide-hydroxide, MnO(OH), crystallizing in the orthorhombic system and isomorphous with diaspore and goethite. Crystals are prismatic and deeply striated parallel to their length; they are often grouped together in bundles. The color is dark steel-grey to iron-black, and the luster brilliant and submetallic. The streak is dark reddish-brown. The hardness is 4, and the specific gravity is 4.3. There is a perfect cleavage parallel to the brachypinacoid, and less-perfect cleavage parallel to the prism faces. Twinned crystals are not infrequent.
The mineral contains 89.7% manganese sesquioxide; it dissolves in hydrochloric acid with evolution of chlorine. The best crystallized specimens are those from Ilfeld in the Harz, where the mineral occurs with calcite and barite in veins traversing porphyry. Crystals have also been found at Ilmenau in Thuringia, Neukirch near Schlett stadt in Alsace (newkirkite), Granam near Towie in Aberdeenshire, and in Upton Pyne near Exeter, UK and Negaunee in Michigan, United States. As an ore of manganese it is much less abundant than pyrolusite or psilomelane.
The name manganite was given by W. Flaidinger in 1827.