Manganite is a black mineral consisting of hydrated manganese (III) oxide (MnO[OH]); an ore of manganese. It
forms as low-temperature veins and secondary deposits, and occurs as bundles
of monoclinic system striated prisms, which are often crusts of small crystals.
It is black or gray in color and has a metallic luster. It alters to pyrolusite.
Manganite is found in western Europe, Michigan and California. Hardness
4, relative density 4.3.
Image source: Mineral Information Institute, courtesy of the Smithsonian
AND PLANETARY SCIENCE