Braunite
Braunite is a silicate mineral containing both di- and tri-valent manganese with the chemical formula: MnMn[O|SiO]. Common impurities include iron, calcium, boron, barium, titanium, aluminium, and magnesium. Braunite forms grey/black tetragonal crystals and has a Mohs hardness of 6 - 6.5. It was named after the Wilhelm von Braun (1790–1872) of Gotha, Thuringia, Germany. A calcium iron bearing variant, named braunite II (formula: Ca(Mn,Fe)SiO), was discovered and described in 1967 from Kalahari, Cape Province, South Africa.