Braunite
Braunite is a silicate mineral containing both di- and tri-valent manganese with the chemical formula:
Mn2+Mn3+6. 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 of Gotha, Thuringia, Germany.
A calcium iron bearing variant, named braunite II, was discovered and described in 1967 from Kalahari, Cape Province, South Africa.
