Braunite
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Braunite is a silicate mineral containing both di- and tri-valent manganese with the chemical formula: Mn2+Mn3+6SiO12. 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 a Mr. Braun of Gotha, Thuringia, Germany.
A calcium iron bearing variant, named braunite II (formula: Ca(Mn3+,Fe3+)14SiO24), was discovered and described in 1967 from Kalahari, Cape Province, South Africa.
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