Braunite

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.

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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.