braunite

Braunite is a manganese silicate mineral with the chemical formula Mn7SiO12. It contains manganese in two oxidation states, Mn2+ and Mn3+. The mineral commonly occurs as black to brownish-black, opaque masses with a dull to submetallic luster; its streak is brownish gray. It typically forms as nodular to massive aggregates, and rare well-formed crystals have been reported.

Braunite's structure comprises interconnected Mn–O polyhedra linked to silicate tetrahedra, giving it a relatively high density. It is moderately hard, with a Mohs hardness of about 5 to 6, and a specific gravity around 4.4 to 4.5. It is insoluble in water and may exhibit alteration to other manganese oxides.

Occurrence and associations: Braunite forms in hydrothermal manganese-rich veins and in metamorphosed or sedimentary manganese deposits. It is commonly associated with quartz, calcite, and other manganese oxides such as psilomelane or pyrolusite, as well as goethite and siderite. It has a worldwide distribution and can be an important source of manganese in certain ore deposits.

Classification and significance: Braunite is a manganese silicate mineral and is valued as a minor ore of manganese in some deposits. It is of interest to mineralogists for its mixed-valence manganese chemistry and its role in the paragenesis of manganese-rich systems.