Braunite

oxide

rhodonite

Formula: Mn2+Mn3+6O8(SiO4)
Multiple oxide, manganese mineral
Specific gravity: 4.75
Hardness: 6 to 6½
Streak: Black
Colour: Black
Environments:

Sedimentary environments
Metamorphic environments
Hydrothermal environments

Braunite is formed by metamorphism of manganese silicates and oxides, or as a product of weathering (HOM)/Webmin). Associated minerals are pyrolusite, jacobsite, hausmannite, bixbyite, rhodonite, spessartine and hematite (HOM). Braunite from arenaceous beds on the farm Weenen, Northern Transvaal, South Africa, occurs in low grade ore of a sedimentary origin with supergene enrichment. Other manganese minerals in this ore are psilomelane, and pyrolusite (AM52.20-30).

Braunite is both an ore-forming mineral and a precursor of supergene oxide ores in manganese deposits worldwide. It is essentially of sedimentary origin. Braunite assemblages are found in rocks of all metamorphic grades:

Greenschist facies, biotite grade: either pyrolusite, braunite and quartz or braunite, bixbyite and quartz

Amphibolite facies facies, garnet and staurolite-kyanite grades: braunite, rhodonite and quartz

Granulite facies, sillimanite grade: either braunite, bixbyite and quartz or braunite, rhodonite and quartz

Progressively higher temperatures resulted in braunite coexisting with more reduced manganese oxides and ultimately with rhodonite or pyroxmangite (AM80.561).

Alteration

braunite to hausmannite, SiO2 and O2
3Mn2+Mn3+6O8(SiO4) ⇌ 7Mn2+Mn3+2O4 +3SiO2 + O2
(AM80.565)

braunite to hausmannite, rhodonite and O2
2Mn2+Mn3+6O8(SiO4) ⇌ 4Mn2+Mn3+2O4 + 2Mn2+SiO3 + O2
A higher temperature favours the forward reaction (AM80.565)

braunite to tephroite, hausmannite and O2
3Mn2+Mn3+6O8(SiO4) ⇌ 3Mn2+2(SiO4) + 5Mn2+Mn3+2O4 + 2O2
(AM80.572)

braunite and CO2 to rhodochrosite, rhodonite and O2
2Mn2+Mn3+6O8(SiO4) + 12CO2 ⇌ 12MnCO3 + 2Mn2+SiO3 + 3O2
A higher temperature favours the forward reaction (AM80.571).

braunite and quartz to rhodonite and O2
2Mn2+Mn3+6O8(SiO4) + 12SiO2 ⇌ 14Mn2+SiO3 + 3O2
A higher temperature favours the forward reaction (AM80.565).

pyrolusite and quartz to braunite and O2
7Mn4+O2 + SiO2 ⇌ Mn2+Mn3+6O8(SiO4) + 2O2
A higher temperature favours the forward reaction (AM80.571).

rhodochrosite, SiO2 and O2 to braunite and CO2
14MnCO3 + 2SiO2 + 3O2 ⇌ 2Mn2+Mn3+6O8(SiO4) ⇌ 14CO2
A higher temperature favours the forward reaction (AM80.571).

rhodonite and braunite to tephroite and O2
10Mn2+SiO3 + 2Mn2+Mn3+6O8(SiO4) ⇌ 12Mn2+2(SiO4) + 3O2
(AM80.571)

Common impurities: Fe,Ca,B,Ba,Ti,Al,Mg

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