Braunite II, andradite garnet, ettringite                          Braunite II on andradite garnet
                       (Wessels mine)                                                 (Wessels mine
      Braunite II crystal (2,7 cm x 1,4 cm)                             specimen (4,1 cm x 3,5 cm)


Crystal system:        Tetragonal        Hardness:        6-6,5

Density:        4.8            Streak:            Black

Cleavage:        Perfect            Composition:        silicate    Mn2+Mn3+6(SiO4)O8

I think it is about time that our MOM features a mineral from a local locality. Many of our interesting discussions at the club regarding minerals from KMF have included the mineral braunite; and more specifically the difference between braunite and braunite II and the other minerals it is associated with and also sometimes mistaken for, such as hausmannite, hematite, bixbyite, manganite, kentrolite. Limited information is available on this mineral but I managed to compile the following.

Braunite was discovered in 1827 and was named after Alderman Wilhelm von Braun (1790-1872) being a minister in Gotha, Thuringia, Germany. He was a strong supporter of geology and mineralogy and supplied the original material for the description of braunite.

Braunite crystallizes in the tetragonal system as pyramidal striated crystals or as masses. It occurs in veins as a secondary mineral formed by weathering and associated with other manganese minerals.

Braunite is a silicate mineral and common impurities include iron, calcium, boron, barium, titanium, aluminum and magnesium. Braunite is found in Germany, Panama, Norway, Sweden, Italy, India, USA, Brazil, Australia, Namibia and South Africa.

Southern African localities:

Braunite

Braunite is one of the most important ore minerals of the manganese fields. It is dominant as a fine-grained form in the Mamatwan-type ore and is found in a coarse recrystallised form in the Wessels-type ore. Single crystals have not yet been found, other than in the Postmasburg manganese field, where braunite occurs with barite at Kapstewel, as small 2-3 mm octahedral crystals in massive braunite.

Other localities are in the Waterberg with the mineral psilomelane, at Weenen in KwaZulu-Natal and at Gendendal (Marico) associated with pyrolusite, psilomelane, dolomite, hausmannite, partridgeite, coronadite, heteraeolite and cryptomelane. I will not even try to pronounce these mineral names. I did not even know they existed, except maybe for the latter that vaguely resembles the name of a “crystal” from a famous super hero’s planet.

The Otjosondu manganese deposit, located 150 km northwest of Okahandja, is the largest manganese deposit in Namibia. The mine is virtually unknown for its collector-type minerals. Yet various minerals are found here which include braunite, with a high barium component. Probably the world’s largest euhedral crystal of braunite was collected at this location and forms part of the Desmond Sacco collection. The main crystal on this specimen measures 5,3 cm.

Braunite II

Braunite II is the name given to a silica-deficient analogue of braunite, in which the c-axis is twice the length of that of braunite.  Braunite II is not regarded as a distinct mineral species, but rather as a compositional variety of braunite that was first discovered at Black Rock (De Villiers, 1945) and described as ferrian braunite. It was renamed braunite II by De Villliers and Herbstein in 1967 and its crystal structure was determined by De Villiers in the 1980s. It is a calcium iron bearing variant with the formula Ca (Mn3+, Fe3+)14SiO24. Black Rock mine is the type locality for this mineral.

Metallic–grey braunite II crystals have come from Wessels, Black Rock (up to 2 cm) and N’Chwanning II. The crystals are typically bi-pyramidal with complex terminations. In 1995, crystals up to 3 cm were found at Wessels mine. These were associated with red andradite garnet, hausmannite and occasionally yellow ettringite. This is the only locality in the world where macroscopic braunite II has been found. – JDJ

             
    
Two specimens with braunite II, hausmannite and andradite garnets probably also from Wessels.
Specimen on the left measures 13 cm x 11,5 cm and specimen on the right 20 cm x 11 cm.


References
Bruce Cairncross, Roger Dixon, Minerals of South Africa.
Bruce Cairncross, 2000 – The Desmond Sacco Collection – Focus on Southern Africa.
Bruce Cairncross, Nicolas Beukes, Jens Gutzmer, 1997 – The Manganese Adventure.
Michael O’Donoghue, 1885 – The Encyclopedia of Minerals & Gemstones.
http://en.wikipedia.org
http://webmineral.com
www.mindat.org