( ~ Copyright the Mineralogical Society

Some observations on the mineralogy and genesis of braunite ALTHOUGH braunite 3Mn20 3-MnSiO 3 is best known from its occurrence in regionally meta-morphosed manganese deposits uch as the gon-dites and kodurites of India (Roy, 1981) it also occurs in hydrothermal veins (Frenzel, 1980) and in unmetamorphosed sediments (De La Hunty, 1963). A minor occurrence in the lower Cretaceous sedimentary manganese ores of Groote Eylandt is interesting as it indicates a sabkha-type origin for the mineral, and suggests a possible origin for the presence of small amounts of boron commonly found in braunite. The presence of braunite was first noted by the writer in 1975 (Ostwald, 1980). The occurrence of irregular zones of braunite around sand grains in manganiferous clays suggested that it was a reaction product, possibly produced during sedi-mentation of MnO2 colloids. Occurrences of skele-tal crystals in dense cryptomelane were more difficult to explain. Recently further occurrences of braunite and cryptomelane showing chevron layer-ing have been noted and examined. Typical occur-rences are shown in fig. 1. Under the optical microscope braunite is brown-ish grey against pale grey cryptomelane, and it commonly occurs as parallel, tabular layers alter-nating with cryptomelane, and with definite angular morphology. Fortification-type structures also occur, with re-entrant angles common. The chemical compositions of the braunite and F1G. 1. Examples of (a) chevron layered structures and (b) fortification structure of braunite (dark grey) and cryptomelane (pale grey). Reflected light ( x 300)