Abstract

Asbestiform manjiroite and todorokite occur as infill of bedding parallel extension veinlets in supergene altered braunite-kutnahorite lutite at the Smartt Mine in the Kalahari manganese field, South Africa. Geological evidence and preliminary geochronological studies suggest that hydraulic extension and formation of the asbestiform Mn oxihydroxides took place during Tertiary uplift and tilting of the South African Subcontinent. Careful chemical studies of both minerals confirm the role of Mg as a minor but essential constituent in the todorokite structure that is located on a distinct – but as yet undetermined – crystallographic site within the tunnels of the todorokite framework structure. This site is distinct from the one occupied by large cations such as Ca, Na and K.

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