SUDOITE IN THE ARCHEAN WITWATERSRAND BASIN

Sudoite, with almost ideal composition of Mg2Al4Si3O10(OH)8, has been found in the Khaki shale of the Witwatersrand Basin, South Africa. The sudoite occurs in aggregates of elongate bundles alternating with mus covite and rarely pyrophyllite, in an assemblage of sudoite - pyrophyl lite - muscovite - quartz - rutile - pyrite. The stability of sudoite is attributed to a particularly Mg rich bulk rock composition, elevate d sulphur activity, and metamorphic conditions just above the kaolinit e to pyrophyllite transition. Nearby low variance chlorite, chloritoid - and pyrophyllite-bearing assemblages represent differing bulk rock c ompositions at potentially very similar metamorphic conditions. As the first find of sudoite in the Archaean, this occurrence in the Witwate rsrand Basin extends the metamorphic setting and age in which sudoite is found to well beyond Phanerozoic belts of crustal thickening.