Titania in Australian massive silcretes

Selected silcretes formed by cementation of stream sediments and having >90 wt% SiO₂, have been examined optically and by scanning electron microscopy. Such silcretes have quartz framework grains cemented by a plasma of quartz and anatase. Both the plasma quartz and the anatase are euhedral where they line cavities in the silcrete. Such quartz is typically up to 5 µm in diameter; the anatase crystals are platy on (001) and 50–100 nm in diameter. Comparison of the SiO₂, TiO₂ and Zr content of 138 silcretes with that of 2345 Australian stream sediments suggests the source of titania in silcretes is endogenous. The morphology of the quartz and anatase leads to the conclusion that both precipitated in situ.