Abstract

Five types of silcrete are recognised in Australia and Southern Africa: terrazzo, conglomeratic, Albertinia, opaline and fine-grained massive, and quartzitic. Of these the first is the primary and most abundant form. The second and third types are derivatives of it, but the final two are distinct and much rarer. The terrazzo variety is mostly pale buff or yellowish grey, but may also be green or purplish brown. It consists of a framework of angular to subrounded quartz grains, some showing effects of solution, set in a matrix of amorphous, cherty or opaline silica containing abundant leucoxene. The leucoxene is a marked characteristic, and commonly forms colloform structures. The conglomeratic type consists of brecciated terrazzo type in a silcrete matrix of similar colour or bright red. In the Albertinia type quartz detritus is almost absent. The rarer forms are mostly pure common opal or chalcedony, either massive or porous. The quartzitic type is the response of a sedimentary orthoquartzite to the silcrete-forming process. Many occurrences can be explained in terms of profile formation, silica being made available from elsewhere within the profile, and attention is drawn to the profile displayed on Spilsby Island, South Australia. Silica-rich solutions rising to meet downward percolating solutions with NaCl, Na 2 SO 4 , Fe 2 O 3 or MgO or of lower pH, could cause the silica deposition. Very widespread occurrences may have formed by accumulation through evapo-transpiration. The quartzitic type forms by authigenic overgrowths in detrital quartz grains.

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