Abstract--The formation of iddingsite by the oxidative weathering of FoB0 olivine begins by solution of Mg from planar fissures, 20 ~ wide and spaced 200 ~ apart, parallel to (001). Oxidation of Fe within the remaining olivine provides nuclei for the topotactic growth of goethite. Cleavage cracks <50 ~ in diameter allow Na, AI, and Ca from adjacent minerals, particularly plagioclase, to enter the altering olivine while Mg and Si diffuse away. In the early stages of weathering, strips of Fe-rich smectite (saponite), 20-50 & wide and 1-7 layers thick, form bridges 50-100 ~. long across the planar fissures. Dioctahedral smectite crystallizes on the margins of wider cleavage-controlled fissures; with further weathering halloysite is formed away from the fissure walls. In the ultimate stages of alteration, the saponite and dioctahedral smectite are lost, leaving a porous, oriented aggregate of goethite crystals each measuring about 50 • 100 • 200 A (X, Y, Z, respectively), with sporadic veins of halloysite crossing the pseudomorph.

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During 1987, the Australian Bureau of Mineral Resources conducted a multidisciplinary investigation of the modern phosphorites on the continental margin of southeastern Australia between 28 and 32°S. The objectives of the work were to examine the processes controlling the cycling of organic carbon and bioactive elements, nitrogen, phosphorus, sulphur and iron in the sediments, and to investigate the roles which these processes played in the formation of the modern phosphorites. Bacterial productivities, sulphate-reduction rates, sedimentary oxygen and pore-water concentrations of nitrate, ammonia, phosphate, iron, sulphate and fluoride were measured at sea. The highest rates of microbial productivity were found in the surficial (0–20 mm) sediments of the modern phosphorite zone in 350–460 m water depth. These rates were about double those in shallower shelf (<300 m) sediments and 3–4 fold those rates in mid-slope (600–1000 m) sediments. Aerobic and anaerobic oxidation rates of organic matter, calculated from sediment oxygen profiles and sulphate-reduction rates were highest in the surface sediments in the modern phosphorite zone. The recycling of sedimentary iron, via reductive dissolution of iron oxyhydroxides and reprecipitation at the oxic/anoxic boundary results in a near-surface sedimentary trap for iron in the phosphorite zone sediments. Phosphate released from organic matter in the interfacial sediments, and fluoride from seawater, are scavenged by iron oxyhydroxides in the top few centimetres of sediment. Phosphorus, in this way, is decoupled from organic carbon in the near-surface sediments and linked to the redox cycling of iron. Phosphate and fluoride scavenged onto iron oxyhydroxides, and concentrated in the surficial sediments, are subsequently released to pore waters in the anoxic sediments when iron oxyhydroxides are buried and dissolve. The recycling process releases phosphate and fluoride for incorporation into apatite; fluoride is depleted from pore waters at depths <18 cm, phosphorite nodules form within anoxic sediments at depths <18 cm and continue to accumulate iron and phosphorus while resident in the mixed layer. Combinations of rapid sediment mixing rates, a slow sedimentation rate and a mixed layer to about 18 cm result in an average particle residence time in the phosphorite zone sediments which is about ten-fold that of the mid-slope sediments. Long residence times and rapid mixing promote the oxidation of organic carbon and release of phosphate, while the continuous recycling of iron and phosphate concentrates the phosphorus for apatite precipitation and accumulation into phosphorite nodules. Phosphorite nodules are not found in mid-slope sediments probably because of combinations of relatively rapid sedimentation rates, ineffective iron, phosphorus and fluoride recycling and trapping mechanisms, plus dilution and dissemination of any incipient apatite.

Detailed studies of morphological, micromorphological and geochemical characteristics of silcretes in the deep bleached and weathered regolith across a large area of inland Australia have provided a new interpretation of the history of the regolith and its climatic and morphological evolution during the Tertiary. Pedogenic silcretes have distinctive morphological and mineralogical features caused by a succession of phases of silica dissolution and recrystallization resulting from multiple episodes of water infiltration and percolation under alternately wet and dry climates. These are the oldest of the regolith features. Deep, bleached profiles formed over a wide area in a variety of substrates ranging from Precambrian granites to Palaeozoic sandstones, Cretaceous sediments and Tertiary deposits, and represent the second major stage in regolith development. These profiles, in which kaolinite coexists with gypsum, alunite and opal, formed by reaction of the substrates with saline groundwaters, the water-table levels of which progressively fell over the region. Extensive networks of termite burrows constructed to great depth in the bleached regolith followed the water tables down. The climate was warm and dry with a high water deficit. Groundwater silcretes formed near-horizontal lenses and pods of porcellanite and jasper in the bleached regolith. They preserve the primary fabric of the host rock. Groundwater silcretes post-date the construction of termite burrows and were formed during a rise in groundwater tables across the landscape, in places to near-surface environments in broad landscape depressions. The climate was more humid but the presence of gypsum during silicification demonstrates that the groundwaters were still saline. Red-brown hardpans are the youngest silicification features and represent periods of successive infiltration and percolation, and waterlogging, during high rainfall or flood events. They are confined to low regions in the landscape. Mineralogical and geochemical analysis of the bleached profiles, together with geochemical modelling, suggests that ferrolysis is the most likely cause of acidity in groundwater leading to the development of the bleach profiles and/or alunite. Present-day groundwater tables are both at low levels and sulphate-rich. It is possible that acidic alteration leading to bleaching is still active around the extensive playa landscapes in the region.

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