This paper summarizes the distribution and characteristics of the lateritic regolith of the Darling Range and presents models for its formation and evolution. Typical, complete, weathering profiles on granite average about 20 m in thickness and consist of gravelly soil, lateritic duricrust, saprolite and saprock. Lateritic duricrusts occupy gently sloping to horizontal upland areas and are either residual, or locally transported and recemented. In much of the Darling Range, the lower part of the duricrust, especially on hill slopes rather than crests or valley floors, is highly aluminous and forms an extensive resource of bauxite. Fragmental, fragmental-pisolitic, pisolitic and vesicular types can be identified on the basis of secondary structures. Fragmental duricrust largely consists of gibbsite, hematite, goethite and quartz and has resulted from direct gibbsitization of saprock or bedrock without forming the kaolinite-rich deep saprolite. Outcrops of duricrust with relict bedrock textures are common. In contrast, pisolitic duricrust with hematitemaghemite and χ-alumina rich mineralogy have a more complex history than fragmental duricrust with simple mineralogy. Vesicular duricrust is goethite-rich and is formed by the ferruginization of sandy detritus and quartz pebbles. The profiles show no condensed sequences, and individual rock types are traceable geochemically and mineralogically, but with increasing difficulty, from bedrock to the surface. The concentrations of Fe, Al, Si, Ti, V, Cr and residual quartz, particularly in fragmental duricrust, can be used to identify bedrock. Deep weathering profiles at Jarrahdale and Boddington yield late Tertiary palaeomagnetic ages and it appears that modification of these profiles to form bauxite is continuing today.
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Saprolite formed by lateritic isovolumetric chemical weathering of granite contains pseudomorphs of primary mineral grains which can be separated and analysed. For most elements the chemical composition of rock and saprolite matches the sum of the contributions by the separated primary mineral grains in the rock and their pseudomorphs in saprolite. This agreement allows an element 'budget' to be calculated, which shows that the behaviour of some elements during alteration of granite is determined by the mineralogy of the primary mineral grains and their various alteration products. Most of the Si, Ba, Sr, Pb and Cu present in feldspars in granite was lost on weathering to halloysite, whereas Ga, Al and V were retained. Most of the Zn, Mn and Co that was present in biotite in granite was also lost during alteration, whereas the pseudomorphs retained Fe, Cr and Ni. Zn, Fe, Mn, Cr and V present in magnetite were wholely retained within the hematite pseudomorphs, whereas Ni was lost. Thus Zn and Mn exhibit a mobile behaviour when initially present in biotite and a residual behaviour when present in magnetite. Ni shows an opposite behaviour; it is mobile in magnetite but residual in biotite. These differences are a consequence of the formation of different alteration products from the primary minerals.
Abstract Thirty-two heavy-mineral separates, each comprising approximately 2000 grains, were analyzed using the Automatic Geological Scanning Electron Microscope (AutoGeoSEM). This analysis was undertaken to study the influence of depositional environment on the distribution of heavy minerals throughout a regressive Pliocene coastal sequence. The majority of these heavy minerals were separated from the Loxton-Parilla sands, the host of the Bondi Main heavy-mineral beach placer deposit in the Murray Basin of southeastern Australia. The remaining heavy minerals were separated from a number of other Neogene units encountered at the deposit. The heavy-mineral suite is dominated by stable to ultrastable minerals such as zircon, rutile, and tourmaline, along with ilmenite, various intermediate titanates, and anatase. Sillimanite, andalusite, topaz, chrome spinel/chromite, monazite, and muscovite also occur. These heavy minerals are heterogeneously distributed throughout both the Loxton-Parilla sands and the remaining Neogene units. The main process controlling the heterogeneous distribution of these heavy minerals throughout the Loxton-Parilla sands appears to have been the hydraulic conditions, which are intimately linked with the environment of deposition. Consequently, the dense heavy minerals (density > ~ 3.5 g/cm3) are concentrated in relatively high-energy environments such as the breaker zone and swash zone. The less dense heavy minerals (density < ~ 3.5 g/cm3) are concentrated in the shoaling zone and shelf environments. The erratic distribution of the various heavy minerals in some of the other Neogene units is also apparently due to varying depositional conditions. Postdepositional weathering has altered the heavy-mineral assemblages in all units. Accordingly, some primary minerals, such as ilmenite, appear to have altered to secondary minerals, such as pseudorutile and anatase.
Abstract Dehydroxylation of synthetic and natural goethites with a range of Al-substitution from 0–28 mole% was investigated with a view to predicting the behaviour of soil goethites heated by bush fires. Hematites formed at temperatures ≤ 500°C retain the initial Al-content of the precursor goethite up to a maximum of 28 mole% Al. Loss of Al from the hematite structure occurred at 700°C for synthetic hematites with levels of substitution ≥18 mole% Al, but no crystalline alumina phase was present. Crystallization of corundum occurred for synthetic Al-goethites with levels of substitution ≥18 mole% when heated at 900°C. Formation of corundum reduces the maximum level of Al-substitution in hematites to ∼ 12 mole%. The exsolved corundum occurs as aggregated, platy crystals, 40–45 nm in diameter, containing a maximum of 7 mole% Fe. Al-substituted maghemite (7 mole% Al) formed from high Al-goethites heated at 900°C. Although some corundum in soils may be produced by heating of aluminous goethite by fires, the absence of corundum in natural Al-goethites calcined at 900°C, and the very high temperature (900°C) at which corundum formed from synthetic goethite suggest that other sources of Al may be required as precursors for the corundum formed by heating of soils.
The morphologically distinct materials in Darling Range lateritic duricrust (i.e. loose and cemented pisoliths, concretions, matrix, pisolith coatings and void coatings) exhibit goethite/(goethite + hematite) ratios ranging from 0.15 for individual pisoliths to 1.0 for void coatings. Mole % Al substitution ranged from 20 to 34% in goethite and from 2 to 15% in hematite. Goethite and hematite in pisoliths and concretions were mostly highly Al substituted. Al substitution in goethite was positively related (P < 0.01) to Al substitution in hematite. Al substitution in maghemite was less than 5%. Goethite crystals ranged in size from 130 to 260 A. Hematite crystals ranged from 140 to 520 A, and were systematically smaller in pisoliths. Crystal size of goethite and hematite decreased with increasing Al substitution. Hematite crystals were usually about 50% larger than goethite crystals in the same sample, and crystal sizes of goethite and hematite were positively correlated (P < 0.01). Goethite and hematite occurred as aggregates of subrounded platy crystals. Differences in the properties of goethite and hematite between morphologically distinct materials in single hand specimens are indicative of the complex history of these duricrusts.
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