Rob Fitzpatrick

New data and a revised structural model for ferrihydrite

Synthetic 2-line and 6-line ferrihydrite samples prepared from ferric nitrate solutions have the bulk compositions Fe4(O,OH,H2O)12 and Fe4,6(O,OH,H2O)12, respectively. The composition depends on crystal size, which averages 20 Å for 2-line and 35 Å for 6-line ferrihydrite. X-ray absorption edge spectra indicate the presence of tetrahedral Fe3+, a conclusion supported by heating experiments which show the development of maghemite after heating to 300°C in the presence of N2, followed by the formation of hematite at higher temperatures. These two reactions are recorded on differential thermal analysis traces by exotherms at 350° and 450°C. Transmission electron microscopy shows that 2-line ferrihydrite has no Z-axis regularity but does show hexagonal 2.54-Å lattice fringes. Six-line ferrihydrite forms faceted crystals having 9.4-Å c-parameter only detectable in dark field. In bright field, 2.54-Å lattice fringes indicate greater atomic regularity than in 2-line ferrihydrite. Analysis of the X-ray powder diffraction pattern of 6-line ferrihydrite suggests a structure based on double-hexagonal close-packed oxygens, containing 36% Fe in tetrahedral sites. Selective chemical dissolution, surface area measurements, and magnetic susceptibility are consistent with the recorded properties of ferrihydrite.

Interpretation of soil features produced by ancient and modern processes in degraded landscapes: V. Development of saline sulfidic features in non-tidal seepage areas

Abstract Saline sulfidic soils are usually associated with tidal flushing zones. However, less developed saline sulfidic soils have recently been identified in non-tidal seepage and marsh areas strongly affected by waterlogging, dryland salinity and erosion in the Mediterranean climatic region ( > 600 mm per annum winter rainfall maximum) of the Mt Lofty Ranges, South Australia. A conceptual sub-model is developed to explain the formation of these saline sulfidic soils by using data from detailed pedological, mineralogical, hydrological and physico-chemical investigations. The following three saline sulfidic features were recognised both within the subsurface soil layers and on the soil surface: (i) black sulfidic materials, (ii) iron-rich gelatinous precipitates and (iii) salt-iron crusts. The abundance of these features and their distribution in relation to each other were used to establish a chronological order of formation. For each saline sulfidic feature, specific minerals (e.g. pyrite framboids, ferrihydrite and schwertmannite) were identified and their conditions of formation (e.g. Eh and pH) established. Their development is closely associated with two water-flow systems: a rising saline sulfatic groundwater aquifer and the seasonal discharge of fresh water via a perched water table. The conceptual sub-model shows how these two water-flow systems and three biomineralization processes lead to the formation of these saline sulfidic marsh soils. It illustrates how the following three main conditions control the formation of these non-tidal saline sulfidic marsh soils: (i) the development of aquic and saline conditions throughout the solum, (ii) the accumulation of organic matter from which Fe and S reducing/oxidising bacteria derive their energy, and (iii) a continuous supply of Fe and S in groundwater aquifers. In the Mt Lofty Ranges, such conditions are due to rising saline sulfatic groundwater tables following land clearing since European settlement and contemporary weathering of pyrite lenses in the underlying rock.

THERMAL AND MINERAL PROPERTIES OF Al-, Cr-, Mn-, Ni- AND Ti-SUBSTITUTED GOETHITE

Mineralogical and thermal characteristics of synthetic Al-, Cr-, Mn-, Ni- and Ti-bearing goethites, synthesized via alkaline hydrolysis of metal-ferrihydrite gels, were investigated by powder X-ray diffraction and differential thermal analysis. Shifts in unit-cell dimensions were consistent with size of substituent metal ions and confirmed the incorporation of Al3+, Cr3+, Mn3+, Ni2+ and Ti4+ in the goethite structure. A weight loss of 6.2 wt.% for goethite containing 12.2 mol.% Ti, being significantly less than for stoichiometric goethite, is consistent with the replacement of Fe by Ti in the goethite structure coupled with the substitution of O2− ions for OH− (i.e. proton loss). These data provide the first confirmation of the direct replacement of Fe by Ti within goethite. Formation of multiple dehydroxylation endotherms for goethite containing 4.5 mol.% Al, 15.3 mol.% Mn and 12.2 mol.% Ti was not attributed to the decomposition of surface OH groups or related simply to the crystallinity of precursor goethite (‘high-a’ vs. ‘low-a’) as defined by the magnitude of a. Instead, endotherm doublet formation was associated with weight loss due to the dehydroxylation of goethite remaining after initial phase transformation to protohematite and to the evolution of OH− associated with the rapid increase in crystallite size of protohematite directed primarily along the a direction. Development of the first endotherm is due to initial dehydroxylation and transformation to protohematite. With continued heating of well ordered goethite or goethite containing moderate to high levels of substituent cations, domain growth along the a direction is delayed or inhibited to a critical point that provides enough thermal energy to enable goethite transformation to proceed to completion and for proto-hematite domain growth to occur. This results in the formation of a second endotherm. For less well ordered goethite and/or goethite containing only low levels of foreign metal cations, protohematite domain growth is not inhibited and proceeds continuously with heating to give only a single endotherm.

Climate-driven mobilisation of acid and metals from acid sulfate soils

The recent drought in south-eastern Australia has exposed to air, large areas of acid sulfate soils within the River Murray system. Oxidation of these soils has the potential to release acidity, nutrients and metals. The present study investigated the mobilisation of these substances following the rewetting of dried soils with River Murray water. Trace metal concentrations were at background levels in most soils. During 24-h mobilisation tests, the water pH was effectively buffered to the pH of the soil. The release of nutrients was low. Metal release was rapid and the dissolved concentrations of many metals exceeded the Australian water quality guidelines (WQGs) in most tests. The concentrations of dissolved Al, Cu and Zn were often greater than 100× the WQGs and strong relationships existed between dissolved metal release and soil pH. Attenuation of dissolved metal concentrations through co-precipitation and adsorption to Al and Fe precipitates was an important process during mixing of acidic, metal-rich waters with River Murray water. The study demonstrated that the rewetting of dried acid sulfate soils may release significant quantities of metals and a high level of land and water management is required to counter the effects of such climate change events.

Demands on Soil Classification and Soil Survey Strategies: Special-Purpose Soil Classification Systems for Local Practical Use

Classifying soils for a particular purpose involves the ordering of soils into groups with similar properties and for potential end uses. The classification of soil is a terrific conceptual and practical challenge, especially in arid environments. The challenge may spur on, or it may deter scientists or end users with an interest in soils. If a classification system proves to be relevant and user-friendly, it stimulates and encourages further work because it is recognised for its inherent capacity to create order and enhance the useful understanding and mapping of soils. General-purpose, internationally recognised soil classification systems such as Soil Taxonomy and the World Reference Base and other nationally recognised classification systems (e.g. Australian or South African) have proved to be tremendously useful for soil classification and advancing understanding of soils across the world. However, because the use of these general-purpose classifications requires considerable expertise and experience, there is a need for complementary special-purpose classification systems that are specifically tailored, for example, to particular environmental problems, land uses or local regions and that use plain language descriptions for soil types. General-purpose classification systems often lag in the incorporation of new terminologies, for example, classification of acid sulfate soils in the Murray-Darling Basin, Australia, has led to descriptions of soil types with subaqueous properties (submerged underwater), monosulfidic materials and hypersulfidic materials, to enable assessment of environmental risk and management options. In addition, new challenges face general-purpose soil classification systems, especially in response to the following questions most frequently asked by soil users: (1) what soil properties are changing vertically and laterally in landscapes and with time, especially in acid sulfate soils? and (2) what are the most suitable approaches for characterising, monitoring, predicting and managing soil changes for environmental impact assessments, pollution incidents, waste management, product development and technology support? The purpose of this chapter is to address these challenges by presenting new ideas and concepts on how best to predict and solve practical problems by focussing on the development of special-purpose or more technical soil classification systems, which use plain language names for soil types. To demonstrate the critical importance of developing special-purpose technical soil classifications, the following five case studies are presented, which tackle difficult problems involving highly complex issues: (1 and 2) soil and water degradation in large aquatic environments from the River Murray and Lower Lakes region in South Australia (changing climatic and anthropogenic modified environments) and from the Mesopotamian marshlands in Iraq (anthropogenic modified arid environment); (3) acid sulfate soil as a new geochemical sampling medium for mineral exploration; (4) soil damage to the Australian telecommunication optic fibre cable network from shrink-swell soils and soil corrosion; and (5) soil landscape features to assist police in locating buried objects in complex terrain.

Sulfidic materials in dryland river wetlands

Due to a combination of river regulation, dryland salinity and irrigation return, lower River Murray floodplains (Australia) and associated wetlands are undergoing salinisation. It was hypothesised that salinisation would provide suitable conditions for the accumulation of sulfidic materials (soils and sediments enriched in sulfides, such as pyrite) in these wetlands. A survey of nine floodplain wetlands representing a salinity gradient from fresh to hypersaline determined that surface sediment sulfide concentrations varied from <0.05% to ~1%. Saline and permanently flooded wetlands tended to have greater sulfide concentrations than freshwater ones or those with more regular wetting–drying regimes. The acidification risk associated with the sulfidic materials was evaluated using field peroxide oxidations tests and laboratory measurements of net acid generation potential. Although sulfide concentration was elevated in many wetlands, the acidification risk was low because of elevated carbonate concentration (up to 30% as CaCO3) in the sediments. One exception was Bottle Bend Lagoon (New South Wales), which had acidified during a draw-down event in 2002 and was found to have both actual and potential acid sulfate soils at the time of the survey (2003). Potential acid sulfate soils also occurred locally in the hypersaline Loveday Disposal Basin. The other environmental risks associated with sulfidic materials could not be reliably evaluated because no guideline exists to assess them. These include the deoxygenation risk following sediment resuspension and the generation of foul odours during drying events. The remediation of wetland salinity in the Murray–Darling Basin will require that the risks associated with disturbing sulfidic materials during management actions be evaluated.

Acidification of floodplains due to river level decline during drought

A severe drought from 2007 to 2010 resulted in the lowest river levels (1.75 m decline from average) in over 90 years of records at the end of the Murray-Darling Basin in South Australia. Due to the low river level and inability to apply irrigation, the groundwater depth on the adjacent agricultural flood plain also declined substantially (1-1.5 m) and the alluvial clay subsoils dried and cracked. Sulfidic material (pH>4, predominantly in the form of pyrite, FeS2) in these subsoils oxidised to form sulfuric material (pH<4) over an estimated 3300 ha on 13 floodplains. Much of the acidity in the deeply cracked contaminated soil layers was in available form (in pore water and on cation exchange sites), with some layers having retained acidity (iron oxyhydroxysulfate mineral jarosite). Post drought, the rapid raising of surface and ground water levels mobilised acidity in acid sulfate soil profiles to the floodplain drainage channels and this was transported back to the river via pumping. The drainage water exhibited low pH (2-5) with high soluble metal (Al, Co, Mn, Fe, Mn, Ni, and Zn) concentrations, in exceedance of guidelines for ecosystem protection. Irrigation increased the short-term transport of acidity, however loads were generally greater in the non-irrigation (winter) season when rainfall is highest (0.0026 tonnes acidity/ha/day) than in the irrigation (spring-summer) season (0.0013 tonnes acidity/ha/day). Measured reductions in groundwater acidity and increases in pH have been observed over time but severe acidification persisted in floodplain sediments and waters for over two years post-drought. Results from 2-dimensional modelling of the river-floodplain hydrological processes were consistent with field measurements during the drying phase and illustrated how the declining river levels led to floodplain acidification. A modelled management scenario demonstrated how river level stabilisation and limited irrigation could have prevented, or greatly lessened the severity of the acidification.

PROPERTIES AND ACID DISSOLUTION OF METAL-SUBSTITUTED HEMATITES

The dissolution in 1 M HC1 of Al-, Mn-, and Ni-substituted hematites and the influence of metal substitution on dissolution rate and kinetics of dissolution were investigated. The inhomogeneous dissolution of most of the hematites investigated was well described by the Avrami-Erofe’ev rate equation, kt = √[-ln(l − α)], where k is the dissolution rate in time, t, and α is the Fe dissolved. Dissolution of Al-substituted hematite occurred mostly by edge attack and hole formation normal to (001), with the rate of dissolution, k, directly related to surface area (SA). Dissolution of rhombohedral Mn- and Ni-bearing hematites occurred at domain boundaries, crystal edges, and corners with k unrelated to SA. The morphology of Mn- and Ni-substituted hematites changed during dissolution with clover-leaf-like forms developing as dissolution proceeded, whereas the original plate-like morphology of Al-bearing hematite was generally retained. Acid attack of platy and rhomboidal hematite is influenced by the direct (e.g., metaloxygen bond energy, hematite crystallinity) and indirect (e.g., crystal size and shape) affects associated with incorporation of foreign ions within hematite.

Petrology and mineralogy of ‘laterites’ in southern and eastern Australia and southern Africa

Abstract Field observations of ‘laterites’ in southern and eastern Australia and in southern Africa reveal a variety of ferruginous horizons and crusts referred to herein as ferricretes. Their geomorphic and stratigraphic relationships with bedrock, sediments and soils indicate formation throughout long intervals of geological time in landscapes which are also characterised by zones of bleached and iron-mottled materials. There does not appear to be a genetic relationship between the ferricretes and the weathered zones in the sense of the so-called ‘laterite profile’. Many of the ferricretes form part of existing soil profiles. Petrographic studies of a variety of ferricretes have identified three broad categories: (a) ferruginised bedrock; (b) Fe-impregnated and -indurated sediments, including sands, clays and organic sediments; and (c) ferricretes of complex sedimentary and pedogenic origin. Type-(a) and -(b) ferricretes characteristically have simple fabrics, often with single-generation, secondary Fe-oxides. Type-(c) ferricretes have complex fabrics, with many generations of hematite, goethite and in some variants, gibbsite, in the matrix and in ferruginous clasts and pisoliths. Maghemite is a common constituent of the pisoliths. The characteristics of ironstone gravelly duplex soils , which are common in the contemporary landscapes, provide the framework for a model involving multiple stages in the development of these ferricretes. The origins of the various secondary oxide minerals in ferricretes are assessed on the basis of knowledge about the formation of these minerals in pedogenic environments. Examples are given of the intricate patterns of distribution of the minerals in thin section from which definitive data may be obtained on environmental conditions for integration with field-based geomorphic studies.

Micromorphological evidence for mineral weathering pathways in a coastal acid sulfate soil sequence with Mediterranean-type climate, South Australia

Soil micromorphology, using light microscopy and scanning electron microscopy (SEM), was used to describe detailed soil morphological and compositional changes and determine mineral weathering pathways in acid sulfate soils (ASS) from the following 2 contrasting coastal environments in Barker Inlet, South Australia: (i) a tidal mangrove forest with sulfidic material at St Kilda, and (ii) a former supratidal samphire area at Gillman that was drained in 1954 causing sulfuric material to form from sulfidic material. Pyrite framboids and cubes were identified in sulfidic material from both sites and are associated with sapric and hemic materials. Gypsum crystals, interpreted as a product of sulfide oxidation, were observed to have formed in lenticular voids within organic matter in the tidal mangrove soils at St Kilda. Sulfide oxidation was extensive in the drained soil at Gillman, evidenced by the formation of iron oxyhydroxide pseudomorphs (goethite crystallites and framboids) after pyrite and jarosite, and of gypsum crystals. Gypsum crystals occur where a local source of calcium such as shells or calcareous sand is present. Sporadic oxidation episodes are indicated by the formation of iron oxide and jarosite coatings around coarse biogenic voids. These observations indicate that mineral transformation pathways are strongly influenced by soil physico-chemical characteristics (i.e. oxidation rate, Eh, pH, soil solution chemistry, mineralogy, and spatial distribution of sulfides). This information has been used to illustrate the interrelationships of pyrite, carbonate, gypsum, jarosite, and organic matter and help predict soil evolution under changing hydro-geochemical, redoximorphic, and thermal conditions in soils from coastal environments.