Goethite Grains Research Articles

Evolution of the Piauí Laterite, Brazil: Mineralogical, Geochemical and Geomicrobiological Mechanisms for Cobalt and Nickel Enrichment

The Piauí laterite (NE Brazil) was initially evaluated for Ni but also contains economic concentrations of Co. Our investigations aimed to characterise the Co enrichment within the deposit; by understanding the mineralogy we can better design mineral processing to target Co recovery. The laterite is heterogeneous on the mineralogical and lithological scale differing from the classic schematic profiles of nickel laterites, and while there is a clear transition from saprolite to more ferruginous units, the deposit also contains lateral and vertical variations that are associated with both the original intrusive complex and also the nature of fluid flow, redox cycling and fluctuating groundwater tables. The deposit is well described by the following six mineralogical and geochemical units: SAPFE, a clay bearing ferruginous saprolite; SAPSILFE, a silica dominated ferruginous saprolite; SAPMG, a green magnesium rich chlorite dominated saprolite; SAPAL, a white-green high aluminium, low magnesium saprolite; saprock, a serpentine and chlorite dominated saprolite and the serpentinite protolith. Not all of these units are ‘ore bearing’. Ni is concentrated in a range of nickeliferous phyllosilicates (0.1–25 wt%) including serpentines, talc and pimelite, goethite (up to 9 wt%), magnetite (2.8–14 wt%) and Mn oxy-hydroxides (0.35–19 wt%). Lower levels of Ni are present in ilmenites, chromites, chlorite and distinct small horizons of nickeliferous silica (up to 3 wt% Ni). With respect to Co, the only significant chemical correlation is with Mn, and Mn oxy-hydroxides contain up to 14 wt% Co. Cobalt is only present in goethite when Mn is also present, and these goethite grains contain an average of 0.19 wt% Co (up to a maximum of 0.65 wt%). The other main Co bearing minerals are magnetite (0.41–1.89 wt%), chlorite (up to 0.45 wt%) and ilmenite (up to 0.35 wt%). Chemically there are three types of Mn oxy-hydroxide, asbolane, asbolane-lithiophorite intermediates and romanechite. Spatially resolved X-ray absorption spectroscopy analysis suggests that the Co is present primarily as octahedrally bound Co3+ substituted directly into the MnO6 layers of the asbolane-lithiophorite intermediates. However significant levels of Co2+ are evident within the asbolane-lithiophorite intermediates, structurally bound along with Ni in the interlayer between successive MnO6 layers. The laterite microbial community contains prokaryotes and few fungi, with the highest abundance and diversity closest to ground level. Microorganisms capable of metal redox cycling were identified to be present, but microcosm experiments of different horizons within the deposit demonstrated that stimulated biogeochemical cycling did not contribute to Co mobilisation. Correlations between Co and Mn are likely to be a relic of parent rock weathering rather than due to biogeochemical processes; a conclusion that agrees well with the mineralogical associations.

Determination of rare earth elements in Fe-minerals using external calibration by LA-ICP-MS and application on Cauê Iron Formation (Brazil)

The rare earth elements (REE) composition in Fe-mineral phases is an important tool in iron formation studies to obtain information about parent rocks and environmental and paragenetic processes. However, the determination of REE presents some difficulties, such as the low concentration of these elements, matrix complexity and lack of iron matrix certified reference materials. The aim of the present work is to propose an analytical method to determine the REE plus Y (REE + Y) contents at trace levels in Fe-(hydr)oxides by the laser ablation ICP-quadrupoleMS technique, using external calibration. The calibration curves were obtained from analyses of reference materials with different matrices, and the analytical conditions were checked on the NIST 614 glass. The linearity (R2 ≥ 0.98), limit of detection (0.002–0.044 μg g−1), limit of quantification (0.008–0.146 μg g−1), recovery (88.4–112.4%), and intraday (0.1–14.1%) and interday (1.6–17.8%) precision were systematically assessed. The results obtained showed that the method is fit for the purpose and showed evidence of a nonsignificant interference of the matrix. Thus, the developed procedure was applied in the analyses of magnetite, martite, hematite, and goethite grains from Cauê Iron Formation (Brazil). The REE + Y patterns of the minerals are consistent with the previous study of bulk analyses on whole rocks and highlight the postdepositional signature of these elements in banded iron formations.

Mineralogical characterization of low-grade nickel laterites from the North Oman Mountains: Using mineral liberation analyses – scanning electron microscopy-based automated quantitative mineralogy

Afterwards to the Late Cretaceous obduction of the Tethyan Semail ophiolite suite intense chemical weathering involved the Oman Mountains region. This weathering produced complex mineral and chemical assemblages that led to the formation of a Ni lateritic paleosol. In the present study, five laterite sections were sampled and studied by using a quantitative mineralogical and petrographic approach. Mineral liberation analyses (MLA) were carried out for the first time for several selected samples.The MLA results, supported by the Electron probe micro-analyzer (EPMA) and Energy-dispersive X-ray spectroscopy (EDX) analyses, revealed that the main ore minerals of the analyzed laterite are chlorite and goethite as the main Ni-bearing hosts. Magnetite and chromite are also present as by-product minerals.The Ni-bearing chlorite in the Saqah-3 and Al Russayl profiles have coarser grains compared to those in the other Saqah localities, while the goethite grains are coarse in most of the analyzed samples. The presence of approximately 60 to 80% of both the Ni-bearing minerals (chlorite and goethite) as liberated phases (>95% free) is a potential indicator for their future exploration.Chromite and magnetite grains are present in a wide range of grain sizes. Seventy percent of the chromite grains in Saqah-4 and Al Russayl profiles and 70% of the magnetite in Saqah-2 occur as liberated phases (>95%) compared to their presence in other profiles, where only half of them occur as liberated phases.The grain-size-liberation class relationships showed that Ni-bearing mineral phases (chlorite and goethite) are present as liberated phases (high-grade materials) followed by chromite and magnetite, which are interlocked with other phases to lower grade materials.A better understanding of the mineralogical and geochemical quantitative characterization, such as grain size, modal analyses, mineral association, and percentage of interlock between minerals in the deposit, can avoid the metallurgical problems, thus possibly reducing the operational costs and improving the recovery of metals.The above-obtained MLA parameters can build a good feasibility study during the exploration stage of these low-grade Ni deposits in the Northern Oman Mountains, as well as later during their beneficiation and processing methods, if other economic and mining factors are deemed favorable.

The weathering of granitic rocks in a hyper-arid and hypothermal environment: A case study from the Sør-Rondane Mountains, East Antarctica

Weathering processes in hyper-arid and hypothermal environments are key to our understanding of geomorphological processes and landscape evolution in Antarctica and likely on Mars. A number of studies have focused on the weathering of basaltic rocks in Antarctica; however, the nature of the weathering of quartzofeldspathic rocks, a common rock type on Earth, has received less attention and remains unclear. Here, we explore the weathering of granitic rocks, which are a type of quartzofeldspathic rock, and the associated weathering products, in the Sør-Rondane Mountains of East Antarctica to obtain quantitative information regarding the physical and chemical weathering of granitic rocks in hyper-arid and hypothermal environments. Various petrological techniques were used to analyze the physical and chemical weathering of the rocks. Loss on ignition (LOI) values and the major element composition obtained from the crust and core of the rock samples indicate that the production of hydrous clay minerals in this area is very limited. Examination under a microscope, together with laser-Raman microspectroscopy, of thin sections from the crust and core of the samples indicate that goethite grains form mainly in veins within the crust, which is consistent with the contrasts seen in the Fe3+/Fe2+ and color strength index (CSI) values obtained from the core and crust. A negative correlation between rock hardness and degree of weathering, as measured from the crust of the samples, indicates that the weathered samples are softer than the core. This indicates that cracking and subsequent vein formation cause a reduction in the hardness of the crust and an increase in staining via the formation of goethite grains that develop in the veins. Furthermore, electron probe microanalyzer (EPMA) analysis indicates that the original Fe–Ti oxide grains are altered by weathering and change to hematite or a non-stoichiometric Fe–Ti compound associated with ilmenite grains in the case of a greater degree of weathering. These findings reveal that the weathering of granitic rocks in a hyper-arid and hypothermal environment is controlled mainly by oxidation, including iron hydroxide formation in veins formed by the thermal expansion and contraction of rock-forming minerals, frost shattering and/or salt fretting, and Fe–Ti oxide alteration in the rock interior. Importantly, these physical and chemical weathering processes are unlikely to require a liquid water supply from the environment (snow or vapor). Hence, the restricted supply of liquid water and salt is the most probable explanation for the extremely slow weathering rates in the Sør-Rondane Mountains. We therefore conclude that the weathering process observed in this area is a typical feature of quartzofeldspathic rocks under hyper-arid and hypothermal conditions, and potentially also on Mars. In situ color measurements essentially express the amount of iron hydroxide on the rock surface; thus, in situ and satellite measurements of the color of exposed rocks in Antarctica can potentially provide quantitative information regarding the degree of weathering.

Late Mio-Pliocene chemical weathering of the Yulong porphyry Cu deposit in the eastern Tibetan Plateau constrained by goethite (U–Th)/He dating: Implication for Asian summer monsoon

Chemical weathering has provided a potentially important feedback between tectonic forcing and climate evolution of the Asian continent, although precise constraints on the timing and history of weathering are only variably documented. Here, we use goethite (U–Th)/He and 4He/3He geochronology to constrain the timing and rates of chemical weathering at the Yulong porphyry Cu deposit on the eastern Tibetan Plateau. Goethite grains have (U–Th)/He ages ranging from 6.73±0.51 to 0.53±0.04 Ma that correlate with independent paleoclimatic proxies inferred from supergene Mn-oxides and loess deposits under variable tectonic regimes and vegetation zones over the southeastern Asia. This correlation indicates that regional climatic conditions, especially monsoonal precipitation, controlled chemical weathering and goethite precipitation in a vast area of southeastern Asia. The goethite ages suggest that the Asian summer monsoon was relatively strong from 7 to 4.6 Ma, but weakened between 4.6 and 4 Ma, and then significantly intensified from 4 to 2 Ma. The precipitation ages of goethites collected along a 100-m-thick weathering profile decrease with depth, and indicate a downward propagation of the weathering front at rates of <6.7, 53.5±10.8, and 4.8±0.6 m/Ma during the intervals of 7–4, 4–2, and 2–0.7 Ma, respectively. The rapid propagation of weathering front during 4–2 Ma was caused by abrupt lowering of the water table, which was possibly related to local surface uplift or reorganization of the river systems in southeastern Tibet during this period.

Goethite (U–Th)/He geochronology and precipitation mechanisms during weathering of basalts

(U–Th)/He geochronology of 33 goethite grains from in situ ferruginous duricrusts overlying the Paraná flood basalt in the Guarapuava region, Paraná, Brazil, reveals ages ranging from 3.6±0.4 to 0.4±0.1Ma. Thirty-one grains from detrital fragments of ferruginous duricrust yield ages in the 6.2±0.6 to 0.7±0.1Ma range. The results show that goethites from detrital blocks are generally older than those from the in situ ferruginous layers and that all the goethites from the Guarapuava sites are relatively young (≤6.2±0.6Ma). Goethites show variable U and Th contents depending on the mode of precipitation. Goethite that forms by progressive ferrugization and pseudomorphic replacement of basalts are rich in Th, U-poor, contain various mineral inclusions (e.g., kaolinite, rutile, ilmenite) and display a distinct “checkered” appearance. Colloform goethite precipitated within cavities and desiccation cracks contain higher U, show low Th/U values, and are free of mineral contaminants, suggesting direct precipitation from solution. The geochronological results reveal a dynamic evolution for the soils overlying the Paraná basalt, where recurrent duricrust formation and dismantling lead to the evolution of regional colluvial soil profiles while simultaneously preventing the evolution of chemically stratified lateritic weathering profiles. The modes of formation and U, Th, and He contents of goethite at Guarapuava provide useful constraints on the expected compositions and modes of formation of goethite formed on basaltic soils on Mars.

Combined 40Ar/39Ar and (U–Th)/He geochronological constraints on long-term landscape evolution of the Second Paraná Plateau and its ruiniform surface features, Paraná, Brazil

Regional correlation of dated weathered land surfaces provides the necessary constraints to test long-term continental landscape evolution models, but major challenges remain in properly dating these surfaces. The geomorphological province of Second Paraná Plateau, Paraná State, Brazil, is a high elevation (ca. 800m) land surface characterized by widely distributed deep saprolites and scattered lateritic profiles (e.g., Vila Velha and Serra das Almas). Prolonged exposure to weathering and erosion has promoted the pseudo-karstic and ruiniform features that are characteristic of this landscape. In this study, 40Ar/39Ar laser incremental heating geochronology on 22 grains of supergene Mn oxyhydroxides from lateritic profiles at Vila Velha yielded results ranging from 17.2±0.7 to 9.1±0.7Ma. (U–Th)/He geochronology on 28 goethite grains from the same profile yielded results ranging from 36.4±3.6 to 1.0±0.1Ma, with an age cluster lying within the 17.2±0.7 to 7.9±0.8Ma interval. (U–Th)/He geochronology on 17 goethite grains from the Serra das Almas lateritic profile, located 20km from Vila Velha, yield results ranging from 35.1±3.5 to 14.1±1.4Ma. The combined results for the two sites reveal a common weathering history that started ca. 35Ma, suggesting that the Second Paraná Plateau results from regional fluvial incision and denudation before ~35Ma, followed by a decline in denudation rates and proportionally more intense weathering. Consistent with the laterite profile central ages, weathering was particularly intense during the Miocene (17–8Ma). Denudation intensified after the Pliocene.

(U–Th)/He geochronology of goethite and the origin and evolution of cangas

(U–Th)/He geochronology of 147 grains of goethite cements extracted from ferruginous duricrusts (cangas) developed on banded iron-formations from the Quadrilátero Ferrífero region, Minas Gerais, Brazil, records a history of protracted mineral dissolution–reprecipitation that started at ca. 48.1±4.8Ma and continues intermittently until the Present. A large majority of the samples (more than 30%) are younger than 2Ma, revealing active mineral dissolution–reprecipitation in the recent past. Within cangas, goethite cements are younger near the surface and become progressively older towards the bottom of the weathering profile, indicating that iron is more effectively cycled in the parts of the weathering profile more strongly affected by biogenic activity. (U–Th)/He geochronology of 14 goethite grains from saprolites in the same profiles yield results ranging from 55.3±5.5 to 25.7±2.6Ma. For a single weathering profile, goethite cements from cangas are invariably younger than goethite grains from the underlying saprolite, indicating that the duricrust and the saprolite behave as independent and separate systems responding to different environmental controls. Thorium shows conservative behaviour during goethite dissolution–reprecipitation, and it is enriched towards the surface of the weathering profile. Uranium, on the other hand, is preferentially leached from the surface into the saprolite or out of the weathering profile. Recurrent goethite dissolution–reprecipitation lends great textural complexities to cangas, but it is also responsible for its capacity to reheal when physically disrupted. This self-healing property accounts for canga’s role in armoring banded iron-formation landscapes.

The origin of magnetic remanence in stalagmites: Observations from electron microscopy and rock magnetism

Speleothems, especially stalagmites, hold great potential as recorders of the Earth's ancient magnetic field. However, our understanding of the magnetic mineral assemblages within speleothems is not well developed. We present tandem rock magnetic characterization of bulk samples and electron microscopy of magnetic extracts from five North American stalagmites. Magnetite and goethite are present in all samples, albeit in varying abundances and morphologies. Magnetite grains are likely detrital, showing evidence of transport and weathering, including plumose markings and etch pits. These grains are unlikely to have precipitated abiotically in the cave environment. Titanomagnetite and exsolved intergrowths of Fe‐Ti oxides indicate that a significant portion of the magnetic mineral assemblage is allochthonous and was transported into the caves. These Ti‐rich grains display a range of morphologies from euhedral to well rounded, indicating that earlier morphological models for speleothem magnetization do not apply in all geologic settings. Goethite was observed in three morphologies: isolated needles, needle aggregates, and polycrystalline aggregates of randomly oriented nanoscale grains. The magnetite and titanomagnetite, as well as their partially oxidized equivalents, likely hold a depositional remanent magnetization (DRM), whereas if goethite grains were precipitated in situ on the stalagmite drip surface, they are more likely to hold a chemical remanent magnetization (CRM) because of their small size and low saturation magnetization. Until the remanence of goethite can be shown to be paleomagnetically meaningful, we propose that paleomagnetic studies of speleothems include a 150°C thermal demagnetization step to remove any goethite remanence.

Sorption and Redox Reactions of As(III) and As(V) within Secondary Mineral Coatings on Aquifer Sediment Grains

Important reactive phenomena that affect the transport and fate of many elements occur at the mineral-water interface (MWI), including sorption and redox reactions. Fundamental knowledge of these phenomena are often based on observations of ideal mineral-water systems, for example, studies of molecular scale reactions on single crystal faces or the surfaces of pure mineral powders. Much less is understood about MWI in natural environments, which typically have nanometer to micrometer scale secondary mineral coatings on the surfaces of primary mineral grains. We examined sediment grain coatings from a well-characterized field site to determine the causes of rate limitations for arsenic (As) sorption and redox processes within the coatings. Sediments were obtained from the USGS field research site on Cape Cod, MA, and exposed to synthetic contaminated groundwater solutions. Uptake of As(III) and As(V) into the coatings was studied with a combination of electron microscopy and synchrotron techniques to assess concentration gradients and reactive processes, including electron transfer reactions. Transmission electron microscopy (TEM) and X-ray microprobe (XMP) analyses indicated that As was primarily associated with micrometer- to submicrometer aggregates of Mn-bearing nanoparticulate goethite. As(III) oxidation by this phase was observed but limited by the extent of exposed surface area of the goethite grains to the exterior of the mineral coatings. Secondary mineral coatings are potentially both sinks and sources of contaminants depending on the history of a contaminated site, and may need to be included explicitly in reactive transport models.

Magnetic interactions between nanoparticles

We present a short overview of the influence of inter-particle interactions on the properties of magnetic nanoparticles. Strong magnetic dipole interactions between ferromagnetic or ferrimagnetic particles, that would be superparamagnetic if isolated, can result in a collective state of nanoparticles. This collective state has many similarities to spin-glasses. In samples of aggregated magnetic nanoparticles, exchange interactions are often important and this can also lead to a strong suppression of superparamagnetic relaxation. The temperature dependence of the order parameter in samples of strongly interacting hematite nanoparticles or goethite grains is well described by a simple mean field model. Exchange interactions between nanoparticles with different orientations of the easy axes can also result in a rotation of the sub-lattice magnetization directions.

On the goethite to hematite phase transformation

Abstract This study deals with some microstructural and crystallographic aspects of the thermally induced transformation of goethite (α-FeOOH) into hematite (α-Fe2O3), occurring at about 300 °C. Powder specimens of goethite have been annealed in air at different temperatures, ranging from 200 °C up to 1,000 °C. The resulting products have been analyzed for a complete characterization of the changes brought about by the thermal treatments, using a multianalytical approach, based on: thermogravimetry, differential thermal analysis, transmission electron microscopy, Raman spectroscopy, and X-ray diffraction. At lower temperatures, the transition to hematite produces no important changes in size and shape of the original goethite grains. Recrystallization, and partial sintering, occurs only at temperatures in excess of 800 °C. The relevant evolution of pores present in both phases has been also considered, as it may provide important indications on the actual formation mechanism of hematite.

Application of white-beam X-ray microdiffraction for the study of mineralogical phase identification in ancient Egyptian pigments

High-brightness synchrotron X-rays together with precision achromatic focusing optics on beamline 7.3.3 at the Advanced Light Source have been applied for Laue microdiffraction analysis of mineralogical phases in Egyptian pigments. Although this task is usually performed using monochromatic X-ray diffraction, the Laue technique was both faster and more reliable for the present sample. In this approach, white-beam diffraction patterns are collected as the sample is raster scanned across the incident beam (0.8 µm × 0.8 µm). The complex Laue diffraction patterns arising from illumination of multiple grains are indexed using the white-beam crystallographic software packageXMAS, enabling a mineralogical map as a function of sample position. This methodology has been applied to determine the mineralogy of colour pigments taken from the ancient Egyptian coffin of Tjeseb, a priestess of the Apis bull dating from the Third Intermediate to Late period, 25th Dynasty to early 26th Dynasty (747 to 600 BC). For all pigments, a ground layer of calcite and quartz was identified. For the blue pigment, cuprorivaite (CuCaSi4O10) was found to be the primary colouring agent with a grain size ranging from ∼10 to 50 µm. In the green and yellow samples, malachite [Cu2(OH)2CO3] and goethite [FeO(OH)] were identified, respectively. Grain sizes from these pigments were significantly smaller. It was possible to index some malachite grains up to ∼20 µm in size, while the majority of goethite grains displayed a nanocrystalline particle size. The inability to obtain a complete mineralogical map for goethite highlights the fact that the incident probe size is considerably larger than the grain size. This limit will continue to improve as the present trend is toward focusing optics approaching the diffraction limit (∼1000× smaller beam area).

Influence of the borohydride concentration on the composition of the amorphous Fe–B alloy produced by chemical reduction of synthetic, nano-sized iron oxide particles: Part II. Goethite

A systematic study of the effect of the NaBH 4 concentration upon the composites produced by chemical reduction of nano-sized goethite (α-FeOOH) particles suspended in water is presented. Amorphous Fe–B alloys with boron contents between 2 and 8 at.% have been produced. The obtained samples were characterized by X-ray diffraction, wet chemical analyses, thermal analyses, scanning electron microscopy, transmission Mössbauer spectroscopy (TMS) and integral low-energy electron Mössbauer spectroscopy (ILEEMS). It was observed that the boron content of this amorphous composite sensitively depends on the borohydride concentration. The TMS analyses have shown that the contribution of the Fe 1− x B x phase, which is present as a superposition of a broad sextet and a doublet, accounts for about 38% of the total spectral area. ILEEMS has further revealed that the reduction process mainly affects the surface layers of the goethite grains, without causing any changes in both particle shape and sizes.

(U–Th)/He and 40Ar/ 39Ar geochronology of weathering, Hamersley Province, Australia: Implications for weathering history and landscape evolution

(U–Th)/He dating of goethite, when combined with quantification of diffusive 4He loss by the 4He/3He methodology, provides reliable corrected ages for minerals precipitated in weathering profiles. We have combined (U–Th)/He dating of supergene goethite with 40Ar/39Ar dating of supergene manganese oxides to study the weathering history and landscape evolution in the Hamersley Province, northwestern Australia. Incremental heating 40Ar/39Ar analysis of 187 grains of Mn oxides from 65 samples (44 hand specimens) collected from weathering profiles at seven field sites across the Hamersley Province yield precipitation ages ranging from 63.4 ± 0.9 to 1.5 ± 0.2 Ma. These results, combined with previous results of 40Ar/39Ar dating of Mn oxides (Vasconcelos, 1998 Vasconcelos, P.V., 1998. Unpub. report, pp. 1–278.Vasconcelos, 1998 and Cochrane, 2003), reveal a protracted and episodic history of weathering and landscape evolution, which was already ongoing in Late Cretaceous and spans the Palaeogene and Neogene. Seventy-three grains of goethite from 39 samples extracted from 21 hand specimens, collected from the same field sites where the Mn oxides originated, were dated by the (U–Th)/He method. Internally consistent (U–Th)/He ages, which range from 84.3 ± 12.2 to 3.3 ± 0.5 Ma, have been obtained for most samples when corrections are applied for 10% helium diffusive loss. The geochronological results obtained show remarkable similarity in the distribution of ages associated with supergene mineral precipitation. The widespread occurrence of iron oxides such as goethite in soils and weathering profiles and the successful application of (U–Th)/He dating of goethite offers great opportunities for extracting the wealth of palaeoclimatic and palaeoenvironmental information recorded by these profiles on the surface of terrestrial planets such as Earth and Mars.

Laser oxygen isotope analysis of weathering goethite from the lateritic profile of Yaou, French Guiana: paleoweathering and paleoclimatic implications

Oxygen isotopic compositions of weathering goethite pseudomorphs after pyrite in the lateritic profile of Yaou, French Guiana, were measured by laser fluorination. The laterite at Yaou is composed of a 25 m thick saprolite overlain by a 3 m thick latosol. Petrographic observations indicate that the pseudomorphic replacement of pyrite by goethite occurs at or near the weathering front and is complete. The goethite pseudomorphs are essentially devoid of Al and are progressively dissolved and partially replaced by a microcrystalline plasma of kaolinite and goethite in the upper horizons (latosol) of the profile. The laser technique used in this study permitted the determination of the δ 18O value of individual grains of goethite and the investigation of grain-to-grain variation within a single sample (horizon) as well as vertical variation with depth. In the latosol, the goethite δ 18O values range from 0.8 to 3.2%‰ and exhibit significant grain-to-grain and vertical variation. This most likely reflects undetected intragrain contamination with microcrystalline kaolinite. In the saprolite, the goethite δ 18O values are consistent, ranging from 1.0 to 1.8%‰ between 3 and 18 m depth, and from 2.5 to 3.3‰ between 23 m depth and the weathering front at 28 m. Oxygen isotope compositions of present groundwater, “soil water” and rainfall at Yaou were also measured, and compared with calculated isotopic compositions for goethite-forming waters. Goethite pseudomorphs in the lower part of the saprolite are in isotopic equilibrium with present groundwater, indicating that they may be forming today or that they formed in the recent past under climatic conditions similar to present conditions. In contrast, goethite pseudomorphs found higher in the profile are not in isotopic equilibrium with present water but with a water depleted in 18O by ≈ 1.50% relative to present groundwater. It is suggested that these low-δ 18O pseudomorphs are probably older and formed under different paleoclimatic conditions, at a time when climatic regimes were possibly more monsoonal than today. This is in agreement with models of climatic evolution derived from pedological considerations. The results of this study suggest that goethite pseudomorphs in ancient saprolites may preserve their original O isotopic composition acquired at the weathering front, and may, therefore, be used as indicators of changes in weathering-climatic conditions during the evolution of a profile.

Magnetic properties of chemical remanent magnetization in synthetic and natural goethite: Prospects for a natural remanent magnetization/thermoremenant magnetization ratio paleomagnetic stability test?

Magnetic properties of chemical remanent magnetization in synthetic and natural goethite: Prospects for a natural remanent magnetization/thermoremenant magnetization ratio paleomagnetic stability test?

Magnetic properties of natural goethite-III. Magnetic behaviour and properties of minerals originating from goethite dehydration during thermal demagnetization

The magnetic behaviour during stepwise demagnetization of a set of artificial samples, containing well-defined natural goethite from five localities (chemical composition, crystallite size and grain size are known) is reported. Differential Scanning Calorimetry measurements indicated that the goethites converted to haematite between 260° and 360°C. In this temperature range a small decrease in initial susceptibility and an additional remanence decay leading toward an extra bending point in the thermal decay curve are observed. Because titanomagnetite or pyrrhotite are absent in the samples, this extra bending point is tentatively interpreted as being due to recrystallization of the minor haematite already present in the original goethite concentrates, facilitated by water made available through the goethite/haematite conversion. At much higher temperatures, at some 600°C, a self-reversal of the remanence is observed in most specimens. The haematite derived from the goethites between 260° and 360°C appeared to be prone to further alteration at successive higher demagnetization temperatures. Chemical alterations in the magnetic mineral suite, starting at temperatures from 380°C upward—depending on the grain size of the original goethite—were inferred from an increase in initial susceptibility. The susceptibility behaviour showed an appreciable variation, due to the creation of varying trace amounts of magnetite, which appeared to be already present after the 400°C step. After the final 685°C demagnetization step the magnetic mineralogy was usually dominated by magnetite. The observed variation is thought to be the result of local within-specimen differences in availability of the amount and possibly also the composition of the vapour phase. The vapour phase is due to chemical reactions between the matrix constituents: waterglass and calcite, leading to a CO2/H2O vapour phase. Reducing capacity, necessary for the magnetite formation, is envisaged to be created by decomposition of trace amounts of organic matter at relatively low temperatures, up to some 400°C. At higher temperatures (over 550°C) decomposition of traces of ferrous iron bearing clayminerals presumably donates the ferrous iron. Differences in the magnetite/haematite ratio as well as in the properties of both minerals were assessed in some detail with more or less routine rockmagnetic methods: acquisition of the isothermal remanent magnetization at room temperature (in fields up to 11.2MA m-1 or 14 T), AF demagnetization and low-temperature cycling of the saturation remanence. This was done after the 400°C step to study the magnetic properties of the newly formed haematite and after the 685°C step to evaluate the properties of the magnetite and more evolved haematite. After the 400°C step the haematite appeared to be very fine-crystalline and magnetically extremely hard. Its saturation remanence is considerably lower than that of well-crystalline haematite. After the 685°C step its hardness has decreased and its saturation remanence slightly increased. Chemical differences between the goethites do not show up as distinct haematite properties, only Mn-bearing goethite yields a softer haematite type. The magnetite formed in all goethite samples seems to have more or less identical properties depending on its thermal growth history. It has a maximum blocking temperature of some 560°C, which does not point to a high vacancy content. High values for the magnetite/haematite saturation remanence ratios indicate magnetite grain sizes from very close to the superparamagnetic threshold size to approximately 1µm, i.e. the goethite grains have been converted to intimately intergrown haematite/magnetite aggregates.

The application of analytical transmission electron microscopy to the study of oolitic ironstones: a preliminary study

Summary Analytical transmission electron microscopy (ATEM) has been used by the author to obtain textural, crystallographic and quantitative chemical information from examples of berthierine-rich and chamosite-rich oolitic ironstones. Individual phases were often found to be too small to have been readily analysed by other techniques. Berthierine laths (of the order of 20 × 500 to 1000 nm) and goethite grains (of the order of 25 nm in diameter) were found to exhibit unorientated microtextures, which were independent of both depositional and oolitic fabrics. Chemical analyses of berthierines revealed fairly uniform compositions regardless of textural association. Chemical data from the chamosite-rich samples appeared analogous to that of the berthierine-rich material, although average crystal sizes were found to be much larger (chamosite laths varied from 0.1 × 1.5 µm to 0.5 × 2 mµ). The compositional data suggest that the chamosite-rich oolitic ironstones are simply the late stage diagenetic (or low grade metamorphic) recrystallized equivalents of berthierine-rich precursors. No direct evidence indicative of the mode of origin for berthierine and goethite ooliths was found. It is suggested that ancient ironstone ooliths were probably formed via similar mechanisms to those which generate modern and ancient carbonate ooliths, although obviously the original sediment and interstitial fluids would have been very different.