Formation Of Gibbsite Research Articles

SYNTHESIS AND CHARACTERIZATION OF ZEOLITES FROM COAL FLY ASH WASTE

South Sumatra had coal production through PT Bukit Asam Tbk, which had coal production since 1950. Tanjung Enim Steam Power Plant (PLTU) is the largest coal ash producer because coal is the primary fuel. Coal combustion by-products include coal fly ash (CFA) and coal bottom ash (CBA). The current utilization of the CFA Tanjung Enim Steam Power Plant is for a cement mixture of PT Semen Baturaja and planting media. This work attempts to optimize zeolite synthesis from CFA by examining the effects of hydrothermal duration on reducing coal waste. This research studies the effect of hydrothermal time with time variations of 5, 12, and 24 on the morphology and phase of zeolite obtained. CFA from the Tanjung Enim Steam Power Plant contains SiO₂ and Al₂O₃, which account for 47.7% and 28.7% of the total composition, respectively. The SEM characterization result shows that the synthesized zeolite forms aggregates with a particle size of about 8-15 μm. Based on XRD characterization of CFA hydrothermal time of 5 hours, the dominant phase is the gibbsite phase, but there is a sodalite phase. The 12-hour hydrothermal time showed the formation of quartz, gibbsite, and sodalite phases. The 24-hour hydrothermal time shows that the dominant phase is sodalite, but there are gibbsite and quartz phases. The peak of the quartz phase decreases the longer the hydrothermal time. In this study, the duration of the hydrothermal process affects the formation of the zeolite phase.

Preparation of Near-Spherical α-Al2O3 Particles by Enhanced Precipitation of Gibbsite from Sodium Aluminate Solution Using Highly Dispersed Al2O3 as Seeds.

The near-spherical α-Al2O3 particles (∼200 nm) were prepared for the first time through the precipitation of gibbsite from a supersaturated sodium aluminate solution, with the addition of an acidic solution containing highly dispersed nanosized Al2O3 seeds. The near-spherical α-Al2O3 were achieved by employing seeds at 1100 °C, with the initial nucleation temperature being only 900 °C, due to the highly homogeneous dispersion of the seeds into gibbsite formation by in situ growth that can effectively enhance the role of the seeds in the phase transition. Moreover, the precipitation rate of sodium aluminate solution was significantly accelerated by 17-fold. This acceleration was due to the slightly acidic solution breaking the stability of sodium aluminate solution through a neutralization reaction on the seed surface, leading to the rapid formation of gibbsite nuclei on the seed surface. Our findings provide a feasible approach for the fabrication of well-dispersed α-Al2O3 particles and make the industrial production of well-dispersed α-Al2O3 from sodium aluminate solution a commercial reality.

Relict soil features in cave sediments record periods of wet climate and dense vegetation over the last 100 kyr in a present-day semiarid region of northeast Brazil

The present-day semiarid region of NE Brazil experienced multiple periods of increased precipitation during the Quaternary. They were likely linked to global-scale events (e.g., Heinrich Events) and drove changes in vegetation dynamics, leading to the expansion of rainforest vegetation. Here, we hypothesize that soil features preserved in cave sediments records paleoenvironmental changes induced by global-scale events that affected NE Brazil during the Late Pleistocene - Early Holocene. To test this hypothesis, we analyzed the elemental and isotopic geochemistry composition, bulk sediment mineralogy, soil micromorphological features, and fauna fossil type and content of a well-preserved 6 m cave sediment profile spanning 103 kyr – 8 kyr in Toca de Cima do Pilão at the Serra da Capivara National Park. Our approach provides new information on the history of soil formation, vegetation, and climate of NE Brazil during the Late Pleistocene - Early Holocene. The results demonstrated that severe weathering and dense vegetation was the dominant paleoenvironmental conditions during wet periods, when latosolization was the main soil forming process. The evidence for wet and intense weathering periods was the co-existence of speleothems layers with Ferralsol features, such as runiquartz, granular microstructure, Fe nodules and gibbsite formation. Some of these features were found in layers deposited in synchrony with Heinrich Stadial and Younger Dryas periods, suggesting that past soil processes in NE Brazil were affected by those global-scale events. We propose five phases of latosolization in our study area during the last 100 kyr. The first was identified around 80 kyr, the second occurred around 29 kyr. The third coincided with Heinrich Stadial 1 (16 kyr) and the fourth likely occur during the Younger Dryas, and the fifth phase occurred during the middle Holocene.

The Problem of the Formation of Boehmite and Gibbsite in Bauxite-Bearing Lateritic Profiles

The study of a large amount of factual material about the formation conditions of gibbsite or boehmite and their mutual transformations in lateritic bauxites allowed us to solve the problem of boehmite formation and its spatial and genetic relationship to gibbsite. The boehmite formation occurred only during the formation of sedimentary-lateritic bauxites from alluvial and lacustrine sediments that underwent bleaching and resilification at the stage of sediment flooding, as well as on bleached bedrock under sedimentary-lateritic bauxites. The most intense boehmite formation occurred at a depth of 20–30 m from the surface and was accompanied by an alumina input, which was realized here in the form of boehmite along with the gibbsite formation due to the hydrolytic decomposition of kaolinite and partially due to the input alumina. In the upper profiles, the recrystallization of bauxites occurs with the replacement of dissolving boehmite with gibbsite, with a decrease in the total Al2O3 content and an increase in the amount of crystallization water. In classic lateritic bauxites, the boehmite content is highly insignificant; that is, its formation in them practically does not occur or is very much inhibited.

Soil Chemistry and Clay Mineralogy of an Alluvial Chronosequence from the North Carolina Sandhills of the Upper Coastal Plain, USA

Temporal changes in soil development were assessed on fluvial terraces of the Little River in the upper Coastal Plain of North Carolina. We examined five profiles from each of six surfaces spanning about 100,000 years. Soil-age relationships were evaluated with inter-surface clay mineral comparisons and regression of chemical properties versus previously reported optically-stimulated luminescence ages using the most developed subsoil horizon per profile. Bases to alumina (Bases/Al2O3) ratios have negative correlations with age, whereas dithionite-Fe (FeD) concentrations are positively correlated with time and differentiate floodplain (<200 yr BP) from terrace (≥10 ± 2 ka) soils and T4 pedons (75 ± 10 ka) from younger (T1-T3b, 10 ± 2–55 ± 15 ka) and older (T5b, 94 ± 16 ka) profiles. Entisols develop into Ultisols with exponentially decreasing Bases/Al2O3 ratios, reflecting rapid weatherable mineral depletion and alumina enrichment during argillic horizon development in the first 13–21 kyr of pedogenesis. Increasing FeD represents transformation and illuviation of free Fe inherited from parent sediments. Within ~80–110 kyr, a mixed clay mineral assemblage becomes dominated by kaolinite and gibbsite. Argillic horizons form by illuviation, secondary mineral transformations, and potentially, a bioturbation-translocation mechanism, in which clays distributed within generally sandy deposits are transported to surface horizons by ants and termites and later illuviated to subsoils. T5b profiles have FeD concentrations similar to, and gibbsite abundances greater than, those of pedons on 0.6–1.6 Ma terraces along Coastal Plain rivers that also drain the Appalachian Piedmont. This is likely because the greater permeability and lower weatherable mineral contents of sandy, Coastal Plain-sourced Little River alluvium favor more rapid weathering, gibbsite formation, and Fe translocation than the finer-grained, mineralogically mixed sediments of Piedmont-draining rivers. Therefore, recognizing provenance-related textural and mineralogical distinctions is crucial for evaluating regional chronosequences.

Origin of the chemical composition of São Francisco ultra-fresh thermal water (Itabira region, Minas Gerais, Brazil)

Geochemical studies documented the composition of ultra-fresh thermal water in the Fazenda Giráu, near Itabira (Minas Gerais state, Brazil) for the first time. The studied water is characterized by temperature of 35 °C, very low electric conductivity (approx. 20 μS/cm), uncommon Mg–Ca–HCO3–NO3 hydrochemical type, and an elevated radon concentration (about 500 Bq/L). The presence of tritium suggests the modern age of studied water, contained in highly resistant quartzite-dominated aquifer rocks. Application of a geothermometric approach gave an estimation of the equilibrium temperature in the geothermal system at about 42 °C. Inverse geochemical modelling quantified water-rock reactions responsible for the observed groundwater chemistry. Decomposition of soil organic matter and decay of primary (feldspars, biotites, muscovite) and secondary (montmorillonites, illite) aluminosilicate minerals accompanied by kaolinite and gibbsite formation are the dominant processes forming chemical composition of water. The investigated water has balneotherapeutic potential due its elevated temperature and high concentrations of radon and silicic acid.

Dolerite-related gibbsite genesis in Australian Banded iron formations

The Pilbara region of Western Australia represents the world’s largest supply of quality iron ores predominantly sourced from supergene and hypogene enriched Banded Iron Formations. Aluminium is a major contaminant in these iron ores and its mineralogical host impacts the efficiency and productivity of the steel-making process. The aluminium can either be hosted by gangue minerals such as kaolinite (Al2Si2O5(OH)4) or be adsorbed or substituted into the crystal lattices of iron oxides. Gibbsite (Al(OH)3) is widely considered a low abundance gangue mineral in this region, posing a minimal risk to iron ore quality. As a result, investigations into the genesis of gibbsite are scarce and its spatial distribution in Pilbara iron ores is poorly understood.This latest research is prompted by discrepancies between predicted and actual aluminum distribution in world class Pilbara iron ore deposits, and reveals that the current means of prediction does not adequately account for the highly aluminous mineral gibbsite. A micromorphological and geochemical study of iron ore samples from mining benches at various depths found that significant gibbsite forms in some iron ore deposits due to localized alumina remobilization from dolerites. The study documents for the first time the micron-scale transformation of gibbsite to kaolinite in iron ores. Structure, hydrogeology and geomorphology were found to control the formation and transformation of gibbsite and explain why gibbsite is sometimes abundant in iron ore mineralization proximal to dolerite dykes and at other times completely absent.Improved fundamental understanding allows gibbsite distribution to be derived from normative mineralogy calculations, and suggests gibbsite is locally more abundant in Australian BIF-hosted iron ores than previously thought. This finding has important metallurgical implications given that ores are selected for beneficiation based on predicted aluminium deportment and its textural associations with the iron oxides.

Natural carbonation-induced phase and molecular evolution of alkali-activated slag: Effect of activator composition and curing temperature

In this work, the evolution of reacted phase assemblage and molecular structure of alkali-activated slag (AAS) exposed to atmospheric natural carbonation is studied. The hardened AAS pastes with various activator types (i.e., sodium hydroxide, sodium carbonate, sodium sulfate, potassium hydroxide, and potassium carbonate solutions) and cured under two temperature conditions (20 °C and 80 °C) are evaluated. The results show that natural carbonation leads to a strong decalcification and silicate polymerization of calcium-aluminosilicate-hydrate (C-A-S-H) and precipitation of various calcium carbonate polymorphs depending on activator composition. The sulfate-activated slag shows the poorest carbonation resistance likely due to its limited CO2 absorption capacity, manifested with the formation of calcium carbonate polymorphs, gypsum, gibbsite, and calcium-deficit aluminosilicate gels. While high-temperature curing tends to increase the crystallinity level of C-A-S-H, it has little effect on its carbonation resistance in terms of phase and molecular stability.

Influence of mixing solution on characteristics of calcium aluminate cement modified with sodium polyphosphate

This study investigated characteristics of a calcium aluminate cement modified with a phosphate (CAP) by changing an amount and concentration of mixing solution with sodium polyphosphate. When the amount of mixing solution was increased with a constant amount of sodium polyphosphate, an enhanced consumption of monocalcium aluminate was observed compared with gehlenite in calcium aluminate cement. Formation of gibbsite, Al(OH)3, was also increased as a hydration product in the CAP and a reduction of water in the amorphous gel phase. When the amount of mixing solution was increased with a constant concentration of sodium polyphosphate, the enhanced consumption of monocalcium aluminate was not observed. Neither gibbsite nor any other crystalline hydration products were identified in this series. In addition, unreacted sodium polyphosphate remained in the system. The increased formation of gibbsite and the possible reduction of water from the amorphous gel phase appears to contribute to the improvement of the microstructure in the products.

Lateritic weathering of trachyte, and bauxite formation in West Cameroon: Morphological and geochemical evolution

Bauxites and Fe laterites were formed on Neogene volcanics from Fongo-Tongo region in the highlands of western Cameroon, and are distributed on plateaus, slopes and downslope surfaces. Bauxitic profiles result from intense in-situ weathering of trachytes that implied depletion of silica and labile elements from the saprolite, while alumina relatively accumulated from parent minerals pseudomorphosis by primary gibbsite formation. During ongoing lateritization and bauxite maturation, important leaching and illuviation processes resulted in secondary gibbsite crystallizations. Late incision and dissection of upper bauxitic plateau resulted in degradation and dismantling of bauxitic duricrusts with Fe-depletion and increasing silica. Compared to trachyte, bauxitic duricrusts are relatively enriched in Nb, Zr, Ga, Ni, Cu, Co, V, Cr, As, Pb, Th, Hf, U and Ta, while Y, Sr, Rb, Ba and Zn are depleted. Trace elements contents depend on relative proportions of gibbsite, kaolinite, iron oxides and anatase and their affinity with these minerals across the weathering sequence. The overall REE composition and C1-Chondrite normalized REE patterns highlight significant fractionations with enrichment in the upslope profile and labile behavior in other profiles of the sequence. REE behavior and Eu/Ce anomalies are controlled by intensity of weathering and lateritic processes during the evolution of profiles. Our results document two major bauxitic phases in the Fongo-Tongo area, i.e., mid-Miocene primary in situ bauxitic weathering of trachyte and late Miocene secondary bauxitization of previously formed bauxitic profiles that led to alumina enrichment up to 53.50 wt%. Combined together, the morphological distribution and geochemical composition of the studied bauxitic profiles constitute guides for bauxite exploration, and more generally document the dynamics and morphogenetic evolution of lateritic landforms in West Cameroon.

Element mobilization and redistribution under extreme tropical weathering of basalts from the Hainan Island, South China

Chemical weathering of rocks has substantial influence on the global geochemical cycle. In this paper, the geochemical profile of a well-developed basalt weathering profile (>15 m thick, including soil, saprolite, semi-weathered rock and fresh basalt) on the Island of Hainan (South China) was presented. The soil and saprolite samples from this profile are characterized by high Al2O3 and Fe2O3 concentrations (up to 32.3% and 28.5%, respectively). The mineral assemblage is dominated by kaolinite, Fe-oxides/-hydroxides and gibbsite (or boehmite), indicating extensive desilicate and ferrallitic weathering. The acidic and organic-rich environment in the soil horizon may have promoted elemental remobilization and leaching.The strongest SiO2 depletion and Al2O3 enrichment at about 2.4 m deep indicate that the main kaolinite hydrolysis and gibbsite formation occurred near the soil-saprolite interface. The mild Sr reconcentration at about 3.9 m and 7.1 m deep may be attributed to secondary carbonate precipitation. Mn-oxides/-hydroxides precipitated at 6.1 m deep, accompanied by the strongest enrichment of Ba and Co. Uranium is mildly enriched in the middle part (about 7.1 m and 9.1 m deep) of the weathering profile, and the enrichment may have been caused by the decomposition of uranyl carbonates or the accumulation of zircon. Immobile element (i.e., Zr, Hf, Nb, Ta, Th and Ti) distributions at different depths are mainly controlled by secondary Fe-oxides/-hydroxides, and follow the stability sequence of Nb ≈ Ta ≈ Th > Zr ≈ Hf > Ti. The limited thickness (∼15 cm) of the semi-weathered basalt horizon at the rock-regolith interface (15.28 m deep) suggests that plagioclase and pyroxene are readily altered to kaolinite, smectite and Fe-oxides under tropical climate. The marked enrichment of transitional metals (such as Cu, Zn, Ni, and Sc) along the rock-regolith interface may have associated mainly with increasing pH values, as well as the dissolution of primary apatite and formation of secondary phosphates. Our findings highlight the importance of secondary phosphates in the redistribution of transition metals, and in the possible Mg, Cu, and Ni isotopic fractionation under extreme weathering of basalt in tropic climate.

Phase transformation processes in karst-type bauxite deposit from Yunnan area, China

Bauxite is the only resource of aluminum metal and forms through weathering, leaching, and deposition (i.e., bauxitization processes). Here we characterize the bauxite deposit in Yunnan area, China, through mineralogical and textural characteristics using XRD, SEM-EDX, and micro-Raman analyses. The Al-oxyhydroxide polymorphs (AlO(OH)) such as diaspore and boehmite are the dominant phases in most of the samples. Gibbsite (Al(OH)3) is the main mineral in two samples from Quaternary bauxite, and these three Al-rich minerals mixed with goethite, hematite, kaolinite, and small amounts of TiO2 polymorphs (anatase and rutile). Considering the textural characteristics and the stratigraphy of the bauxite deposit, some of the Al-oxyhydroxides and Ti-oxides are inferred to be products of the bauxitization in the lateritic soil during late Permian weathering. In the subsequent transgression stage, the earlier formed (Al-rich minerals and anatase) phases were transformed to diaspore or boehmite as spherulitic form and to rutile, respectively, with the processes of diagenesis, burial metamorphism, tectonic movements and magmatic activity. Following crustal uplift in the Yunnan Province after the late Triassic, diaspore was transformed into boehmite and the bauxite deposit formed earlier was exposed again to weathering resulting in the formation of gibbsite, hematite, goethite, kaolinite, and anatase, with diaspore, boehmite, and rutile remaining as relics. Our study provides insights into phase transformation during bauxitization and subsequent processes.

The production of hydrogen as an alternative energy carrier from aluminium waste

BackgroundAluminium dross is a valuable resource that is often redirected to landfill as there are no real viable solutions for the utilisation of this industrial waste. A study has been conducted to provide a recycling process where the dross is reacted with an alkaline solution in order to generate hydrogen with bayerite and gibbsite products.MethodsSamples of aluminium dross were obtained from two sources: aluminium dross obtained from an aluminium recycling facility, recycled dross (RD) in Qatar and aluminous dross samples which were to be directed to landfill, landfill dross (LD), also in Qatar. Quantities of each sample were weighed in amounts that would contain equal amounts of aluminium reacting with an aqueous NaOH solution to generate hydrogen. The generated hydrogen has been analysed and compared with that reported in the literature.ResultsCertainly, the lower purity sample LD was treated at a lower standard than the recycled dross. The LD and RD samples generated 0.15 and 0.5 g/l Al, respectively, with the landfilled dross achieving a maximum flow rate of 0.8 l/min compared to an RD which generated hydrogen at 2 l/min.The results proved that both forms of aluminium dross possess the potential to provide an acceptable volume of hydrogen at relatively consistent flow rates. The RD sample provided higher flow rates, and the LD sample generated hydrogen at a lower but consistent flow rate for a longer period and at a volume rather close to that of the RD sample. XRD analysis of the resulting product also yielded promising results with the formation of bayerite and gibbsite, which would provide additional side products of market value; if this process will be conducted on a larger scale.ConclusionsThis study has shown very promising results, with both dross samples allowing for an acceptable production of hydrogen. It has shown that the utilisation of dross can be a potentially economically viable process for a product that provides clean, renewable energy and residual aluminous products of a real market value.

An assessment of the hydrogeochemistry of two wetlands located in Bihar State in the subtropical climatic zone of India

A total of 30 water samples and 8 sediment samples were collected and chemically analysed for major ions (Na+, K+, Mg2+, Ca2+, HCO3 −, SO4 2−, Cl−, NO3 −, silica, PO4 3−, F−), trace elements (Fe, Mn, Ni, Cd, Zn, Pb, Cu), minerals and nutrients to determine the factors that control the chemistry of water in the Kawar-Tal and Kusheshwar-Asthan wetlands in the Bihar State of India. These analyses indicate that Ca2+ and HCO3 − ions are the most dominant ions in both of the wetlands. It also indicates rock weathering is a major source of ions in these wetlands, particularly the dissolution of carbonate minerals. The hydrogeochemistry of water in Kusheshwar-Asthan is favouring kaolinite formation and in Kawar-Tal favouring kaolinite–gibbsite formation. Quartz (~36%), clay minerals (~21%) and chlorite (~10%) are the dominant minerals in both the wetlands. Orthoclase (~12.49%), calcite (~7.51%) and illite (4.89%) minerals are only available in Kawar-Tal surface sediment, while albite (6.29%) and biotite (~13.6%) minerals are only available in Kusheshwar-Asthan. Total carbon (~3%), inorganic carbon (0.9%), organic carbon (~2.1%), total sulphur (~0.0008%), nitrogen (~0.55%) and phosphate (~0.96%) are available in Kawar-Tal surface sediments, while total carbon (~2.38%), inorganic carbon (0.55%), organic carbon (~1.84%), total sulphur (~0.001%), nitrogen (~0.62%) and phosphate (~0.64%) are available in Kusheshwar-Asthan surface sediments. The study indicates wetlands are rich in nutrient for biological activities and are sufficient to support the biodiversity, but few locations are influenced by anthropogenic activities which cause the increase of sulphur, chloride, iron and lead.

The Piriá aluminous lateritic profile: mineralogy, geochemistry and parent rock

ABSTRACT: Relatively small aluminous lateritic deposits are abundant in the northeast and northwest parts of the Pará and Maranhão states, respectively. Most of them hosts aluminum phosphate mineralization forming hills and plateaus that stand out in the topography of the undulating plains of this region. The Piriá ridge is one of those topographic features, covered by lateritic iron crusts that have been studied in the 1970s as part of iron ore exploration campaigns and recently for phosphates prospection. This study improves the knowledge about the evolution of the lateritic Piriá deposit and demonstrates its relationship with the most evolved laterites of the Amazon, known as mature laterites, which formed major ore deposits during the Paleogene. Samples of a 17 meter-deep borehole were investigated through mineralogical (X-ray diffraction -XRD, optical and electron microscopy) and chemical methods (inductively coupled plasma mass spectrometry - ICP-MS, inductively coupled plasma optical emission spectrometry - ICP-OES and X-ray fluorescence - XRF). The studied lateritic profile comprises a clay bauxitic horizon overlaid by an aluminous iron crust. Upwardly continuous dissolution of kaolinite occurs with the formation of gibbsite, as the result of intensive leaching, resulting in a higher Al2O3 content in the crust. The continuous formation of hematite from goethite resulted from the transition to more arid conditions. Anatase is a newly formed mineral (100-400 nm crystallites), showing a gradual increase, following the increase in TiO2 content, which is high and indicative of a mafic parent rock, confirmed by the Ti × Zr dispersion pattern. Prominent zoning in the lateritic profile is characterized by the mineralization in bauxite and augelite and abrupt chemical transition between the horizons, marked by a decrease in Si and increase in Fe content from the bottom to the top of the profile. These features are compatible and indicative of mature laterites formed in Amazon during the Paleogene.

Do spectral gamma ray data really reflect humid–arid palaeoclimates? A test from Palaeogene Interbasaltic weathered horizons at the Giant's Causeway, N. Ireland

Since 1999, the rapid, inexpensive and non-destructive use of Th/K and Th/U ratios from spectral gamma ray measurements have been used as a proxy for changes in palaeo-hinterland weathering. This model is tested here by analysis of in situ palaeoweathering horizons where clay mineral contents are well-known. A residual palaeoweathered horizon of Palaeogene laterite (developed on basalt) has been logged at 14 locations across N. Ireland using spectral gamma ray detectors. The results are compared to published elemental and mineralogical data. While the model of K and U loss during the early stages of weathering to smectite and kaolinite is supported, the formation (during progressively more advanced weathering) of gibbsite and iron oxides has reversed the predicted pattern and caused U and Th retention in the weathering profile. The severity (duration, humidity) of weathering and palaeoweathering may be estimated using Th/K ratios as a proxy. The use of Th/U ratios is more problematic should detrital gibbsite (or similar clays) or iron oxides be detected. Mineralogical analysis is needed in order to evaluate the hosts for K, U and Th: nonetheless, the spectral gamma ray machine offers a real-time, inexpensive and effective tool for the preliminary or conjunctive assessment of degrees of weathering or palaeoweathering.

Effect of weathering transformations of coal combustion residuals on trace element mobility in view of the environmental safety and sustainability of their disposal and use. I. Hydrogeochemical processes controlling pH and phase stability

Coal combustion residuals (CCRs) are one of the most abundant high-volume waste materials disposed in impoundments worldwide. Some methods of CCR recycling, e.g. their use as structural fill for low lying areas or as soil amendment, also expose this material to atmospheric conditions. Combustion processes result in concentration of trace elements in CCRs at about an order of magnitude compared to coal. In order to assess an effect of long-term weathering transformations of CCRs on trace element binding/release, a study has been carried out. It is based on the chemical composition of real pore solutions extracted from the most abundant primary alkaline Class F bituminous CCRs, 0 to >40 years old, sampled from the surface layer and vertical profiles at four different impoundments. In this part of the study, results of a hydrogeochemical simulation of the saturation state of real pore solutions with respect to mineral phases of CCRs with use of the PHREEQC program, related to actual pH values reflecting the full cycle of weathering transformations, have been discussed. This study is the first geochemical proof of the general trend towards a progressive acidification up to pH < 4 of primary alkaline CCRs due to release of protons during internal processes of formation of gibbsite and aluminosilicate minerals, buffered by carbonates at the alkaline - near-neutral stages, and followed by parallel dissolution and buffering by aluminosilicates at pH < 7 after carbonate depletion, to the level up to pH∼3.5–4.0. The intrinsic geochemical changes have resulted in the different susceptibility of trace elements to release and associated changes in risk to the environment at consecutive stages of weathering.

Temperature Effect on the Synthesis of Gibbsite and Boehmite

Abstract To investigate the temperature dependency of gibbsite and boehmite formation, the Gibbs free energy change (ΔG) was calculated at different temperatures. Calculated Gibbs free energy shows a positive value at 323 K, and gibbsite formation is preferred. On the other hand, it shows a negative ΔG value at 371 K, and boehmite formation is preferred. These predictions were characterized by transmittance electron microscopy and X-ray diffraction.

Facies-related baryte mineralization bearing Cu–Zn sulfides in Miocene estuarine deposits of the upper Rhein Graben (Wetterau, Central Germany)

Baryte with or without base metal sulfides is quite common in sediments deposited in open marine environments or in continental sedimentary basins. Its precipitation is caused by hydrothermal processes, related to diagenesis, and frequently mediated by biogenic processes. The current study is focused on siliciclastic sandstones of Miocene (Aquitanian) age in an estuarine environment in the Wetterau region of the Rhein Graben, central Germany. In the estuarine environment only the central basin and the landward delta are host to a diagenetic and subsequent hydrothermal mineralization.Diagenesis took place under near-ambient (T≈25°C) conditions and resulted in strong pyritization (−0.75<Eh<+0.25, pH>5) in the central basin. Diagenesis is more landward represented by a pervasive silicification (pH<12) in deltaic sandstones.Epigenetic mineralization (100°–130°C) with pyrite in the central basins was succeeded by Cu–Zn–(Sb) minerals (0.75<Eh<0/5<pH<11), silicification and kaolinization (2<pH<9.5) and eventually by the formation of gibbsite (3<pH<8). At the transition from the delta to the estuarine funnel, baryte is of very widespread occurrence. Its variegated texture and crystal morphology allow for a precise determination of the hydraulic system as marine phreatic, freshwater phreatic, and freshwater vadose. The narrow size of the rift graben and its sealing against the open sea fostered concentration of Ba and enhanced the redox processes. Hypogene brines along with Miocene volcanic activity provided the metals, and marine ingressions in this transitional environment supplied the sulfur. Sulfides were concentrated in the finer-grained rocks because of their enrichment in organic material, while sulfates accumulated in the more permeable coarser sandstones.

The riverine silicon isotope composition of the Amazon Basin

We present here the first large-scale study of riverine silicon isotope signatures in the Amazon Basin. The Amazon and five of its main tributaries were studied at different seasons of the annual hydrological cycle. The δ30Si signature of the dissolved silicon (DSi) exported to the estuary (weighted for DSi flux) for the period considered is estimated at +0.92‰. A river cross-section shows the homogeneity of the Amazon River regarding DSi concentration and isotope ratio. The biogenic silica (BSi) concentration measured in surface water from all rivers is generally small compared to the DSi reservoir but large variations exist between rivers. Very low isotope signatures were measured in the upper Rio Negro (δ30Si=+0.05±0.06‰), which we explain both by an equilibrium between clay formation and dissolution and by gibbsite formation. The Si isotope fractionation in the Andean tributaries and the Amazon main stem can be explained by clay formation and follow either a Rayleigh or a batch equilibrium fractionation model. Our results also suggest that the formation of 2:1 clays induces a fractionation factor similar to that of kaolinite formation.