Ion exchange properties of manganese and iron minerals in oceanic micronodules

The results of experimental studies of ion exchange properties of manganese and iron minerals in micronodules (MN) from diverse bioproductive zones of the World Ocean are considered. It was found that the sorption behavior of these minerals is similar to that of ore minerals from ferromanganese nodules (FMN) and low-temperature hydrothermal crusts. The exchange complex of minerals in the MN includes the major (Na⁺, K⁺, Ca²⁺, Mg²⁺, and Mn²⁺) and the subordinate (Ni²⁺, Cu²⁺, Co²⁺, Pb²⁺, and others) cations. Reactivity of theses cations increases from Pb²⁺ and Co²⁺ to Na⁺ and Ca²⁺. Exchange capacity of MN minerals increases from the alkali to heavy metal cations. Capacity of iron and manganese minerals in the oceanic MN increases in the following series: goethite < goethite + birnessite < todorokite + asbolane-buserite + birnessite < asbolane-buserite + birnessite < birnessite + asbolane-buserite < birnessite + vernadite Fe-vernadite + Mn-feroxyhyte. The data obtained supplement the available information on the ion exchange properties of oceanic ferromanganese sediments and refine the role of sorption processes in the redistribution of metal cations at the bottom (ooze) water-sediment interface during the MN formation and growth.     

Comparison of numerical simulation of solute transport with observed experimental data in a silt loam subsoil

 Solute transport experiments were conducted on loamy soils of north-eastern Iowa, USA, and the results were compared with a numerical solution of a classical advection-dispersion transport model developed in this study. Flow experiments in the laboratory on undisturbed soil columns showed a flow rate of water much higher than was estimated from the soil properties and grain-size analysis data, suggesting preferential flow regime in the soil. In contrast, the relative concentration peaks of Cl^– and Br^– in the effluent were only approximately 70% of those predicted by the classical advection-dispersion equation (ADE). In addition, the experimental breakthrough curves (BTCs) showed greater tailings of these ions than the model solution. These observations suggest a loss of solute mass during transport from the dynamic flowing regions to a stagnant, immobile water phase in the soil matrix. Experiments in small disturbed soil columns showed that movement of Cl^– and Br^– is in good agreement with predictions of the classical ADE when the tracers are applied as a continuous source. However, in the case of a pulse source, the BTCs of Cl^– and Br^– matched the model only in the ascending part of the curves. Such variation indicates greater retardation of these ions than that of simulation, probably caused by the decrease in soil permeability due to cation exchange reactions in the soil involving monovalent and divalent cation pairs such as K⁺–Ca²⁺ and K⁺–Mg²⁺. In addition, retardation occurred as a result of the continuous saturation of soil columns which seemed to have caused an expansion of clay minerals, thus resulting in decreased soil permeability. In both the continuous and the pulse-source experiments, K⁺ was not detected in the effluent samples, which seemed to have been lost in exchange reactions and adsorption. Received: 20 November 1998 · Accepted: 19 July 1999     

Groundwater Evolution and Mineral Alteration Reactions in the Basaltic Rock Sequence of Mt. Wasserkuppe, Germany: A Case Study

Groundwater sampling was accomplished in the basaltic sequence of the Rh?n mountain range, Germany, in order to investigate hydrochemical groundwater evolution and to delineate mineral alteration reactions involved in natural weathering. The hydrochemical compositions of near-surface groundwaters indicate a Ca/Mg–HCO₃ type with near-neutral pH and evolve to a Na–HCO₃ type with high pH at greater depth. Column experiments were performed with basaltic and phonolitic rock samples to determine individual mineral alteration reactions. The basic reactions could be related to the alteration of olivine, Ca-pyroxene, plagioclase, pyrrhotite, and feldspathoids under formation of secondary clay minerals (smectites, illite) and goethite. The mineral alteration reactions deduced from the leaching experiments by inverse modelling were found to be consistent with the mineral reactions associated with the natural groundwaters. The reactions calculated for groundwater evolution involve the alteration of primary and secondary minerals to produce low-T mineral phase. The conversion of secondary Na-beidellite to illite occurs at a later stage of groundwater evolution, reducing the concentrations of K⁺ and Mg²⁺. Near-surface groundwaters do not indicate significant cation exchange. Initial cation exchange requires elevated pH values, with Mg²⁺ removed from solution preferred to Ca²⁺. Na-alkalisation of the groundwaters at greater depth suggests the exchange of Na⁺ for Mg²⁺ and Ca²⁺ on Na-beidellite, supported by cation exchange on coatings of iron hydroxides as alteration products. Among the mature high-pH groundwater at greater depth, the dissolution of anorthite and albite has significant effect on groundwater composition.     

Effects of rock fragments on infiltration and evaporation in hilly purple soils of Sichuan Basin,China

This paper presents the results of laboratory experiments showing the effects of rock fragments contained in three different purple soils of the Sichuan basin of southwest China. The experiments investigated how these rock fragments alter the soil’s physical, chemical, and agronomical characteristics such as infiltration and evaporation. We found that the infiltration rate, whether horizontal or vertical, in the three soils has the following order: gray brown purple soil < reddish brown purple soil < brown purple soil. With increasing rock fragment contents the accumulated infiltration decreases, while the total time decreases first and then increases. The minimum occurs at approximately 10–20% of fragment content by weight. The infiltration rate also changes with the distance. In the 0–5 cm range, the initial infiltration rate increases with increasing rock fragment contents, while in the 5–10 cm range, the slope of infiltration curve increases with increasing rock fragment contents. With increasing distance, the slope gradually decreases and finally reaches a stable value. The presence of rock fragments reduces soil water content, the minimal value appearing when the rock fragments were on top of the soil column (soil + rock sample), decreasing with increasing rock fragments for other samples mixed with fragments. Under the constant 40°C temperature, the accumulated evaporation and evaporation rate are minimal for soils covered by rock fragments, and the accumulated evaporation decreases with increasing rock fragment for other soil samples. However, the evaporation rate increases with increasing rock fragments in the first 4 days and decreases thereafter.     

The sorption of toxic elements onto natural zeolite,synthetic goethite and modified powdered block carbon

Inorganic elements analyses of Carapicuíba lake reveal that As, Cr, Pb and Mn are above the recommended drinking water standards. The mean total concentrations of toxic elements in surface water decrease in the order Mn > Cr > Pb > As. At elevated concentrations, toxic elements like Cr can accumulate in soils and enter the food chain, leading to serious health hazards and threatening the long-term sustainability of the local ecosystem. Absorbing materials has often been used to improve water quality. In this investigation three types of material were studied: the natural zeolite (mordenite); synthetic goethite and the powdered block carbon modified. The adsorption of Pb²⁺ and Mn²⁺ onto natural zeolite as a function of their concentrations was studied at 24°C by varying the metal concentration from 100 to 400 mg L⁻¹ while keeping all other parameters constant. The low-cost zeolites removed Pb from water without any pretreatment at pH values <6. The maximum adsorption attained was as follows: Pb²⁺ 78.7% and Mn²⁺ 19.6%. The modified powdered block carbon effectively removed As(V) and Cr(VI) while goethite removed more chromate than arsenate in the pH range 5–6. Results of this study will be used to evaluate the application these materials for the treatment of the Carapicuíba lake’s water.     

Experimental study on the competitive adsorption of metal ions onto minerals

The study on the competitive adsorption shows that the magnitude order of metal ions adsorbed onto oxide and silicate minerals in near-neutral solution with low ionic strength is in mole/nm² as follows: CaCO₃ > quarte > hydromuscovite > kaolinite > Ca-montmorillonite > goethite > gibbsite. These minerals can be divided into three groups according to their surface equilibrium constantsK _M of the adsorption reactions, which are the function of the dielectric constants ε of the absorbent minerals. The relationships between constantsK _M and mineral dielectric constants ε are described as follows: lgK _M ¹ = 7.813-26.15/ε lgK _M ² = 9.030-26.15/ε lgK _M ³ =11.63-26.15/ε for the adsorption reaction: >SO^- + Mⁿ⁺≥SOM^n-1)+ (n = 1, 2, 3) The first group of minerals include quartz, goethite, 1:1 phyllosilicates and other oxide minerals; the second: gibbsite, brucite and 2:1 phyllosilicates; the third: carbonate, sulphate and phosphorate minerals. The appearance reaction constants have a variation of magnitude ±0.5 for different metal ions with the same mineral. This project was financially supported by the National Natural Science Foundation of China (No. 49572091).     

Salinization of groundwater in arid and semi-arid zones: an example from Tajarak,western Iran

Study of the groundwater samples from Tajarak area, western Iran, was carried out in order to assess their chemical compositions and suitability for agricultural purposes. All of the groundwaters are grouped into two categories: relatively low mineralized of Ca–HCO₃ and Na–HCO₃ types and high mineralized waters of Na–SO₄ and Na–Cl types. The chemical evolution of groundwater is primarily controlled by water–rock interactions mainly weathering of aluminosilicates, dissolution of carbonate minerals and cation exchange reactions. Calculated values of pCO₂ for the groundwater samples range from 2.34 × 10⁻⁴ to 1.07 × 10⁻¹ with a mean value of 1.41 × 10⁻² (atm), which is above the pCO₂ of the earth’s atmosphere (10^−3.5). The groundwater is oversaturated with respect to calcite, aragonite and dolomite and undersaturated with respect to gypsum, anhydrite and halite. According to the EC and SAR the most dominant classes (C3-S1, C4-S1 and C4-S2) were found. With respect to adjusted SAR (adj SAR), the sodium (Na⁺) content in 90% of water samples in group A is regarded as low and can be used for irrigation in almost all soils with little danger of the development of harmful levels of exchangeable Na⁺, while in 40 and 37% of water samples in group B the intensity of problem is moderate and high, respectively. Such water, when used for irrigation will lead to cation exchange and Na⁺ is adsorbed on clay minerals while calcium (Ca²⁺) and magnesium (Mg²⁺) are released to the liquid phase. The salinity hazard is regarded as medium to high and special management for salinity control is required. Thus, the water quality for irrigation is low, providing the necessary drainage to avoid the build-up of toxic salt concentrations.     

Impact of slate quarrying on soil properties in semi-arid Mahendragarh in India

Slate quarrying in Mahendragarh district of Haryana state has resulted in changes in soil properties. Most of the mining area is devoid of vegetation. The soil in and around the mining area (0–1 km) is alkaline (pH 11.2–11.7) but non-saline (electrical conductivity < 4). The alkaline nature of the soil was attributed to the high concentrations of hydroxyl (OH⁻), carbonate (CO₃²⁻) and bicarbonate (HCO₃ ⁻) present in minerals of mined materials. Biotite, limonite, kaolinite, gibbsite, muscovite, geothite, dolomite and so on were the chief minerals added to soil through mining. The physical properties of soil, i.e. porosity, water-holding capacity (WHC), bulk density and particle density represented poor soil health in mining area (34.4, 29.8%, 1.636, 2.496 g/cc, respectively) and they improved with distance away from it (46.4, 38.3%, 1.070, 2.180 g/cc, respectively, at a distance of 1 km). Porosity and WHC were found to be a function of increased organic matter away from the mining area. CO₃²⁻, HCO₃⁻, phosphate (PO₄³⁻), lead (Pb) and iron (Fe) were more in mining area and decreased with distance. On the other hand, sodium (Na⁺), potassium (K⁺), chloride (Cl⁻), sulphate (SO₄²⁻), organic carbon, total Kjeldahl’s nitrogen, cation exchange capacity, chromium (Cr) and cadmium (Cd) increased with distance from mining area. High concentration of heavy metals in mining area was a cause of concern (0.93 μg/g Cd, 22.35 μg/g Cr, 26.25 μg/g Pb, 1,383.75 μg/g Fe). The change in physico-chemical properties could be because of the addition of chemical constituents that are a part of major minerals present in mined material. The soil away from mining area represented comparatively better properties.     

德兴铜矿地区土壤的基本特征及其对重金属元素的吸附研究

本文首先分析了江西德兴铜矿区周围土壤的微量元素和矿物组成特征,结果显示该地区重金属元素富集,且表层土中重金属元素含量与粘土矿物相对含量变化具有较好的一致性。室内土柱淋滤实验结果表明,当总淋滤时间为451 h时,土壤对Pb²⁺的总吸附量为2 584.75 mg/kg,淋滤实验的前半期存在多种竞争吸附机制,后半期土壤对Pb²⁺的吸附基本达到动态平衡。淋滤后土壤矿物的相对质量分数发生了改变,粘土矿物有所减少。粘土矿物在不同土壤层对Pb²⁺的吸附能力也各异。     

Goethite adsorption of Cu(II), Pb(II), Cd(II), and Zn(II) in the presence of sulfate: Properties of the ternary complex

Adsorption of Cu²⁺, Zn²⁺, Cd²⁺, and Pb²⁺ onto goethite is enhanced in the presence of sulfate. This effect, which has also been observed on ferrihydrite, is not predicted by the diffuse layer model (DLM) using adsorption constants derived from single sorbate systems. However, by including ternary surface complexes with the stoichiometry FeOHMSO₄, where FeOH is a surface adsorption site and M²⁺ is a cation, the effect of SO₄²⁻ on cation adsorption was accurately predicted for the range of cation, goethite and SO₄²⁻ concentrations studied. While the DLM does not provide direct molecular scale insights into adsorption reactions there are several properties of ternary complexes that are evident from examining trends in their formation constants. There is a linear relationship between ternary complex formation constants and cation adsorption constants, which is consistent with previous spectroscopic evidence indicating ternary complexes involve cation binding to the oxide surface. Comparing the data from this work to previous studies on ferrihydrite suggests that ternary complex formation on ferrihydrite involves complexes with the same or similar structure as those observed on goethite. In addition, it is evident that ternary complex formation constants are larger where there is a stronger metal-ligand interaction. This is also consistent with spectroscopic studies of goethite-M²⁺-SO₄²⁻ and phthalate systems showing surface species with metal-ligand bonding. Recommended values of ternary complex formation constants for use in SO₄-rich environments, such as acid mine drainage, are presented.     

Hydrogeochemical evaluation of groundwater in the lower Offin basin,Ghana

Alumino-silicate mineral dissolution, cation exchange, reductive dissolution of hematite and goethite, oxidation of pyrite and arsenopyrite are processes that influence groundwater quality in the Offin Basin. The main aim of this study was to characterise groundwater and delineate relevant water–rock interactions that control the evolution of water quality in Offin Basin, a major gold mining area in Ghana. Boreholes, dug wells, springs and mine drainage samples were analysed for major ions, minor and trace elements. Major ion study results show that the groundwater is, principally, Ca–Mg–HCO₃ or Na–Mg–Ca–HCO₃ in character, mildly acidic and low in conductivity. Groundwater acidification is principally due to natural biogeochemical processes. Though acidic, the groundwater has positive acid neutralising potential provided by the dissolution of alumino-silicates and mafic rocks. Trace elements’ loading (except arsenic and iron) of groundwater is generally low. Reductive dissolution of iron minerals in the presence of organic matter is responsible for high-iron concentration in areas underlain by granitoids. Elsewhere pyrite and arsenopyrite oxidation is the plausible process for iron and arsenic mobilisation. Approximately 19 and 46% of the boreholes have arsenic and iron concentrations exceeding the WHO’s (Guidelines for drinking water quality. Final task group meeting. WHO Press, World Health Organization, Geneva, 2004) maximum acceptable limits of 10 μg l⁻¹ and 0.3 mg l⁻¹, for drinking water.     

A theoretical study of the absorption spectra of Pb+ and Pb3+ in site K+ of microcline: application to the color of amazonite

The blue-green color of amazonite has been assigned by various authors to ions Pb⁺ (6 s)² (6 p) and/or Pb³⁺ (6 s) in site of K⁺ of microcline. Owing to the complex which forms between the ion Pb³⁺ and the lone pairs of the oxygen atoms surrounding it, the peripheral electron of Pb³⁺ passes on the levels (6 p) of the latter, which results in a great similarity of the spectra of Pb⁺ and Pb³⁺ in amazonite (the transition energies are multiplied by a factor greater than 1), whereas, in the isolated state, these spectra are completely different from one another. An analytical development of the crystal field around a site K⁺ is established. Under the effect of the crystal field, the transition ² P _1/2→² P _3/2 (6 p) is split into two double transitions. The lower transition only falls in the visible domain (1.6–1.8 eV for Pb⁺), the second in U−V. The green color would arise from the ion Pb⁺, whereas the blue one would be attributed to the ion Pb³⁺. Received: 23 January 1997 / Revised, accepted: 10 September 1997     

Influence of hydrogeochemical processes on temporal changes in groundwater quality in a part of Nalgonda district, Andhra Pradesh, India

Geochemical processes that take place in the aquifer have played a major role in spatial and temporal variations of groundwater quality. This study was carried out with an objective of identifying the hydrogeochemical processes that controls the groundwater quality in a weathered hard rock aquifer in a part of Nalgonda district, Andhra Pradesh, India. Groundwater samples were collected from 45 wells once every 2 months from March 2008 to September 2009. Chemical parameters of groundwater such as groundwater level, EC and pH were measured insitu. The major ion concentrations such as Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl⁻, and SO₄ ²⁻ were analyzed using ion chromatograph. CO₃ ⁻ and HCO₃ ⁻ concentration was determined by acid–base titration. The abundance of major cation concentration in groundwater is as Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ while that of anions is HCO₃ ⁻ > SO₄ ²⁻ > Cl⁻ > CO₃ ⁻. Ca–HCO₃, Na–Cl, Ca–Na–HCO₃ and Ca–Mg–Cl are the dominant groundwater types in this area. Relation between temporal variation in groundwater level and saturation index of minerals reveals the evaporation process. The ion-exchange process controls the concentration of ions such as calcium, magnesium and sodium. The ionic ratio of Ca/Mg explains the contribution of calcite and dolomite to groundwater. In general, the geochemical processes and temporal variation of groundwater in this area are influenced by evaporation processes, ion exchange and dissolution of minerals.     

Water–Rock Interactions: An Investigation of the Relationships Between Mineralogy and Groundwater Composition and Flow in a Subtropical Basalt Aquifer

A holistic study of the composition of the basalt groundwaters of the Atherton Tablelands region in Queensland, Australia was undertaken to elucidate possible mechanisms for the evolution of these very low salinity, silica- and bicarbonate-rich groundwaters. It is proposed that aluminosilicate mineral weathering is the major contributing process to the overall composition of the basalt groundwaters. The groundwaters approach equilibrium with respect to the primary minerals with increasing pH and are mostly in equilibrium with the major secondary minerals (kaolinite and smectite), and other secondary phases such as goethite, hematite, and gibbsite, which are common accessory minerals in the Atherton basalts. The mineralogy of the basalt rocks, which has been examined using X-ray diffraction and whole rock geochemistry methods, supports the proposed model for the hydrogeochemical evolution of these groundwaters: precipitation + CO₂ (atmospheric + soil) + pyroxene + feldspars + olivine yields H₄SiO₄, HCO₃ ⁻, Mg²⁺, Na⁺, Ca²⁺ + kaolinite and smectite clays + amorphous or crystalline silica + accessory minerals (hematite, goethite, gibbsite, carbonates, zeolites, and pyrite). The variations in the mineralogical content of these basalts also provide insights into the controls on groundwater storage and movement in this aquifer system. The fresh and weathered vesicular basalts are considered to be important in terms of zones of groundwater occurrence, while the fractures in the massive basalt are important pathways for groundwater movement.     

Minéralogie et bilan de matière de l''altération supergène d''un basalte alcalin du Moyen Atlas Marocain

In the Middle Atlas of Morocco, alkali basaltic flows record successive weathering phases during the Quaternary. In fresh basalt interior and intermediate external zones, the first weathering stage is characterised by glass dissolution and the formation of a Si-Al poorly-crystallised product. Advanced weathering phases are characterised by 10 Å halloysite, kaolinite and goethite, located within the primary minerals or as secondary products in fissures. Olivine and iddingsite are transformed into Si-rich goethite, plagioclase into halloysite and pyroxene into a mixture of halloysite + geothite. Dissolution of Ti-magnetite and ilmenite yielded Ti-rich products. In these conditions, the weathering of basalts and development of a soil matrix are accompanied by the elimination of certain chemical elements, such as Si, Ca, Na and K, and the concentration of Fe and Al. In the soil, clay minerals such as illite and vermiculite, do not have any genetic relationship with weathered basalt and were probably introduced externally.     

The crystal structure of arsentsumebite, Pb2Cu[(As, S)O4]2(OH)

Summary The crystal structure of arsentsumebite, ideally, Pb₂Cu[(As, S)O₄]₂(OH), monoclinic, space group P2₁/m, a = 7.804(8), b = 5.890(6), c = 8.964(8) ?, β = 112.29(6)°, V = 381.2 ?³, Z = 2, d_calc. = 6.481 has been refined to R = 0.053 for 898 unique reflections with I> 2σ(I). Arsentsumebite belongs to the brackebuschite group of lead minerals with the general formula Pb₂ Me(XO₄)₂(Z) where Me = Cu²⁺, Mn²⁺, Zn²⁺, Fe²⁺, Fe³⁺; X = S, Cr, V, As, P; Z = OH, H₂O. Members of this group include tsumebite, Pb₂Cu(SO₄)(PO₄)(OH), vauquelinite, Pb₂Cu(CrO₄)(PO₄)(OH), brackebuschite, Pb₂ (Mn, Fe)(VO₄)₂(OH), arsenbracke buschite, Pb₂(Fe, Zn)(AsO₄)₂(OH, H₂O), fornacite, Pb₂Cu(AsO₄)(CrO₄)(OH), and feinglosite, Pb₂(Zn, Fe)[(As, S)O₄]₂(H₂O). Arsentsumebite and all other group members contain M = M–T chains where M = M means edge-sharing between MO₆ octahedra and M–T represents corner sharing between octahedra and XO₄ tetrahedra. A structural relationship exists to tsumcorite, Pb(Zn, Fe)₂(AsO₄)₂ (OH, H₂O)₂ and tsumcorite-group minerals Me(1)Me(2)₂(XO₄)₂(OH, H₂O)₂. Received June 24, 2000; revised version accepted February 8, 2001     

U–Pb systematics of the McClure Mountain syenite: thermochronological constraints on the age of the 40Ar/39Ar standard MMhb

Recent advances in U–Pb geochronology allow unprecedented levels of precision in the determination of geological ages. However, increased precision has also illuminated the importance of understanding subtle sources of open-system behavior such as Pb-loss, inheritance, intermediate daughter product disequilibria, and the accuracy of the model assumptions for initial Pb. Deconvolution of these effects allows a much richer understanding of the power and limitations of U–Pb geochronology and thermochronology. In this study, we report high-precision ID-TIMS U–Pb data from zircon, baddelleyite, titanite and apatite from the McClure Mountain syenite, from which the ⁴⁰Ar/³⁹Ar hornblende standard MMhb is derived. We find that excess ²⁰⁶Pb in zircon due to inclusions of high-Th minerals and elevated Th/U in titanite and apatite jeopardize the utility of the ²³⁸U–²⁰⁶Pb system in this rock. Strongly air-abraded zircons give dates that are younger than chemical-abraded zircons, which yield a statistically robust ²⁰⁷Pb/²³⁵U date of 523.98±0.12 Ma that is interpreted as the crystallization age. We explore the best method of Pb_c correction in titanite and apatite by analyzing the U–Pb isotopes of K-feldspar and using 2-D and 3-D regression methods—the latter of which yields the best results in each case. However, the calculated compositions of Pb_c for titanite, apatite and K-feldspar are different, implying that using a single Pb_c correction for multiple U–Pb thermochronometers may be inaccurate. The U–Pb thermochronological results are used to predict a closure time for Ar in hornblende of 522.98±1.00 Ma. Widely cited K–Ar and ⁴⁰Ar/³⁹Ar dates overlap with the U–Pb date, and relatively large errors make it impossible to verify whether U–Pb dates are systematically ≤1% older than K–Ar and ⁴⁰Ar/³⁹Ar dates.     

Genesis of sediment-hosted stratiform copper–cobalt mineralization at Luiswishi and Kamoto, Katanga Copperbelt (Democratic Republic of Congo)

The sediment-hosted stratiform Cu–Co mineralization of the Luiswishi and Kamoto deposits in the Katangan Copperbelt is hosted by the Neoproterozoic Mines Subgroup. Two main hypogene Cu–Co sulfide mineralization stages and associated gangue minerals (dolomite and quartz) are distinguished. The first is an early diagenetic, typical stratiform mineralization with fine-grained minerals, whereas the second is a multistage syn-orogenic stratiform to stratabound mineralization with coarse-grained minerals. For both stages, the main hypogene Cu–Co sulfide minerals are chalcopyrite, bornite, carrollite, and chalcocite. These minerals are in many places replaced by supergene sulfides (e.g., digenite and covellite), especially near the surface, and are completely oxidized in the weathered superficial zone and in surface outcrops, with malachite, heterogenite, chrysocolla, and azurite as the main oxidation products. The hypogene sulfides of the first Cu–Co stage display δ³⁴S values (−10.3‰ to +3.1‰ Vienna Canyon Diablo Troilite (V-CDT)), which partly overlap with the δ³⁴S signature of framboidal pyrites (−28.7‰ to 4.2‰ V-CDT) and have ∆³⁴S_SO4-Sulfides in the range of 14.4‰ to 27.8‰. This fractionation is consistent with bacterial sulfate reduction (BSR). The hypogene sulfides of the second Cu–Co stage display δ³⁴S signatures that are either similar (−13.1‰ to +5.2‰ V-CDT) to the δ³⁴S values of the sulfides of the first Cu–Co stage or comparable (+18.6‰ to +21.0‰ V-CDT) to the δ³⁴S of Neoproterozoic seawater. This indicates that the sulfides of the second stage obtained their sulfur by both remobilization from early diagenetic sulfides and from thermochemical sulfate reduction (TSR). The carbon (−9.9‰ to −1.4‰ Vienna Pee Dee Belemnite (V-PDB)) and oxygen (−14.3‰ to −7.7‰ V-PDB) isotope signatures of dolomites associated with the first Cu–Co stage are in agreement with the interpretation that these dolomites are by-products of BSR. The carbon (−8.6‰ to +0.3‰ V-PDB) and oxygen (−24.0‰ to −10.3‰ V-PDB) isotope signatures of dolomites associated with the second Cu–Co stage are mostly similar to the δ¹³C (−7.1‰ to +1.3‰ V-PDB) and δ¹⁸O (−14.5‰ to −7.2‰ V-PDB) of the host rock and of the dolomites of the first Cu–Co stage. This indicates that the dolomites of the second Cu–Co stage precipitated from a high-temperature, host rock-buffered fluid, possibly under the influence of TSR. The dolomites associated with the first Cu–Co stage are characterized by significantly radiogenic Sr isotope signatures (0.70987 to 0.73576) that show a good correspondence with the Sr isotope signatures of the granitic basement rocks at an age of ca. 816 Ma. This indicates that the mineralizing fluid of the first Cu–Co stage has most likely leached radiogenic Sr and Cu–Co metals by interaction with the underlying basement rocks and/or with arenitic sedimentary rocks derived from such a basement. In contrast, the Sr isotope signatures (0.70883 to 0.71215) of the dolomites associated with the second stage show a good correspondence with the ⁸⁷Sr/⁸⁶Sr ratios (0.70723 to 0.70927) of poorly mineralized/barren host rocks at ca. 590 Ma. This indicates that the fluid of the second Cu–Co stage was likely a remobilizing fluid that significantly interacted with the country rocks and possibly did not mobilize additional metals from the basement rocks.     

Oxygen isotope fractionation between hydroxide minerals and water

The modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxide minerals. The results suggest the following sequence of ¹⁸O-enrichment in the common hydroxides: limonite > gibbsite > goethite > brucite > diaspore. The hydroxides are significantly enriched in ¹⁸O relative to the corresponding oxides. The sequence of ¹⁸O-enrichment in the hydroxides and oxides of trivalent cations is as follows: M(OH)₃ > MO(OH) > M₂O₃. There are also considerable fractionations within the polymorphos of Al(OH)₃. The internally consistent fractionation factors for hydroxide–water systems are obtained for the temperature range of 0 to 1200 °C, which are comparable with the data derived from synthesis experiments and natural samples at surficial temperatures. Temperature dependence of oxygen isotope fractionations between goethite, gibbsite, boehmite and diaspore and water are significant enough for the purpose of geothermometry. Thus the hydroxide–water pairs hold great promise of serving as reliable paleothermometers in surficial geological environments. Received: 22 January 1997 / Revised, accepted: 2 June 1997     

Arsenic adsorption on nanocrystalline goethite: the natural example of bolar earths from Mt Amiata (Central Italy)

Bolar earths deposits from Mt Amiata (Central Italy) consist of nanosized pseudo-spherical goethite, with average crystal size of 10–15 nm (as determined by X-ray powder diffraction and transmission electron microscopy observations), possibly associated to amorphous silica and minor sheet silicates, quartz and feldspars. Chemical analyses revealed high As contents (up to 7.4 wt% As₂O₅), thus indicating the occurrence of a potentially dangerous contaminant. Arsenic doesn’t occur as a specific As phase, but it is strictly associated with goethite nanocrystals. Eh and pH measurements suggest that As occurs as arsenate anions (H₂AsO₄⁻ and HAsO₄²⁻), which are easily and strongly adsorbed to goethite surfaces. The high specific surface area, resulting from goethite nanosize, and the absence of competitive anions explain the extremely efficient adsorption of arsenate and the anomalously high As content in bolar earths. Overall physical/chemical data suggest stable arsenate adsorption, with very limited risk for As release to the environment.