Structure of the orthoclase (001)- and (010)-water interfaces by high-resolution X-ray reflectivity

High-resolution in situ X-ray specular reflectivity was used to measure the structures of orthoclase (001) and (010) cleavage surfaces in contact with deionized water at 25°C. X-ray reflectivity data demonstrate a high degree of structural similarity between these two orthoclase-water interfaces. Both interfacial structures include cleavage along the plane of minimal bond breakage resulting in surfaces terminated by non-bridging oxygens; structured water within 5 Å of the orthoclase surface (consisting of adsorbed species at the surface and layered water above the surface), with a featureless water profile beyond 5 Å; substitution of outermost K⁺ ions by an oxygen containing species (presumably H₃O⁺); and small structural displacements of the near surface atoms. The interfacial water structure, in comparison with recent results for other mineral-water interfaces, is intermediate between the minimal structure found at calcite-, barite-, and quartz-water interfaces and the more extensive structure found at the muscovite-water interface.     

Changes of semifusinite and fusinite surface roughness during heat treatment determined by atomic force microscopy

This study describes changes of surface roughness of semifusinite and fusinite as an indicator of structural alteration resulting from heat treatment at 400–1200 °C. Surface roughness has been investigated by atomic force microscopy of inertinite concentrates from coking coals (vitrinite reflectance R_r = 1.07%–1.41%) from the Upper Silesian Coal Basin of Poland (Namurian C — Westphalian A). Unheated fusinite has a higher surface roughness than semifusinite from the same coal. The average surface roughness of semifusinite decreases with the Swelling Index of the parent coal. Heating increases the surface roughness of semifusinite and fusinite. Increase in the average surface roughness is stronger for semifusinite than fusinite and correlates to increasing reflectance of these macerals. The surface roughness of semifusinite correlates to the relative mass loss of the inertinite concentrates during heating. After heating to 1200 °C fusinite has a lower average surface roughness than semifusinite from the same coal. Consequently, average surface roughness can be used as a measure of structural alteration of inertinite group macerals during heat treatment.     

Basal plane reactivity of phyllosilicates studied in situ by hydrothermal atomic force microscopy (HAFM)

The basal plane reactivities of the sheet silicates apophyllite and phlogopite have been studied by hydrothermal atomic force microscopy (HAFM) in situ in aqueous solutions at temperatures from 20 to 140 °C. At pH 4-5.6 (T = 20-100 °C), the apophyllite basal surface undergoes a swelling process which forms square hillocks on the surface. The reaction comprises three sequential morphological transformations that cause swelling to increase from 0.15 to 2.5 nm. In the first two transformations, interlayer cations are replaced by hydronium ions from the solution; the third transformation involves a depolymerization and partial cross-linking of the protonated silicate sheets. The reaction of phlogopite with acidic aqueous solutions (pH 1.5-2) at high temperature (T = 100-140 °C) causes the nucleation of numerous monolayer etch pits on the pristine surface. Where the 2D pits recur at the same lateral position, they can accumulate to a total pit depth of up to 50 nm. The formation of an altered layer has also been detected at these conditions. The alteration affects the uppermost 4-5 layers. The layers are expanded, corrugated, highly unstable, and readily peel off the surface. Etch pit formation has been detected even underneath the altered layer. On the basis of HAFM data, dissolution rates and activation energies were calculated. The presented data show that the basal surface of phlogopite plays an important role in the dissolution process at least at elevated temperatures and that the absolute amount of released material has comparable contributions from both basal surfaces and edge surfaces.     

Effects of citrate on the dissolution and transformation of biotite,analyzed by chemical and atomic force microscopy

The aim of this study was to investigate the dissolution and transformation characteristics of phyllosilicate under low molecular weight organic acids in the farmland environment (pH 4.0–8.0). Changes of dissolution and morphology of biotite were evaluated using chemical extraction experiments and in situ/ex situ atomic force microscopy (AFM) with fluids of citric acid (CA) solution at pH 4.0, 6.0, and 8.0. Results of extracting experiments show that CA solutions contributed to the release rate of potassium (K), silicon (Si), and aluminum (Al) from biotite relative to a control aqueous solution. In situ AFM observations indicate that the dissolution of biotite from the biotite (0 0 1) surface occurred on the terrace, segment, and fringe of pits, while new etch pits did not readily form on biotite (0 0 1) surfaces in aqueous solutions. However, dissolution rates of terraces can be greatly accelerated with the help of citrate. In pH 4.0 CA solution, 70 min dissolution reactions of biotite (0 0 1) surfaces result in more etch pits than in pH 6.0 and 8.0 solutions. In addition, the transformation of biotite occurred simultaneously with the dissolution process. Secondary coating was observed on the biotite (0 0 1) surface after 140 h of immersion in a weak acid environment. Thus, the protons have a dominant role in the dissolution process of biotite with organic (carboxyl) acting as a catalyst under acidic condition. Based on the theory of interactions on a water–mineral interface in a weak acid environment, dissolution of biotite starts from defect/kink sites on the surface, one layer by one layer, and develops along the [h k 0] direction. A secondary coating that forms on the biotite (0 0 1) surface may restrain the formation and growth of etch pits, whereas this process may have a positive role on the stability of soil structure during long-term soil management.     

Molecular dynamics simulations of the orthoclase (0 0 1)- and (0 1 0)-water interfaces

Molecular dynamics simulations of water in contact with the (0 0 1) and (0 1 0) surfaces of orthoclase (KAlSi₃O₈) were carried out to investigate the structure and dynamics of the feldspar-water interface, contrast the intrinsic structural properties of the two surfaces, and provide a basis for future work on the diffusion of ions and molecules in microscopic mineral fractures. Electron density profiles were computed from the molecular dynamics trajectories and compared with those derived experimentally from high-resolution X-ray reflectivity measurements by Fenter and co-workers [Fenter P., Cheng L., Park C., Zhang H. and Sturchio N. C. (2003a) Structure of the orthoclase (0 0 1)- and (0 1 0)-water interfaces by high-resolution X-ray reflectivity. Geochim. Cosmochim. Acta67, 4267-4275]. For each surface, three scenarios were considered whereby the interfacial species is potassium, water, or a hydronium ion. Excellent agreement was obtained for the (0 0 1) surface when potassium is the predominant interfacial species; however, some discrepancies in the position of the interfacial peaks were obtained for the (0 1 0) surface. The two surfaces showed similarities in the extent of water ordering at the interface, the activation energies for water and potassium desorption, and the adsorption localization of interfacial species. However, there are also important differences between the two surfaces in the coordination of a given adsorbed species, adsorption site densities, and the propensity for water molecules in surface cavities and those in the first hydration layer to coordinate to surface bridging oxygen atoms. These differences may have implications for the extent of dissolution in the low-pH regime since hydrolysis of Si(Al)OSi(Al) bonds is a major dissolution mechanism.     

Characterization of magnetite particles in shocked quartz by means of electron- and magnetic force microscopy: Vredefort, South Africa

Submicroscopic opaque particles from highly shocked granite-gneisses close to the core of the Vredefort impact structure have been investigated by means of micro-analytical techniques with high spatial resolution such as electron diffraction, orientation contrast imagery and magnetic force microscopy. The opaque particles have been identified as nano- to micro-sized magnetite that occur in several distinct modes. In one sample magnetite occurs along relict planar deformation features (PDFs) in quartz, generally accepted as typical shock lamellae. The magnetite particles along shock lamellae in quartz grains virtually all show uniform crystallographic orientations. In most instances, the groups of magnetite within different quartz grains are systematically misorientated such that they share a subparallel <101> direction. The magnetite groups of all measured quartz grains thus appear to have a crystallographic preferred orientation in space. In a second sample, orientations of magnetite particles have been measured in microfractures (non-diagnostic of shock) of quartz, albite and in the alteration halos, (e.g. biotite grains breaking down to chlorite). The crystallographic orientations of magnetite particles are diverse, with only a minor portion having a preferred orientation. Scanning electron microscopy shows that magnetite along the relict PDFs is invariably associated with other microcrystalline phases such as quartz, K-feldspar and biotite. Petrographic observations suggest that these microcrystalline phases crystallized from locally formed micro-melts that intruded zones of weakness such as microfractures and PDFs shortly after the shock event. The extremely narrow widths of the PDFs suggest that heat may have dissipated rapidly resulting in melts crystallizing relatively close to where they were generated. Magnetic force microscopy confirms the presence of magnetic particles along PDFs. The smallest particles, <5 μm with high aspect ratios 15:1 usually exhibit intense, uniform magnetic signals characteristic of single-domain magnetite. Consistent offsets between attractive and repulsive magnetic signals of individual single-domain particles suggest consistent directions of magnetization for a large proportion of particles. Received: 16 November 1998 / Accepted: 17 May 1999     

Hydromechanical properties and alteration of natural fracture surfaces in the Soultz granite (Bas-Rhin, France)

Natural fractures are characterized by rough surfaces and complex fluid flows. A large distribution of apertures (residual voids) within their walls and the presence of contact points (in situ normal loads) produce heterogeneous flows (channeling). The resulting permeabilities, porosities or fluid–rock exchange surfaces cannot be realistically modeled by parallel and smooth plate models. Four natural fractures are sampled at different depths and degrees of alteration in the Soultz sandstone and granite (EPS1 drillhole, Soultz-sous-Forêts, Bas-Rhin, France). The fracture surfaces are measured with mechanical profilometry and maps of asperity heights (XYZ). Resulting local apertures (XYe) are then calculated. A statistical study of the surface profiles (XZ) show that the fractures are more or less rough and tortuous according to the types of alteration. Altered samples are characterized by smoother surfaces of fractures. Such differences imply that (i) the average fracture aperture is not representative for the whole fracture and that (ii) the different local apertures should be integrated in hydraulic and mechanical models. A hydraulic model (finite difference calculations) of fluid flow, taking into account the elastic closure (Hertz contact theory) of fractures with depth, is used. Maps of contact points and relative local loads within the fracture planes are compared to flow maps. They show different channeling of fluid flows. Strongly altered fractures are characterized by homogeneous fluxes despite the presence of numerous contact zones during the closure of fracture. By contrast, fresh fractures develop, increasing fluid flow channels with depth.Fracture closure (increasing normal stress) does not systematically increase the channeling of fluid flow. There is evidence for a general smoothing out of the irregularities of the fracture walls due to precipitation of secondary minerals, indicating that the cubic law can be commonly valid, also at great crustal depth but this validity depends on the degree of fracture alteration. Mineralogical and geochemical observations, thus, should be taken into account to perform more accurate permeability calculations and models of fluid circulation in fracture networks.     

Evaluation of natural quartz and zeolitic tuffs for As(V) removal from aqueous solutions: a mechanistic approach

Naturally occurring pyroclastic materials, quartz-rich tuffs (Qz) and stellerite tuffs, were evaluated for their capacities to remove As(V) from aqueous solutions. The mechanism of As(V) uptake was evaluated using sequential extraction of As(V)-loaded adsorbent which entailed that the plausible removal mechanism is sorption to short-range ordered hydrous oxides of iron and aluminum. In addition, buffering effects of adsorbents could form favorable charges upon them through hydrolysis of amphoteric oxides, enhancing the performance of sorption. The influence of anions co-existing with As(V) in water such as carbonate, bicarbonate, nitrate, chloride, phosphate, and sulfate was studied in a batch sorption process. The impact of most anions on As(V) removal was found to be negligible except phosphate. The sorption behavior well fitted to Langmuir and Freundlich models. Estimated maximum sorption capacities of 0.42 and 0.23 mg/L were observed using quartz-rich tuffs and stellerite tuffs, respectively. As(V) concentration of 0.8 mg/L was easily decreased to below the drinking water standard of 0.01 mg/L using Qz adsorbent, whereas 0.1 mg/L As(V) decreased to below this limit upon the use of stellerite tuffs under similar conditions. The buffering capacity of quartz-rich tuffs and stellerite tuffs induced a pH increase to 5.76 and 5.40, respectively, from initial pH of 3.50, which will incur an important asset in real applications.     

The influence of the low-high quartz transformation on recrystallization and grain growth during contact metamorphism (Traversella intrusion, North Italy)

Recrystallization and grain growth of quartz occur only within 35 m of the contact. This limit of recrystallization and grain growth coincides with an inferred maximum temperature of 620 ± 20°C, which at about 6–7 km is the temperature for the low-high quartz transformation. The remarkable coincidence of the temperature of about 620°C for the beginning of recrystallization and grain growth with the transformation temperature of quartz is explained by the atomic displacement mechanism of the low-high quartz transformation. The already disordered structure of the grain boundary becomes even more distorted by the fluctuations of the SiO₄ tetrahedra during the low-high quartz transformation. This results in an increase of the number of atoms which are transferred from one grain into the grain boundary, and from the grain boundary to the other grain. A lowering of the activation energy for grain boundary migration results such that grain boundary migration becomes the most important process for recrystallization and grain growth.     

Enhanced microbial reduction of Cr(VI) and U(VI) by different natural organic matter fractions

Although direct microbial reduction of Cr(VI) and U(VI) is known, few studies have examined the kinetics and the underlying mechanisms of the reduction of these contaminants by different natural organic matter (NOM) fractions in the presence or absence of microorganisms. In this study, NOM was found to chemically reduce Cr(VI) at pH 3, but the reduction rates were negligible at pH ∼7. The abiotic reduction of U(VI) by NOM was not observed, possibly because of the presence of small amounts of nitrate in the reactant solution. However, all NOM fractions, particularly the soil humic acid (HA), enhanced the bioreduction of Cr(VI) or U(VI) in the presence of Shewanella putrefaciens CN32. The reduction rates varied greatly among NOM fractions with different chemical and structural properties: the polyphenolic-rich NOM-PP fraction appeared to be the most reactive in abiotically reducing Cr(VI) at a low pH, but soil HA was more effective in mediating the microbial reduction of Cr(VI) and U(VI) under anaerobic, circumneutral pH conditions. These observations are attributed to an increased solubility and conformational changes of the soil HA with pH and, more importantly, its relatively high contents of polycondensed and conjugated aromatic organic moieties. An important implication of this study is that, depending on chemical and structural properties, different NOM components may play different roles in enhancing the bioreduction of Cr(VI) and U(VI) by microorganisms. Polycondensed aromatic humic materials may be particularly useful in mediating the bioreduction and rapid immobilization of these contaminant metals in soil.     

富含Na~ 、Ca~(2 )、Al~(3 )、Cr~(3 )蒙脱石对垃圾渗滤液中有机物的吸附

由于层间含有高价态金属阳离子的蒙脱石对特定有机物的吸附能力可大大增强 ,故分别用层间含有Na 、Ca2 、Al3 和Cr3 的蒙脱石对垃圾渗滤液中的有机物进行吸附实验 ,研究它们吸附苯酚、二甲苯和COD的能力及离子形态对吸附效果的影响。结果表明 ,含高价金属阳离子的蒙脱石对苯酚和COD的吸附能力较高 ,吸附能力由小到大的顺序为 :Na 相似文献   

The structure of hematite (α-Fe2O3) (001) surfaces in aqueous media: scanning tunneling microscopy and resonant tunneling calculations of coexisting O and Fe terminations

The iron oxide-water interface is of interest not only in geochemical and environmental processes, but also in fields ranging from corrosion to photocatalysis. The structure of α-Fe₂O₃ (001) surfaces is not fully understood, and questions have arisen recently concerning different terminations of (001) terraces; a so-called Fe-termination is expected, but under some conditions an O-termination may also be possible. Ultra-high vacuum (UHV) scanning tunneling microscope (STM) studies report evidence for an O-termination in coexistence with an Fe-termination, but other studies find no evidence for an O-termination. Molecular mechanics studies suggest that an O-termination should be possible in an aqueous environment. An O-termination could result from dissolution; if Fe atoms were to dissolve from an Fe-termination, an O-termination would presumably result (and vice-versa). We imaged hematite (001) surfaces in air and aqueous solution using STM. To aid interpretation of the images, we use a resonant tunneling model (RTM) parameterized using ab initio calculations.Our STM and RTM results are consistent with mixed O- and Fe-terminated (001) surfaces. For acid-etched surfaces we find evidence for a periodic (with wavelength of 2.2 ± 0.2 nm) arrangement of nominal O- and Fe-terminated domains. Two different borders between domains should occur, one in which the Fe-termination is high relative to the O-termination, and the reverse. The different domain borders have significantly different heights, consistent with RTM calculations. This agreement allows us to conclude that the Fe-termination is topographically high on most terraces. Surface domains are observed in aqueous solutions at the atomic scale, and appear to be very unreactive on tens-of-seconds time scales at pH 1.     

Neoarchean (2.5-2.8 Ga) crustal growth of the North China Craton revealed by zircon Hf isotope: A synthesis

The crustal growth of the North China Craton(NCC) during the Neoarchean time(2.5—2.8 Ga) is a hotly controversial topic,with some proposing thai the main crustal growth occurred in the late Neoarchean (2.5—2.6 Ga),in agreement with the time of the magmatism,whereas others suggest that the main crustal accretion took place during early Neoarchean time(2.7—2.8 Ga),consistent with the time of crustalformation of other cratons in the world.Zircon U-Pb ages and Hf isotope compositions can provide rigorous constraints on the time of crustal growth and the evolution and tectonic division of the NCC.In this contribution, we make a comprehensive review of zircon Hf isotope data in combination with zircon U-Pb geochronology and some geochemistry data from various divisions of the NCC with an aim to constrain the Neoarchean crustal growth of the NCC.The results suggest that both 2.7—2.8 Ga and 2.5—2.6 Ga crustal growth are distributed over the NCC and the former is much wider than previously suggested.The Eastern block is characterized by the main 2.7—2.8 Ga crustal growth with local new crustal-formation at 2.5—2.6 Ga,and the Yinshan block is characterized by～2.7 Ga crustal accretion as revealed by Hf-isotope data of detrital zircons from the Zhaertai Group.Detrital zircon data of the Khondalite Belt indicate that the main crustal growth period of the Western block is Paleoproterozoic involving some～2.6 Ga and minor Early- to Middle-Archean crustal components,and the crustal accretion in the Trans-North China Orogen(TNCO) has a wide age range from 2.5 Ga to 2.9 Ga with a notable regional discrepancy.Zircon Hf isotope compositions,coupled with zircon ages and other geochemical data suggest that the southern margin may not be an extension of the TNCO,and the evolution and tectonic division of the NCC is more complex than previously proposed,probably involving multi-stage crustal growth and subduction processes.However, there is no doubt that 2.7—2.8 Ga magmatism and crustal-formation are more widely distributed than previously considered,which is further supported by the data of zircons from Precambrian lower crustal rocks, overlying sedimentary cover,modern river sediments and Late Neoarchean syenogranites.     

中国大陆科学钻探主孔100~2000 m测井磁化率和磁三分量分析

对比分析了中国大陆科学钻探(CCSD)主孔100~2 000 m测井磁化率与岩心样本磁化率的测量结果,二者具有较好的一致性。测井磁化率统计结果表明CCSD主孔100~2 000 m超高压变质岩的磁化率从高至低依次为:蛇纹岩、石英榴辉岩、金红石榴辉岩、正片麻岩、退变质榴辉岩、多硅白云母榴辉岩、副片麻岩、绿泥石角闪岩、角闪岩。由CCSD主孔100~2 000 m磁三分量测量数据得到磁异常的△BZ、△BH和△BT值,结合前人所做的岩石学研究分析发现:主孔530~575 m的磁异常主要由岩石的金红石矿化、钛磁铁矿化和磁铁矿化引起,604~643 m和652~678 m的磁异常主要由岩石的蛇纹石化引起;钻孔中岩层的磁化率对磁三分量异常起着主要的决定作用,高磁化率岩层都表现出较大的磁三分量异常。     

中国大陆科学钻探工程主孔100～2 000 m 岩心自然伽马测量及其地质意义

中国大陆科学钻探工程使用石油大学研制的岩心地面伽马测量仪对主孔2000m岩心进行了连续测量，并记录峰、谷的伽马值及其对应的岩性、构造和矿化等特征。通过系统测量，建立了主孔2000m自然伽马值剖面，发现榴辉岩具有较低的伽马值，并从榴辉岩→角闪岩→花岗片麻岩，伽马值逐渐增大。伽马值与变质岩的岩性具有明显的对应关系。并从放射性元素U、Th和K含量的角度来看岩石中放射性元素的变化规律，探讨这些元素变化的成因，指出变质基(中)性岩原岩反映的岩浆岩成因，不同于变质泥质沉积岩；岩石高Th／U比值与岩石曾经经历了近地表环境有关。主孔深变质岩自然伽马测量的系统研究对主孔岩心深度归位、探讨岩石成因和区域岩石圈热状态和大地构造演化具有重要的作用。