首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 46 毫秒
1.
Molecular dynamics computer simulations of the molecular structure, diffusive dynamics and hydration energetics of water adsorbed on (0 0 1) surfaces of brucite Mg(OH)2, gibbsite Al(OH)3, hydrotalcite Mg2Al(OH)6Cl · 2H2O, muscovite KAl2(Si3Al)O10(OH)2, and talc Mg3Si4O10(OH)2 provide new insight into the relationships between the substrate structure and composition and the molecular-scale structure and properties of the interfacial water. For the three hydroxide phases studied here, the differences in the structural charge on the octahedral sheet, cation occupancies and distributions, and the orientations of OH groups all affect the surface water structure. The density profiles of water molecules perpendicular to the surface are very similar, due to the prevalent importance of H-bonding between the surface and the water and to their similar layered crystal structures. However, the predominant orientations of the surface water molecules and the detailed two-dimensional near-surface structure are quite different. The atomic density profiles and other structural characteristics of water at the two sheet silicate surfaces are very different, because the talc (0 0 1) surface is hydrophobic whereas the muscovite (0 0 1) surface is hydrophilic. At the hydrophilic and electrostatically neutral brucite and gibbsite (0 0 1) surfaces, both donating and accepting H-bonds from the H2O molecules are important for the development of a continuous hydrogen bonding network across the interfacial region. For the hydrophilic but charged hydrotalcite and muscovite (0 0 1) surfaces, only accepting or donating H-bonds from the water molecules contribute to the formation of the H-bonding network at the negatively and positively charged interfaces, respectively. For the hydrophobic talc (0 0 1) surface, H-bonds between water molecules and the surface sites are very weak, and the H-bonds among H2O molecules dominate the interfacial H-bonding network. For all the systems studied, the orientation of the interfacial water molecules in the first few layers is influenced by both the substrate surface charge and the ability by the surfaces to facilitate H-bond formation. The first layer of water molecules at all surfaces is well ordered in the xy plane (parallel to the surface) and the atomic density distributions reflect the substrate crystal structure. The enhanced ordering of water molecules at the interfaces indicates reduced orientational and translational entropy. In thin films, water molecules are more mobile parallel to the surface than perpendicular to it due to spatial constraints. At neutral, hydrophilic substrates, single-monolayer surface coverage stabilizes the adsorbed water molecules and results in a minimum of the surface hydration energy. In contrast, at the charged and hydrophilic muscovite surface, the hydration energy increases monotonically with increasing water coverage over the range of coverages studied. At the neutral and hydrophobic talc surface, the adsorption of H2O is unfavorable at all surface coverages, and the hydration energy decreases monotonically with increasing coverage.  相似文献   

2.
Understanding mechanisms and kinetics of mineral carbonation reactions relevant to sequestering carbon dioxide as a supercritical fluid (scCO2) in geologic formations is crucial to accurately predicting long-term storage risks. Most attention so far has been focused on reactions occurring between silicate minerals and rocks in the aqueous dominated CO2-bearing fluid. However, water-bearing scCO2 also comprises a reactive fluid, and in this situation mineral carbonation mechanisms are poorly understood. Using in situ high-pressure X-ray diffraction, the carbonation of brucite [Mg(OH)2] in wet scCO2 was examined at pressure (82 bar) as a function of water concentration and temperature (50 and 75 °C). Exposing brucite to anhydrous scCO2 at either temperature resulted in little or no detectable reaction over three days. However, addition of trace amounts of water resulted in partial carbonation of brucite into nesquehonite [MgCO3·3H2O] within a few hours at 50 °C. By increasing water content to well above the saturation level of the scCO2, complete conversion of brucite into nesquehonite was observed. Tests conducted at 75 °C resulted in the conversion of brucite into magnesite [MgCO3] instead, apparently through an intermediate nesquehonite step. Raman spectroscopy applied to brucite reacted with 18O-labeled water in scCO2 show it was incorporated into carbonate at a relatively high concentration. This supports a carbonation mechanism with at least one step involving a direct reaction between the mineral and water molecules without mediation by a condensed aqueous layer.  相似文献   

3.
Summary The crystal structure of meta-uranocircite II, Ba(UO2)2(PO4)2·6H2O, has been determined with a synthetic crystal using three-dimensional X-ray techniques.R=0.071 andR w =0.064 were obtained for 1743 observed reflections. Ba(UO2)2(PO4)2·6H2O is monoclinic, space groupP1121/a, a=9.789,b=9.822,c=16.868 Å, =89.95° andZ=4. The structure consists of slightly corrugated UO2PO4 layers parallel (001). The layers are connected by Ba atoms and H2O molecules. Uranium exhibits a (2+4)-coordination with mean U-O bond lengths of 1.78 Å for the uranyl oxygens and 2.28 Å for the phosphate oxygens. The average P-O bond length is 1.52 Å. Barium is coordinated by two uranyl oxygens. two phosphate oxygens and five water molecules. The Ba–O bond lengths vary from 2.74 to 3.11 Å. Two of the six water molecules of the formula are not bonded to barium.
Die Kristallstruktur des Meta-Uranocircits II, Ba(UO2)2(PO4)2·6H2O
Zusammenfassung Die Kristallstruktur des Meta-Uranocircits II, Ba(UO2)2(PO4)2·6H2O, wurde anhand eines künstlichen Kristalls mit dreidimensionalen Röntgendaten bearbeitet und für 1743 Reflexe aufR=0,071 undR w =0,064 verfeinert. Ba(UO2)2(PO4)2·6H2O kristallisiert monoklin in der RaumgruppeP1121/a, a=9,789,b=9,882,c=16,868 Å, =89,95° und einem Zellinhalt von vier Formeleinheiten. Die Struktur besteht aus schwach gewellten UO2PO4-Schichten parallel (001), die durch Ba-Atome und H2O-Moleküle miteinander verknüpft sind. Uran besitzt oktaedrische (2+4)-Koordination mit mittleren U-O-Abständen von 1,78 Å für die Uranylsauerstoffatome und 2,28 Å für die Phosphatsauerstoffatome. Die P-O-Abstände der Phosphattetraeder messen im Mittel 1.52 Å. Barium ist von je zwei Uranyl- und Phosphatsauerstoffatomen sowie von fünf Wassermolekülen koordiniert. Die Ba-O-Abstände betragen 2,74–3,11 Å. Von den sechs H2O-Molekülen der Formel sind zwei nicht an Barium gebunden.

With 3 Figures  相似文献   

4.
Zusammenfassung Die Kristallstruktur des Johannits wurde anhand eines verzwillingten Kristalls von Joachimsthal, Böhmen, mit dreidimensionalen Röntgendaten bestimmt und für 2005 unabhängige Reflexe aufR=0,039 verfeinert. Johannit kristallisiert triklin, RaumgruppeP1, mita=8,903 (2),b=9,499 (2),c=6,812 (2) Å, =109,87 (1) =112,01 (1), =100,40 (1)° undV=469,9 Å3. Chemische Formel und Zellinhalt lauten Cu(UO2)2(OH)2(SO4)2·8H2O, das ist um zwei H2O-Moleküle mehr als bisher angenommen. In der Struktur sind pentagonal dipyramidale (UO2)(OH)2O3-Polyeder paarweise über eine von zwei OH-Gruppen gebildete Kante zu Doppelpolyedern und diese wiederum durch SO4-Gruppen zu (UO2)2(OH)2(SO4)2-Schichten parallel (100) verknüpft. Die Schichten sind parallel über gestreckte Cu(H2O)4O2-Oktaeder und Wassermoleküle miteinander verbunden. Folgende Bindungslängen wurden gefunden: U–O=1,78 Å (2x) und 2,34–2,39 Å (5x); Cu–O=1,97 Å (4x) und 2,40 Å (2x); =1,47 Å; O–O in Wasserstoffbrücken 2,71–2,91 Å (8x) und 3,30 Å.
The crystal structure of johannite, Cu(UO2)2(OH)2(SO4)2·8H2O
Summary The crystal structure of johannite has been determined from threedimensional X-ray data measured on a twinned crystal from Joachimsthal, Böhmen, and has been refined toR=0.039 for 2005 independent reflections. Johannite crystallizes triclinic, space groupP1, witha=8.903 (2),b=9.499 (2),c=6.812 (2) Å, =109.87(1), =112.01(1), =100.40 (1)° andV=469.9 Å3. Chemical formula and cell content are Cu(UO2)2(OH)2(SO4)2·8H2O, by two H2O molecules more than previously assumed. Pairs of pentagonal dipyramidal (UO2) (OH)2O3 polyhedra form double polyhedra by edgesharing via two OH groups. The double polyhedra are linked by the SO4 tetrahedra to form layers (UO2)2(OH)2(SO4)2 parallel zu (100). These layers are interconnected parallel toa by elongated Cu(H2O)4O2 octahedra and water molecules. Following bond lengths have been observed: U–O=1.78 Å (2x) and 2.34–2.39 Å (5x); Cu–O=1.97 Å (4x) and 2.40 Å (2x); =1.47 Å; O–O for hydrogen bonds 2.71–2.91 Å (8x) and 3.30 Å.

Mit 2 Abbildungen  相似文献   

5.
Hydrous minerals within the subducting oceanic slab are important hosts for water. Clarification of the stability field of hydrous minerals helps to understand transport and distribution of water from the surface to the Earth’s interior. We investigated the stability of brucite, a prototype of hydrous minerals, by means of electrical conductivity measurements in both open and closed systems at 3 GPa and temperatures up to 1300 K. Dramatic increase of conductivity in association with characteristic impedance spectra suggests that partial dehydration of single-crystal brucite in the open system with a low water fugacity occurs at 950 K, which is about 300 K lower than those previously defined by phase equilibrium experiments in the closed system. By contrast, brucite completely dehydrates at 1300 K in the closed system, consistent with previous studies. Partial dehydration may generate a highly defective structure but does not lead to the breakdown of brucite to periclase and water immediately. Water activity plays a key role in the stability of hydrous minerals. Low water activity (aH2O) caused by the high wetting behavior of the subducted oceanic slab at the transition zone depth may cause the partial dehydration of the dense hydrous magnesium silicates (DHMSs), which significantly reduces the temperature stability of DHMS (this mechanism has been confirmed by previous study on super hydrous phase B). As a result, the transition zone may serve as a ‘dead zone’ for DHMSs, and most water will be stored in wadsleyite and ringwoodite in the transition zone.  相似文献   

6.
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.  相似文献   

7.
The structure and mechanism of cation sorption at the (0 0 1) muscovite-water interface were investigated in 0.01 and 0.5 m KCl, CsCl, and CaCl2 and 0.01 m BaCl2 solutions at slightly acidic pH by high-resolution X-ray reflectivity. Structural relaxations of atom positions in the 2M1 muscovite were small (?0.07 Å) and occurred over a distance of 30 to 40 Å perpendicular to the interface. Cations in all solutions were sorbed dominantly in the first and second solution layers adjacent to the mineral surface. The derived heights of the first solution layer in KCl and CsCl solutions, 1.67(6)-1.77(7) and 2.15(9)-2.16(2) Å, respectively, differ in magnitude by the approximate difference in crystallographic radii between K and Cs, and correspond closely to the interlayer cation positions in bulk K- and Cs-mica structures. The first solution layer heights in CaCl2 and BaCl2 solutions, 2.46(5)-2.56(11) and 2.02(5) Å, respectively, differ in a sense opposite to that expected based on crystallographic or hydrated radii of the divalent cations. The derived ion heights in all solutions imply that there is no intercalated water layer between the first solution layer and the muscovite surface. Molecular compositions were assigned to the first two solution layers in the electron density profiles using models that constrain the number density of sorbed cations, water molecules, and anions by considering the permanent negative charge of the muscovite and average solution density. The models result in partial charge balance (at least 50%) by cations sorbed in the first two layers in the 0.01 m solutions and approximately full charge balance in the 0.5 m solutions. Damped oscillations of model water density away from the first two solution layers agree with previous X-ray reflectivity results on the muscovite (0 0 1) surface in pure water.  相似文献   

8.
Synchrotron X-ray reflectivity (SXR) was used to measure the thickness of the water film that adsorbs on a {} cleavage surface of calcite (CaCO3) in a sample chamber where relative humidity could be controlled within the range from <4% to 90%. Gases used to carry water vapour were initially either 100% N2 or 100% CO2. The product water film was remarkably constant in thickness at 15.5 Å (±1 Å) and independent of humidity. When N2 was used as the carrier gas, this film displayed a gap in its electron density at between 0.6 and 2 Å distance from the calcite surface, depending on humidity. This implies that a change in the arrangement of water molecules occurs in direct proximity to the surface. This electron density discontinuity was measurably further from the calcite surface, at 3.4 Å, when CO2 was used as the carrier gas. Except for this thin low density region proximate to the calcite surface, the density of the adsorbed water layer was 0.9 g cm−3, therefore suggesting a significant degree of ordering. Atomic force microscopy (AFM) images were completed in conjunction with the SXR measurements on similarly prepared calcite cleavage surfaces. AFM showed that terraces may be atomically flat over 1 μm or more. SXR corroborated this observation, with results showing that carefully cleaved surfaces had a starting root mean square (r.m.s.) roughness of ∼1.2 Å. Diffuse scatter measurements constrained the correlation lengths of these surfaces to be at least 18,000 Å. For comparison with the cleaved samples, a surface was also prepared by chemo-mechanical Syton polishing. This surface gave an r.m.s roughness by SXR that was an order of magnitude higher, equal to 12.1 Å. In this case, diffuse scatter resolved a correlation length of 950 Å, and revealed a fractal dimension that was higher than for the cleaved surface. On Syton polished samples, the water film determined by SXR was about twice as thick as for freshly cleaved surfaces, with a density of 1.0 g cm−3, equal to that of bulk water. However, surface roughness was too large to allow resolution of any gap in the electron density within the water layer proximate to the solid surface. Our AFM observations also confirm previous reports of calcite surface recrystallization. The electron density of the solid surface determined by SXR is indistinguishable from that of calcite, indicating that any material recrystallized within the adsorbed water film is compositionally indistinguishable from the calcite substrate.  相似文献   

9.
Zusammenfassung Hydrothermalsynthese von Be2BO3OH.H2O und Strukturbestimmung der Verbindung nach der trial and error. Methode aus Pulveraufnahmen wurden durchgeführt. Be2BO3OH.H2O krístallisiert in der Raumgruppe P321-D 3 2 a0=4,43 Å, c0=5,34 Å, und repräsentiert einen neuen Strukturtyp mit ausgeprägtem Schichtcharakter. Bor ist von drei Sauerstoffen koordiniert, Beryllium nahezu tetraedrisch von drei Sauerstoffen und einem (OH, H2O). Zwischen letzteren sind Wasserstoffbrücken von 2,72Å Länge ausgebildet; diese bewirken die Bindung zwischen den Schichten.
Summary Be2BO3OH.H2O was synthesized under hydrothermal conditions, and its crystal structure was determined from powder data by the trial and error-method. The compound crystallizes in space group P 321-D 3 2 a0=4,43 Å, c0=5,34 Å, and represents a new structure type with pronounced layers. Boron is coordinated by three oxygens, beryllium almost tetrahedrally by three oxygens and one (OH, H2O). Between the latters, there are hydrogen bridges of 2,72 Å length which cause the chemical bonds between the layers.

Mit 1 Textabbildung  相似文献   

10.
Monte Carlo simulations show that the adsorption position of the Sr2+ or Ba2+ ion on the cleaved muscovite surface is determined by the radius of the ion’s first hydration shell, hydrogen bonding of the first shell water molecules with the basal oxygens of muscovite as well as a requirement of minimization of the number of muscovite’s lattice cations in the ion’s first coordination shell. Accordingly, Sr2+ or Ba2+ adsorbs in ditrigonal cavities at a distance of 1.12 Å or 1.35 Å, respectively, from the basal surface on dehydrated mica and above tetrahedral substitutions at a height of 1.93 ± 0.02 Å or 2.15 ± 0.03 Å, respectively, at the highest simulated water coverage of 28 H2O per ion. The ion’s displacement from a ditrigonal cavity occurs upon adsorption of 2 H2O per ion for Sr2+ and 3 H2O per ion for Ba2+. At a coverage of 28 H2O per ion, outer-sphere adsorption of Sr2+ or Ba2+ at a height of 3.9 ± 0.2 Å or 4.17 ± 0.07 Å, respectively, is possible albeit unfavorable on the free energy scale by 107 ± 7 kJ/mol or 89 ± 13 kJ/mol, respectively, as compared to inner-sphere adsorption. Activation energies for the transformation between inner-sphere and outer-sphere adsorptions are calculated to be 121 ± 3 kJ/mol for Sr2+ and 99 ± 10 kJ/mol for Ba2+. A comparison of these values with those reported recently for Mg2+ and Ca2+ results in an adsorption affinity sequence Mg2+ > Ca2+ > Sr2+ > Ba2+ in agreement with the sequence predicted recently for low dielectric constant solids (which include mica) (Sverjensky, 2006). A recent resonant anomalous X-ray reflectivity study of Sr2+ adsorption on muscovite (Park et al., 2006) has questioned the common assumption (Stumm, 1992), which is supported by the present simulation results, that inner-sphere adsorption is stronger than outer-sphere adsorption. A modification of the cleaved muscovite surface as a result of Sr2+ adsorption in muscovite’s ditrigonal cavities and related destabilization of muscovite’s hydroxyl groups is proposed as a possible reason for this controversy.  相似文献   

11.
This paper describes the first synthesis method of yukonite, its thorough molecular and structural analysis along with natural specimens originating from Tagish Lake (Canada) and Grotta della Monaca (Italy) for comparison, and its structural relation to arseniosiderite. The synthetic and natural yukonites were found to have a range of composition according to the general formula Ca2Fe3-5(AsO4)3(OH)4-10·xH2O where x = 2-11. The synthetic yukonite was found to be equivalent at the atomic, molecular and structural level to the Tagish Lake yukonite. At the molecular level, arseniosiderite, via vibrational spectroscopy, was found to have a H-bonding system as in scorodite and exhibit an extra arsenate mode indicative of groups. Heating experiments along with ATR-IR analysis indicated the presence of structural water and hydroxyl units in arseniosiderite. In yukonite in contrast, a wide diffuse H-bonding environment was observed with only arsenate groups. The presence of both structural water and hydroxyl groups was further verified via ATR-IR spectroscopy. The As K, Fe 2p and Ca 2p XANES spectra of yukonite and arseniosiderite were found to be identical, confirming that the local nature of the As, Fe and Ca atoms in these structures is the same. Diffraction analysis (X-ray and electron) showed that yukonite consist of nano-crystalline domains while in the case of arseniosiderite micro-size single crystal domains exist.  相似文献   

12.
Biogeochemical cycling of zinc is strongly influenced by sorption on birnessite minerals (layer-type MnO2), which are found in diverse terrestrial and aquatic environments. Zinc has been observed to form both tetrahedral (ZnIV) and octahedral (ZnVI) triple-corner-sharing surface complexes (TCS) at Mn(IV) vacancy sites in hexagonal birnessite. The octahedral complex is expected to be similar to that of Zn in the Mn oxide mineral, chalcophanite (ZnMn3O7·3H2O), but the reason for the occurrence of the four-coordinate Zn surface species remains unclear. We address this issue computationally using spin-polarized density functional theory (DFT) to examine the ZnIV-TCS and ZnVI-TCS species. Structural parameters obtained by DFT geometry optimization were in excellent agreement with available experimental data on Zn-birnessites. Total energy, magnetic moments, and electron overlap populations obtained by DFT for isolated ZnIV-TCS revealed that this species is stable in birnessite without a need for Mn(III) substitution in the octahedral sheet and that it is more effective in reducing undersaturation of surface O at a Mn vacancy than is ZnVI-TCS. Comparison between geometry-optimized ZnMn3O7·3H2O (chalcophanite) and the hypothetical monohydrate mineral, ZnMn3O7·H2O, which contains only tetrahedral Zn, showed that the hydration state of Zn significantly affects birnessite structural stability. Finally, our study also revealed that, relative to their positions in an ideal vacancy-free MnO2, Mn nearest to Zn in a TCS surface complex move toward the vacancy by 0.08-0.11 Å, while surface O bordering the vacancy move away from it by 0.16-0.21 Å, in agreement with recent X-ray absorption spectroscopic analyses.  相似文献   

13.
Garnets in UHP eclogites from Bixiling in Dabieshan were investigated by Fourier transform infrared spectroscopy (FTIR). The results indicate that all garnets contain structural water that occurs as hydroxyl (OH−) and non-structural molecular water (H2O) possibly in the form of sub-microscopic fluid inclusions. The structural hydroxyl contents range from 92 to 1735 ppm (H2O wt.) and most are between 200 and 1000 ppm. Therefore, garnet in eclogite can recycle surface water into the mantle. Various water contents were observed among different samples of the same outcrop (∼150 m) and in different domains of the same sample (∼1 cm). This variability in structural H2O contents suggests that the mobility of fluids during UHP metamorphism was very limited, and that both subduction and exhumation processes of UHP rocks occurred in a short time interval.  相似文献   

14.
Zusammenfassung Röntgenbeugungsaufnahmen des Artinits zeigen diffuse Schichtlinien für k=ungerade. Die vorläufige Strukturbestimmung vonde Wolff, die nur die scharfen Reflexe berücksichtigt, wurde verfeinert. Die Struktur besteht danach aus Mg(OH)3(H2O)2O-Oktaedern, die über Kanten zu Ketten mit angehängten Karbonatgruppen verknüpft sind. H2O und CO3 alternieren statistisch in der Kette. Die elektrisch neutralen Ketten sind durch Wasserstoffbrückenbindung zwischen den Karbonatgruppen und Kristallwasser verbunden.Die Fehlordnung kann als eine Lagenstatistik der Karbonatgruppen und eines der drei Kristallwasser beschrieben werden, die geordnete Ketten mit doppelter Identitätsperiode und einer zufälligen Lage bilden. Die Theorie der Röntgenbeugung an solchen Anordnungen wird allgemein diskutiert, und die Gleichungen, die das Verhalten der diffusen Schichtlinie beschreiben, werden abgeleitet. Nach den vorliegenden Untersuchungen gibt es zwei Ordnungsvorgänge mit versehiedenen Wechselwirkungsenergien. Der eine Prozeß führt zu geordneten Schichten, deren gegenseitige Lage rein zufällig ist, der andere, schwerer zu übersehende Ordnungsvorgang ist durch Wechselwirkungen großer Reichweite bestimmt und weicht bemerkenswert von den Bauprinzipien der geordneten Schicht ab.
Summary X-ray diffraction photographs of Artinite exhibit weak diffuse layer-lines for k=odd. The preliminary structure determination byde Wolff, considering the sharp reflection only, has been refined. The structure consists of Mg(OH)3(H2O)2O-octahedra, linked over edges to chains with attached CO3-groups, which alternate statistically with H2O. The electrically neutral chains are linked by hydrogen bonds between the water molecules and the CO3-groups.The disorder phenomenon can be described as statistics of positions of the Carbonate groups and one of the 3 water molecules, which form strictly ordered chains with a doubled identy period and a random position. The theory of X-ray diffraction of such arrangements is generally discussed and some formulae for the diffuse layers in reciprocal space are given. Two ordering processes are found experimentally, which cannot be explained in terms of similar interactions. One of these ordering principles leads to ordered layers arranged at random with respect to their mutual positions. Long range interactions determine the second, more complex ordering process, which shows some remarkable deviations from the disordered layer structure.

Mit 12 Textabbildungen

Herrn Professor Dr.F. Machatschki zum 70. Geburtstag gewidmet.  相似文献   

15.
The sediments in the Salford Quays, a heavily-modified urban water body, contain high levels of organic matter, Fe, Zn and nutrients as a result of past contaminant inputs. Vivianite [Fe3(PO4)2 · 8H2O] has been observed to have precipitated within these sediments during early diagenesis as a result of the release of Fe and P to porewaters. These mineral grains are small (<100 μm) and micron-scale analysis techniques (SEM, electron microprobe, μ-EXAFS, μ-XANES and Raman) have been applied in this study to obtain information upon the structure of this vivianite and the nature of Zn uptake in the mineral. Petrographic observations, and elemental, X-ray diffraction and Raman spectroscopic analysis confirms the presence of vivianite. EXAFS model fitting of the FeK-edge spectra for individual vivianite grains produces Fe–O and Fe–P co-ordination numbers and bond lengths consistent with previous structural studies of vivianite (4O atoms at 1.99–2.05 Å; 2P atoms at 3.17–3.25 Å). One analysed grain displays evidence of a significant Fe3+ component, which is interpreted to have resulted from oxidation during sample handling and/or analysis. EXAFS modelling of the Zn K-edge data, together with linear combination XANES fitting of model compounds, indicates that Zn may be incorporated into the crystal structure of vivianite (4O atoms at 1.97 Å; 2P atoms at 3.17 Å). Low levels of Zn sulphate or Zn-sorbed goethite are also indicated from linear combination XANES fitting and to a limited extent, the EXAFS fitting, the origin of which may either be an oxidation artifact or the inclusion of Zn sulphate into the vivianite grains during precipitation. This study confirms that early diagenetic vivianite may act as a sink for Zn, and potentially other contaminants (e.g. As) during its formation and, therefore, forms an important component of metal cycling in contaminated sediments and waters. Furthermore, for the case of Zn, the EXAFS fits for Zn phosphate suggest this uptake is structural and not via surface adsorption.  相似文献   

16.
 H2O activities in concentrated NaCl solutions were measured in the ranges 600°–900° C and 2–15 kbar and at NaCl concentrations up to halite saturation by depression of the brucite (Mg(OH)2) – periclase (MgO) dehydration equilibrium. Experiments were made in internally heated Ar pressure apparatus at 2 and 4.2 kbar and in 1.91-cm-diameter piston-cylinder apparatus with NaCl pressure medium at 4.2, 7, 10 and 15 kbar. Fluid compositions in equilibrium with brucite and periclase were reversed to closures of less than 2 mol% by measuring weight changes after drying of punctured Pt capsules. Brucite-periclase equilibrium in the binary system was redetermined using coarsely crystalline synthetic brucite and periclase to inhibit back-reaction in quenching. These data lead to a linear expression for the standard Gibbs free energy of the brucite dehydration reaction in the experimental temperature range: ΔG° (±120J)=73418–134.95T(K). Using this function as a baseline, the experimental dehydration points in the system MgO−H2O−NaCl lead to a simple systematic relationship of high-temperature H2O activity in NaCl solution. At low pressure and low fluid densities near 2 kbar the H2O activity is closely approximated by its mole fraction. At pressures of 10 kbar and greater, with fluid densities approaching those of condensed H2O, the H2O activity becomes nearly equal to the square of its mole fraction. Isobaric halite saturation points terminating the univariant brucite-periclase curves were determined at each experimental pressure. The five temperature-composition points in the system NaCl−H2O are in close agreement with the halite saturation curves (liquidus curves) given by existing data from differential thermal analysis to 6 kbar. Solubility of MgO in the vapor phase near halite saturation is much less than one mole percent and could not have influenced our determinations. Activity concentration relations in the experimental P-T range may be retrieved for the binary system H2O-NaCl from our brucite-periclase data and from halite liquidus data with minor extrapolation. At two kbar, solutions closely approach an ideal gas mixture, whereas at 10 kbar and above the solutions closely approximate an ideal fused salt mixture, where the activities of H2O and NaCl correspond to an ideal activity formulation. This profound pressure-induced change of state may be characterized by the activity (a) – concentration (X) expression: a H 2O=X H 2O/(1+αX NaCl), and a NaCl=(1+α)(1+α)[X NaCl/(1+αX NaCl)](1+α). The parameter α is determined by regression of the brucite-periclase H2O activity data: α=exp[A–B/ϱH 2O ]-CP/T, where A=4.226, B=2.9605, C=164.984, and P is in kbar, T is in Kelvins, and ϱH 2O is the density of H2O at given P and T in g/cm3. These formulas reproduce both the H2O activity data and the NaCl activity data with a standard deviation of ±0.010. The thermodynamic behavior of concentrated NaCl solutions at high temperature and pressure is thus much simpler than portrayed by extended Debye-Hückel theory. The low H2O activity at high pressures in concentrated supercritical NaCl solutions (or hydrosaline melts) indicates that such solutions should be feasible as chemically active fluids capable of coexisting with solid rocks and silicate liquids (and a CO2-rich vapor) in many processes of deep crustal and upper mantle metamorphism and metasomatism. Received: 1 September 1995 / Accepted: 24 March 1996  相似文献   

17.
 An experimental technique to make real-time observations at high pressure and temperature of the diamond-forming process in candidate material of mantle fluids as a catalyst has been established for the first time. In situ X-ray diffraction experiments using synchrotron radiation have been performed upon a mixture of brucite [Mg(OH)2] and graphite as starting material. Brucite decomposes into periclase (MgO) and H2O at 3.6 GPa and 1050 °C while no periclase is formed after the decomposition of brucite at 6.2 GPa and 1150 °C, indicating that the solubility of the MgO component in H2O greatly increases with increasing pressure. The conversion of graphite to diamond in aqueous fluid has been observed at 7.7 GPa and 1835 °C. Time-dependent X-ray diffraction profiles for this transformation have been successfully obtained. Received: 17 July 2001 / Accepted: 18 February 2002  相似文献   

18.
Summary Crystals of K2[Co2(SeO3)3]-2H2O and K2[Ni2(SeO3)3]-2H2O were synthesized under low-hydrothermal conditions. Their structures were determined using single crystal X-ray data up to sin / = 0.7Å-1. [Space group P63/m; a = 9.091(3),9.016(2)Å; c = 7.562(2), 7.476(2)Å; Z = 2; RW = 1.6, 2.5%]. The investigations confirmed that K2[Co2(SeO3)3].2H2O and K2[Ni2(SeO3)3]-2H2O represent the first selenites belonging to the zemannite structure type, a framework structure with wide channels running parallel [0001]. In both compounds four maxima were clearly located in the channel by Fourier summations and attributed to two K atoms and two H2O molecules, each with an occupancy factor of 1/6; a possible ordering scheme (full occupancy) with local symmetry 1 and [6]-coordinated K atoms could be derived for the channel atoms.Zusammenfassung Kristalle von K2[Co2(SeO3)3]-2H2O und K2[Ni2(SeO3)3]-2H2O wurden unter niedrig-hydrothermalen Bedingungen synthetisiert. Die Strukturen wurden unter Verwendung von Einkristallröntgendaten bis sin /= 0.7Å-1 bestimmt. [Raumgruppe P63/m; a = 9.091(3), 9.016(2)Å; c = 7.562(2), 7.476(2)Å; Z = 2; RW = 1.6, 2.5%] Die Untersuchungen bestätigten, daß K2[Co2(SeO3)3] - 2H2O und K2 [Ni2(SeO3)3] - 2H2O als erste Selenite dem Strukturtyp des Zemannits angehören, einer Gerüststruktur mit weiten, parallel [0001] verlaufenden Kanälen. In beiden Verbindungen wurden im Kanal vier Maxima durch Fourier-Summationen eindeutig lokalisiert und zwei Kalium-atomen sowie zwei H2O Molekülen, jeweils mit einem Besetzungsfaktor von 1/6, zugeschrieben. Für die Kanalatome konnte ein möglicher Ordnungszustand (volle Besetzung) mit lokaler Symmetrie 1 und [6]-koordinierten Kaliumatomen abgeleitet werden.
Selenite des Zemannittyps: Kristallstrukturen von K2[Co2(SeO3)3] - 2H2O und K2[Ni2(SeO3)3]-2H2O

Dedicated to Prof. Dr. Josef Zemann at the occasion of his 70th birthday

With 2 Figures  相似文献   

19.
Zusammenfassung Rhomboklas, FeH(SO4)2.4H2O, wurde aus einer mit Salpetersäure oxidierten, schwefelsauren, wäßrigen Eisen(II)-sulfatlösung hergestellt. Die Be stimmung der Gitterkonstanten ergab: a0=9,785 Å, b0=18,363 Å, c0=5,431 Å; Raumgruppe: P nma oder P n2a, Z=4.
Lattice constants and space-group of rhomboclase, FeH(SO4)2.4H2O
Summary Rhomboclase, FeH(SO4)2.4H2O was precipitated from a solution of ferrous sulphate in water by oxydation with HNO3 and the addition of excess H2SO4. The lattice constants are a0=9.785 Å, b0=18.363 Å, c0=5.431 Å; space-group P nma or P n2a, Z=4.
  相似文献   

20.
Relative humidity ( P\textH 2 \textO P_{{{\text{H}}_{ 2} {\text{O}}}} , partial pressure of water)-dependent dehydration and accompanying phase transitions in NAT-topology zeolites (natrolite, scolecite, and mesolite) were studied under controlled temperature and known P\textH 2 \textO P_{{{\text{H}}_{ 2} {\text{O}}}} conditions by in situ diffuse-reflectance infrared Fourier transform spectroscopy and parallel X-ray powder diffraction. Dehydration was characterized by the disappearance of internal H2O vibrational modes. The loss of H2O molecules caused a sequence of structural transitions in which the host framework transformation path was coupled primarily via the thermal motion of guest Na+/Ca2+ cations and H2O molecules. The observation of different interactions of H2O molecules and Na+/Ca2+ cations with host aluminosilicate frameworks under high- and low- P\textH 2 \textO P_{{{\text{H}}_{ 2} {\text{O}}}} conditions indicated the development of different local strain fields, arising from cation–H2O interactions in NAT-type channels. These strain fields influence the Si–O/Al–O bond strength and tilting angles within and between tetrahedra as the dehydration temperature is approached. The newly observed infrared bands (at 2,139 cm−1 in natrolite, 2,276 cm−1 in scolecite, and 2,176 and 2,259 cm−1 in mesolite) result from strong cation–H2O–Al–Si framework interactions in NAT-type channels, and these bands can be used to evaluate the energetic evolution of Na+/Ca2+ cations before and after phase transitions, especially for scolecite and mesolite. The 2,176 and 2,259 cm−1 absorption bands in mesolite also appear to be related to Na+/Ca2+ order–disorder that occur when mesolite loses its Ow4 H2O molecules.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号