Number Of Bridging Oxygen Atoms Research Articles

Graphene Oxide-Enhanced Nucleation and Growth of Calcium-Silicate-Hydrate Gel at Nanoscale: A Molecular Dynamics Study.

Graphene oxide (GO) enhances the performance of cement-based materials by optimizing the microstructure of calcium-silicate-hydrate (C-S-H). However, the influence of GO on the nucleation and growth of C-S-H gel at nanoscale is unexplored. This study investigates this mechanism by molecular dynamics simulation at nano scale. Results show that GO can reduce the activation energy during the polymerization reaction of silicon oxide tetrahedra during the reaction process, and can increase the content of polymer Q3 and Q4. The influence of GO with epoxy (-O-), hydroxyl (-OH) and carboxyl (-COOH) groups on the radial distribution function (RDF), mean square displacement (MSD), and atomic spatial distribution of monomers are studied. Results show that GO-OH exhibits excellent performance, with the highest number of bridging oxygen atoms (about 0.6), the lowest Q0 monomer content (just 26.8%), the highest RDF (27.18), and the highest MSD (calcium and silicon content around 20,000 Å2). This paper elucidates the nucleation and growth mechanism of C-S-H influenced by GO to develop high performance cement.

Structure and dynamic viscosity of CaO–SiO2 and CaO–SiO2–B2O3 model slag systems

Correlation dependencies between the dynamic viscosity of slag and its structural parameters were studied to determine an optimal basicity of silicon smelting slag under the addition of boron oxide to eliminate slagging of the bottom of ore-smelting furnaces. Experimental studies were conducted on CaO–SiO2 and CaO– SiO2–B2O3 model slags obtained at 1600°С. Raman spectroscopic analysis was carried out using a Horiba JobinYvon HR800UV analyzer (France). Theoretical calculations of slag viscosity were performed using Urbain and Mills models. During the experiments, the key structural parameters of slag systems varied within the following limits: the experimental Raman spectrum deconvolution function from 1.41 to 2.45 and optical basicity from 0.58 to 0.68. The obtained experimental and theoretical data were related by mathematical dependencies. It was found that the dynamic viscosity of slag can be promptly determined by Raman spectroscopy on the basis of mathematical models. The dependence obtained shows that slag viscosity decreases upon an increase in the number of bridging oxygen atoms in the silicate anion structure. Notably, this decrease in slag viscosity is observed up to the value of the experimental Raman spectrum deconvolution function of ~1.55-1.60 or slag optical basicity of 0.60–0.62. When B2O3 is added, the viscosity undergoes a further decrease. In practice, for CaO–SiO2 slag systems, the use of boroncontaining flux as a liquefying agent is reasonable at CaO/SiO2 = 0.61–0.63 while maintaining the content of B2O3 in the slag at a level of 1%. The two models (classical and modified) proposed by Urbain were established to be more suitable for theoretical calculation of viscosity in CaO–SiO2 and CaO–SiO2–B2O3 systems. Mills’ model is not suitable for these purposes, since the correlation coefficients in the corresponding mathematical model are not sufficiently large. Further research in this direction is required in order to establish appropriate dependencies of slag viscosity on its structural parameters at different temperatures.

Poisson's Ratio of Glasses, Ceramics, and Crystals.

Poisson's ratio is the fundamental metric used to discuss the performance of any material when strained elastically. However, the methods of the determination of Poisson's ratio are not yet discussed well. The first purpose of this paper is to introduce the five kinds of typical experimental methods to measure Poisson's ratio of glasses, ceramics, and crystals. The second purpose is to discuss the experimental results on the variation of Poisson's ratio by composition, temperature, and pressure reviewed for various glasses, ceramics, and crystals, which are not yet reviewed. For example, in oxide glasses, the number of bridging oxygen atoms per glass-forming cation provides a straightforward estimation of network crosslinking using Poisson's ratio. In the structural-phase transition of crystals, Poisson's ratio shows remarkable temperature-dependence in the vicinity of a phase-transition temperature. The mechanism of these variations is discussed from physical and chemical points of view. The first-principles calculation of Poisson's ratio in the newly hypothesized compounds is also described, and its pressure-induced ductile-brittle transition is discussed.

Structure of amorphous materials in the NASICON system NaTi2Si x PO12

The structure of glasses in the sodium (Na) super-ionic conductor (NASICON) system NaTi2Si x PO12 with x = 0.8 and x = 1.0 was explored by combining neutron and high-energy x-ray diffraction with 29Si, 31P and 23Na solid-state nuclear magnetic resonance (NMR) spectroscopy. The 29Si magic angle spinning (MAS) NMR spectra reveal that the silica component remains fully polymerized in the form of Si4 units, i.e. the silicon atoms are bound to four bridging oxygen atoms. The 31P{23Na} rotational echo adiabatic passage double resonance (REAPDOR) NMR data suggest that the 31P MAS NMR line shape originates from four-coordinated P n units, where n = 1, 2 or 3 is the number of bridging oxygen atoms per phosphorus atom. These sites differ in their 31P-23Na dipolar coupling strengths. The results support an intermediate range order scenario of a phosphosilicate mixed network-former glass in which the phosphate groups selectively attract the Na+ modifier ions. Titanium takes a sub-octahedral coordination environment with a mean Ti–O coordination number of 5.17(4) for x = 0.8 and 4.86(4) for x = 1.0. A mismatch between the P–O and Si–O bond lengths of 8% is likely to inhibit the incorporation of silicon into the phosphorus sites of the NASICON crystal structure.

Vibrational characteristics of aluminum–phosphate compounds by an experimental and theoretical approach

Aluminum phosphates are materials with relatively wide potential applications in many industries. The vibrational features of selected compounds were established on Raman and infrared spectroscopy. The experimentally determined spectra are compared to those calculated by ab initio methods. This gives a unique possibility of a proper assignment of the experimental spectral features to specific modes of vibration. In the results, it was evidenced that the spectra are characterized by two specific intense bands in the mid- and high-frequency range due to the P–O–P and P–O bonds in [PO4] tetrahedron vibrations. The position of the high-frequency band is related to the number of bridging oxygen atoms connecting [PO4] tetrahedrons in the unit cell. Additionally, the differences in the spectra were evidenced as a result of different polymorphic forms of the selected compounds. Therefore, the results may be useful in determining the phase composition of polyphase materials or structural features of aluminum–phosphate glasses and glass–ceramic materials.

Discoveries about the structure of alkaline earth-bearing borosilicate glasses doped with TiO2 revealed by Raman spectroscopy

The influence of TiO2 on the glass structure in the CaO-SrO-B2O3-SiO2 (CSrBS) system was examined by Raman spectroscopy. The Raman spectra of these glasses presented bending vibrational modes and stretching vibration of Qn species (where n is the number of bridging oxygen atoms shared between tetrahedra), leading to the identification of Q1, Q2, Q2–A, and Q3 species. The addition of Ti produced two new Raman bands at 756 and 825 cm−1 which were attributed, respectively, to the stretching vibrations of Ti-ONBO and Ti-OBO. Our results suggest that titanium is coordinated mainly in Ti[4] species, with negligible amounts of Ti[5] units. The intensity of Si-O-Si bending bands shows a decrease in intensity, which is attributed to the restriction of bending motions, possibly due to titanium bonding to the network structure through both vertices and edges.

Structure of crystalline and amorphous materials in the NASICON system Na1+xAlxGe2-x(PO4)3.

The structure of crystalline and amorphous materials in the sodium (Na) super-ionic conductor system Na1+xAlxGe2-x(PO4)3 with x = 0, 0.4, and 0.8 was investigated by combining (i) neutron and x-ray powder diffraction and pair-distribution function analysis with (ii) 27Al and 31P magic angle spinning (MAS) and 31P/23Na double-resonance nuclear magnetic resonance (NMR) spectroscopy. A Rietveld analysis of the powder diffraction patterns shows that the x = 0 and x = 0.4 compositions crystallize into space group-type R3̄, whereas the x = 0.8 composition crystallizes into space group-type R3̄c. For the as-prepared glass, the pair-distribution functions and 27Al MAS NMR spectra show the formation of sub-octahedral Ge and Al centered units, which leads to the creation of non-bridging oxygen (NBO) atoms. The influence of these atoms on the ion mobility is discussed. When the as-prepared glass is relaxed by thermal annealing, there is an increase in the Ge and Al coordination numbers that leads to a decrease in the fraction of NBO atoms. A model is proposed for the x = 0 glass in which super-structural units containing octahedral Ge(6) and tetrahedral P(3) motifs are embedded in a matrix of tetrahedral Ge(4) units, where superscripts denote the number of bridging oxygen atoms. The super-structural units can grow in size by a reaction in which NBO atoms on the P(3) motifs are used to convert Ge(4) to Ge(6) units. The resultant P(4) motifs thereby provide the nucleation sites for crystal growth via a homogeneous nucleation mechanism.

Auto-Radiation Damages in Zircons of the Yastrubtsi Ore Occurrence of the Ukrainian Shield According to the Data of NMR and XRD

By the method of XRD and high-resolution nuclear magnetic resonance (MAS NMR), the features of the structure of partially metamict zircons from Yastrubtsi ore occurrence of the Ukrainian Shield were studied. Samples 1 and 2 (depths 627 and 1069 m) belong to middle metamict zircons, and sample 3 (depth 1302.5 m) — to weak metamict zircons. The purpose of this work was to elucidate the features of the structure of partially metamictic zircons from Yastrubtsi ore occurrence of the Ukrainian Shield and the features of their recrystallization. The XRD method showed the presence of diffuse diffraction bands, which indicates the presence of an amorphous phase. For sample 3, these bands are less pronounced. After heating at T = 1100°C, these bands significantly decrease. Based on the XRD data, the degree of crystallinity and unit cell parameters of the samples were calculated. To obtain information on the local structure of the samples, spectra were obtained for the initial and annealed samples. It was shown by MAS NMR that there are regions in the initial structure in which [SiO4] tetrahedra are interconnected. These regions are designated Qn (n is the number of bridging oxygen atoms per Si atom). It has been established that the silica phase is practically absent in the zircon samples. In such samples, mainly Q1-2 and Q3 polymerization of silicon atoms occurs (for sample 3, only Q1-2 polymerization is characteristic). The MAS NMR results are in good agreement with the XRD data.

Polymerization during melting of ortho- and meta-silicates: Effects on Q species stability, heats of fusion, and redox state of mid-ocean range basalts (MORBs)

Abstract29Si NMR and Raman spectroscopic studies demonstrate that fusion of crystalline orthosilicates and metasilicates produces melts more polymerized than their precursor crystals. Forsterite, for example, consists of 100% Q0 species, whereas its melt consists of ~50 mol% of Q1 species (Q = a Si tetrahedron and the superscript indicates the number of bridging oxygen atoms in the tetrahedron). Polymerization during melting can be rationalized from an energetics perspective. Si-NBO-M moieties of Q species are more susceptible to librational, rotational, and vibrational modes than are Si-BO-Si moieties (NBO = non-bridging oxygen; BO = bridging oxygen; M = counter cation). Thermal agitation activates these additional modes, thus increasing the CP and free energy of melts. The reaction of Qn to Qn+1 species during melting eliminates Si-NBO-M moieties and produces Si-O-Si moieties that are less susceptible to the additional modes, thereby minimizing the CP of melts. By decreasing the abundances of Q0, Q1, and Q2 species in favor of Q3 and Q4 species, melts become more stable. In the absence of polymerization, melting temperatures of minerals would be appreciably greater than observed.Polymerization involves formation of Si-O bonds, which are strongly endothermic (Si-O bond dissociation is ~798 kJ/mol). The large heats of fusion (ΔHf) of orthosilicates result primarily from polymerization reactions during melting (ΔHf of forsterite, fayalite, and tephroite are ~142, ~92, and ~90 kJ/mol). The fusion of metasilicates and sorosilicates (e.g., pyroxenes and melilites) involves endothermic polymerization and exothermic depolymerization reactions, although the former dominates. These reactions tend to negate each other during melting, yielding less positive ΔHf values than observed for orthosilicate fusion (e.g., ΔHf of enstatite, diopside, pseudowollastonite, and åkermanite are ~73, ~69, ~57, and ~62 kJ/mol). Where polymerization and depolymerization reactions are absent ΔHf is low and is due mostly to disordering during melting (e.g., ΔHf of cristobalite iŝ8.9 kJ/mol).Experimental evidence indicates that ferric iron is present as a negatively charged oxy-anionic complex in melts (e.g., [FeO2]1–) so that oxidation of Fe2+ should proceed according to: 4Femelt2+ + 1O2 + 6Omelt2−→4[FeO2]melt1−.Free oxygen (O2–), a by-product of polymerization reactions, drives the reaction to the right. Midocean ridge basalts (MORBs) consequently should be more oxidized than their source (e.g., lherzolites) or their residues (e.g., harzburgites). Extraction of melt from the upper mantle and deposition in the crust should produce a crust more oxidized than its upper mantle source. Production of O2– during melting and its presence in alkali-rich magmas also explains the alkali-ferric iron effect.

Study of the Properties of Oxide Melts in a Model Experiment

The relative number of bridging oxygen atoms in Si–O–Si bonds at α = 0.7 linearly decreases in the case of all modifiers. The modifiers are arranged in the following order in decreasing modifier mass loss rate: MgO, Na2O, Li2O, and CaO.

Factors affecting line shapes and intensities of Q3 and Q4 Raman bands of Cs silicate glasses

The Raman spectra of glasses containing 0 to 30 mol% Cs2O have been fit successfully with line shapes of dominantly Lorentzian character for the Q3 species, allowing quantification of Q3 and Q4 species intensities (Q represents a Si tetrahedron and the superscript indicates the number of bridging oxygen atoms, BOs, bonded to Si.) The intensity of the Q4 species A1 symmetric stretch is exceptionally weak in vitreous silica (v-SiO2) but it increases dramatically with addition of small amounts of Cs2O to the glass. We propose that Cs, where in close proximity to BO of Q4 species, promotes the polarizability of Q4 tetrahedra and these primed Q4 species (Q4-p) produce a strong Q4 signal. There are, therefore, two variants of the Q4 species, a Q4-p species which produces a strong signal, and an unprimed species (Q4-u) which yields a very weak signal. The increase in the abundance of the primed Q4 species (Q4-p) can be modelled as a function of alkali content using a simple, upper-bounded growth model:XQ4-p = (1 − e−kx)where XQ4-p is the fraction of polarizable Q4 species, k is a constant and x is the mol% counter oxide in the glass (here Cs2O). Comparison of calculated XQ4-p values with experimental results indicates that its cross-section is similar to that of the Q3 species.There is no evidence for a ~1050 cm−1 band in the 5 mol% Cs2O glass spectrum but in the 30 mol% Cs2O glass spectrum about 11% of spectral intensity is observed at about this frequency. The intensity likely results from development of asymmetry on the Q3 band, which increases with Cs2O content of the glass. The asymmetry results from weakened SiO force constants of some Q3 tetrahedra due to charge transfer via CsBO bonds. As evidence, Si 2p and O 1s X-ray Photoelectron Spectroscopic (XPS) studies demonstrate that the electron density over Si and BO atoms of Q4 species increases with Cs2O content. With charge transfer to tetrahedra, the negative charge accumulates preferentially on Si atoms thus decreasing SiO coulombic interactions, weakening SiO force constants, and shifting the Q3 A1 symmetric stretch vibrational frequencies to lower values (e.g., from ~1100 cm−1 to ~1050 cm−1). The fraction of affected Q3 species increases with alkali content, as does the Q3 peak asymmetry. The Raman shifts of the Q4 species are also affected by increased Cs2O contents.

Viscosity Measurement and Structure Analysis of Cr2O3-Bearing CaO-SiO2-MgO-Al2O3 Slags

In this study, the effects of different Cr2O3 contents and optical basicity (denoted by Λ) on the viscosity and structure of the Cr2O3-bearing CaO-SiO2-MgO-Al2O3 slag were investigated. The viscosities of Cr2O3-bearing CaO-SiO2-MgO-Al2O3 slags in the liquid phase below 1823 K (1550 °C) were measured by rotating-cylinder method, and the structures of the slags were examined via Raman spectroscopy. Three different parameters were used to characterize the structures of the slags. The results showed that the viscosity of the slags increased as the Cr2O3 content increased, but decreased as Λ increased. The Cr3+ ions acted as network formers and increased the degree of polymerization (DOP), and thus, the addition of Cr2O3 to the slag increased the number of bridging oxygen atoms in the silicate structural units. Generally, the viscosity increased by increasing DOP. In addition, there was a linear inverse relationship between the viscous activation energy (Eμ) and Λ. Furthermore, as the Cr2O3 content increased, the gradients of the plots of Eμ vs Λ decreased. This indicates that for a slag with a high Cr2O3 content, trying to improve the fluidity of the slag by increasing Λ has a limited effect.

Feasibility of the Redox Sulfur Recovery Process using Heteropoly Molybdophosphate

The feasibility of the removal of sulfide species using acid solutions of 12-molybdophosphoric acid was studied. The key factor in this purification process is the formation of a hydrogen-bonded structure between the hydrogen sulfide species and the bridging oxygen atom of the MoO6 octahedrons of the molybdophosphate complexes. The number of bridging oxygen atoms depends of the pH. Consequently, the efficiency of the process is also largely dependent on the pH. Most of the literature focuses on the redox potentials for the purification process. The slope of the redox pair S/H2S and the molybdophosphate complexes is -59 pH/mV. The differences between the redox potentials do not fluctuate with the pH, which indicates that the oxidation of H2S by molybdophosphate complexes is spontaneous in all pH intervals but does not explain why the efficiency decreases at high pH. In summary, the Keggin-type structure contributes to the efficient removal of sulfide because this structure contains the maximum number of bridging oxygen atoms. The solubility of H2S gas is lower in acidic media. This drawback severely restricts the performance of the purification process.

Effect of annealing on the elastic moduli of amorphous zirconium tungstate

Amorphous zirconium tungstate (a-ZrW2O8), prepared at 7.7GPa, was subjected to successive heat treatments at increasingly higher temperatures, up to 640°C. Evidence of recrystallization into α-ZrW2O8 was found in the room temperature X-ray powder diffraction pattern of samples previously heated above 600°C. Changes in elastic moduli and linear thermal expansion coefficient upon annealing of a-ZrW2O8 up to 600°C cannot be accounted for by assuming a composite sample consisting of α-ZrW2O8 domains dispersed into an amorphous zirconium tungstate matrix. The irreversible relaxation of a-ZrW2O8 and recrystallization into α-ZrW2O8 is accompanied by an increase of bulk modulus (from 61.8(8)GPa to 74.4(9)GPa) and Poisson's ratio (from 0.267(3) to 0.302(3)) and diminution of density (from 6.54(1)g/cm3 for a-ZrW2O8 to 5.18(1)g/cm3 for α-ZrW2O8). Despite the greater density, a-ZrW2O8 (as well as γ-ZrW2O8, a high pressure polymorph) is more compressible than α-ZrW2O8, possibly owing to the new degrees of freedom provided by lowering symmetry, which allow external stresses to be accommodated by polyhedral tilting. Furthermore, the observed increase of Poisson's ratio upon thermal treatments at increasingly higher temperatures indicates that annealing of a-ZrW2O8 is accompanied by the irreversible reduction in the average number of bridging oxygen atoms.

Computer Simulation of Anionic Structures of Molten CaO-SiO2-P2O5 System

Molecular dynamics simulations were carried out to investigate the anionic structures of the molten CaO-SiO2-P2O5 system. The results show that the average first nearest-neighbor distances for Si-O and P-O pairs are 1.61 and 1.53 A, respectively. As expected, above 98 pct P and 95 pct Si show fourfold coordination and form tetrahedral structures. Due to the high basicity, nonbridging oxygen occupies a predominant position in Si and P tetrahedron. Based on the oxygen number of different types, the structures of both Si and P tetrahedron were classified as Q 0, Q 1, Q 2, Q 3, and Q 4, where the superscript referred to the number of bridging oxygen atoms. With the substitution of P2O5 for SiO2, Q 0 decreased and other type of Q i units increased. For Si tetrahedron, Q 2 and Q 3 show most notable change, for P tetrahedron, Q 1and Q 2 show the most notable change. The change of Q i units for Si tetrahedron is larger than that for P tetrahedron. The concentration of free oxygen decreases remarkably with the increase of P2O5 content. The Si-O-P linkage is energetically more favorable than Si-O-Si and P-O-P linkages. P ion has a tendency to promote the polymerization of phosphosilicate melts.

Reactive simulations of the activation barrier to dissolution of amorphous silica in water

Molecular dynamics simulations employing reactive potentials were used to determine the activation barriers to the dissolution of the amorphous SiO2 surface in the presence of a 2 nm overlayer of water. The potential of mean force calculations of the reactions of water molecules with 15 different starting Q4 sites (Qi is the Si site with i bridging oxygen neighbors) to eventually form the dissolved Q0 site were used to obtain the barriers. Activation barriers for each step in the dissolution process, from the Q4 to Q3 to Q2 to Q1 to Q0 were obtained. Relaxation runs between each reaction step enabled redistribution of the water above the surface in response to the new Qi site configuration. The rate-limiting step observed in the simulations was in both the Q32 reaction (a Q3 site changing to a Q2 site) and the Q21 reaction, each with an average barrier of ∼14.1 kcal mol(-1). However, the barrier for the overall reaction from the Q4 site to a Q0 site, averaged over the maximum barrier for each of the 15 samples, was 15.1 kcal mol(-1). This result is within the lower end of the experimental data, which varies from 14-24 kcal mol(-1), while ab initio calculations using small cluster models obtain values that vary from 18-39 kcal mol(-1). Constraints between the oxygen bridges from the Si site and the connecting silica structure, the presence of pre-reaction strained siloxane bonds, and the location of the reacting Si site within slight concave surface contours all affected the overall activation barriers.

Investigation of the structural environment of Ta in a silicate glass and water system under high P–T conditions

In situ Ta L3-edge XAS measurements have been made from a Ta (~1400ppm)-bearing peraluminous silicate glass+H2O system to 960°C and ~0.6GPa. A white-line doublet separated by ~4eV occurs in the Ta L3-edge XANES and results from octahedral crystal field splitting of the Ta 5d levels due to the local structure surrounding Ta coordinated by Qn-species (n=number of bridging oxygen atoms shared between SiO4 and AlO4 units) in the silicate glass/melt+H2O system. The XANES spectra measured from the hydrous silicate glass/melt and from the silicate-rich aqueous fluid have been analyzed using multi-peak fitting techniques. The white-line doublet intensity varies with increasing P–T conditions of the silicate glass/melt+water system, indicating a shift in the electronic density of states in the vicinity of quasi bound Ta 5d states probed by the 2p3/2 core photoelectron. Ab initio modeling of the XANES indicates that water dissolution causes distortion of local structure surrounding the 6-fold coordinated Ta–Qn clusters in the hydrous silicate glass/melt and in the silicate-rich aqueous fluid. Calculation of the angular-momentum projected density of states (l-DOS) shows that the upper doublet level quasi-bound d-DOS is steadily reduced with increasing distortion of the local structure surrounding Ta–Qn clusters.

Local structure around phosphorus and silicon in the CaO–SiO2–PO2.5 system

The local structure of phosphorus and silicon in the molten CaO–SiO2–PO2.5 slag system was investigated by magic angle spinning nuclear magnetic resonance (MAS-NMR). The 31P MAS-NMR spectra revealed that phosphorus was present primarily as the monophosphate complex ion PO43−, with a small amount of diphosphate ion also present. Their relative ratio to total phosphorus was independent of the phosphate concentration of the sample. In the case of the 29Si MAS-NMR, the mean number of the non-bridging oxygen atoms associated with tetrahedrally coordinated silicon decreased as the phosphate concentration increased at a fixed CaO/SiO2 ratio. This indicates that the nonbridging oxygen atoms around the silicon were replaced by bridging oxygen atoms around the phosphorus as the phosphate concentration in the samples increased.To elucidate the basicity dependence of the structure of slag, the relationship between the structure and optical basicity was also investigated. The relative ratio of Qn (Qn means the silicon atoms tetrahedrally bonded with “n” number of bridging oxygen atoms) strongly depends on the optical basicity. These optical basicity dependencies of the structures of phosphorus and silicon can be explained clearly by the basicity equalization concept (Duffy and Ingram, 1976) [12].

Study of elastic moduli of lithium borobismuthate glasses using ultrasonic technique

Ultrasonic studies on x B2O325 Li2O(75−x) Bi2O3, (0≤x≤75mol%) glasses have been carried out. The elastic moduli of glasses have been investigated using ultrasonic velocity measurements at 4MHz. The results revealed an increase of the ultrasonic velocity and Debye temperature with the increase of the B2O3 content which was attributed to the increase in the packing density, the local contraction of the network around the Bi and Li cations and the increase of the number of bonds per unit volume. Also, the increase of the elastic moduli with the increase in the B2O3 content is affected with the increase in the dissociation energy, the average cross-link density, the increase in the number of bridging oxygen atoms, and the substitution of high bond strength BO with low bond strength BiO. The optical properties such as the refractive index, the energy gap and the optical polarizability were evaluated from the values of the elastic moduli. It was observed that as the bulk modulus increases, the optical energy gap increases and both the refractive index and the optical basicity decrease.

Vibrational, X-ray absorption, and Mössbauer spectra of sulfate minerals from the weathered massive sulfide deposit at Iron Mountain, California

The Iron Mountain Mine Superfund site in California is a prime example of an acid mine drainage (AMD) system with well developed assemblages of sulfate minerals typical for such settings. Here we present and discuss the vibrational (infrared), X-ray absorption, and Mössbauer spectra of a number of these phases, augmented by spectra of a few synthetic sulfates related to the AMD phases. The minerals and related phases studied in this work are (in order of increasing Fe 2O 3/FeO): szomolnokite, rozenite, siderotil, halotrichite, römerite, voltaite, copiapite, monoclinic Fe 2(SO 4) 3, Fe 2(SO 4) 3·5H 2O, kornelite, coquimbite, Fe(SO 4)(OH), jarosite and rhomboclase. Fourier transform infrared spectra in the region 750–4000 cm −1 are presented for all studied phases. Position of the FTIR bands is discussed in terms of the vibrations of sulfate ions, hydroxyl groups, and water molecules. Sulfur K-edge X-ray absorption near-edge structure (XANES) spectra were collected for selected samples. The feature of greatest interest is a series of weak pre-edge peaks whose position is determined by the number of bridging oxygen atoms between Fe 3+ octahedra and sulfate tetrahedra. Mössbauer spectra of selected samples were obtained at room temperature and 80 K for ferric minerals jarosite and rhomboclase and mixed ferric–ferrous minerals römerite, voltaite, and copiapite. Values of Fe 2+/[Fe 2+ + Fe 3+] determined by Mössbauer spectroscopy agree well with those determined by wet chemical analysis. The data presented here can be used as standards in spectroscopic work where spectra of well-characterized compounds are required to identify complex mixtures of minerals and related phases.