Silicate Anions Research Articles

Bis(difluoromethyl)trimethylsilicate Anion: A Key Intermediate in Nucleophilic Difluoromethylation of Enolizable Ketones with Me3 SiCF2 H.

A pentacoordinate bis(difluoromethyl)silicate anion, [Me3 Si(CF2 H)2 ](-) , is observed for the first time by the activation of Me3 SiCF2 H with a nucleophilic alkali-metal salt and 18-crown-6. Further study on its reactivity by tuning the countercation effect led to the discovery and development of an efficient, catalytic nucleophilic difluoromethylation of enolizable ketones with Me3 SiCF2 H by using a combination of CsF and 18-crown-6 as the initiation system. Mechanistic investigations demonstrate that [(18-crown-6)Cs](+) [Me3 Si(CF2 H)2 ](-) is a key intermediate in this catalytic reaction.

Bis(difluoromethyl)trimethylsilicate Anion: A Key Intermediate in Nucleophilic Difluoromethylation of Enolizable Ketones with Me3SiCF2H

AbstractA pentacoordinate bis(difluoromethyl)silicate anion, [Me3Si(CF2H)2]−, is observed for the first time by the activation of Me3SiCF2H with a nucleophilic alkali‐metal salt and 18‐crown‐6. Further study on its reactivity by tuning the countercation effect led to the discovery and development of an efficient, catalytic nucleophilic difluoromethylation of enolizable ketones with Me3SiCF2H by using a combination of CsF and 18‐crown‐6 as the initiation system. Mechanistic investigations demonstrate that [(18‐crown‐6)Cs]+[Me3Si(CF2H)2]− is a key intermediate in this catalytic reaction.

Tubular Chains, Single Layers, and Multiple Chains in Uranyl Silicates: A2[(UO2)Si4O10] (A = Na, K, Rb, Cs)

Four new uranyl silicates with the same framework composition but different crystal structures, Na2[(UO2)Si4O10]·0.5H2O (1), K2[(UO2)Si4O10] (2), Rb2[(UO2)Si4O10] (3), and Cs2[(UO2)Si4O10] (4), have been synthesized under supercritical hydrothermal conditions at 590 °C and 1620 bar and characterized by single-crystal X-ray diffraction and infrared and photoluminescence spectroscopy. Crystal data for compound 1: tetragonal, I4/mcm (No. 140), a = 18.0332(7) Å, c = 7.7752(5) Å, V = 2528.5(2) Å3, Z = 8, R1 = 0.0447; compound 2: monoclinic, P21/n (No. 14), a = 8.2970(17) Å, b = 7.7403(15) Å, c = 8.7906(18) Å, β = 104.83(3)°, V = 545.74(19) Å3, Z = 2, R1 = 0.0140; compound 3: monoclinic, P21/c (No. 14), a = 10.3483(3) Å, b = 23.7297(6) Å, c = 7.6278(2) Å, β = 110.482(1)°, V = 1754.68(8) Å3, Z = 6, R1 = 0.0194; compound 4: orthorhombic, Cmca (No. 64), a = 7.7157(3) Å, b = 19.8719(8) Å, c = 24.0160(10) Å, V = 3682.3(3) Å3, Z = 12, R1 = 0.0277. The silicate anions in the structures of these compounds form tubular chains, single layers, and multiple chains which are linked by UO6 tetragonal bipyramids to form three-dimensional frameworks. The tubular chains and single layers are of the new type with respect to the sequence of vertex orientations. The multiple chain is observed for the first time and contains Q2, Q3, and Q4 Si. The structures of these silicate anions can be correlated with the sizes of alkali metal cations.

Influence of silicate anions structure on desilication in silicate-bearing sodium aluminate solution

The structural changes of silicate anions in the desilication process with the addition of calcium hydrate alumino-carbonate were studied by measuring Raman spectra, infrared spectra and corresponding second derivative spectra. The results show that the desilication ratio in the solution prepared by the addition of sodium silicate (solution-SS) is much greater than that in the solution by the addition of green liquor (solution-GL), and low alumina concentration in the sodium aluminate solutions facilitates the desilication process. It is also shown that alumino-silicate anions in the solution-GL, and Q3 polymeric silicate anions in solution-SS are predominant, respectively. In addition, increasing the concentration of silica favors respectively the formation of the alumino-silicate or the Q3 silicate anions in the solution-GL or the solution-SS. Therefore, it can be inferred that the low desilication ratio in the silicate-bearing aluminate solution is mainly attributed to the existence of alumino-silicate anions.

Influence of reactivity of the colloidal silica on the properties of conjugate phases

The properties of hydrosols (brand Ludox) are studied by dynamic light scattering (photon correlation spectroscopy, PCS) and colorimetric analysis (formation of a colored β-silicomolybdate complex, KMK). Experimental sol samples for silicate modulus M = 50.0 and М = 3.0 M (M = [SiO2]/[Me2O], mol/mol) are obtained by introducing an alkali metal hydroxide (Me). According to the PCS data, the hydrodynamic radius of the sol particles (\({C_{Si{O_2}}}\)= 41.0 g/L) is r = 8.0 nm and increases with the dilution of the sol. These particles’ radii r∞ (at \({C_{Si{O_2}}}\) → 0) for the initial sol are r∞ = 13.8 nm and r∞ = 16.8 nm in sols with alkali metal hydroxides (M = 50). The conjugated phase properties (silica particles and aqueous solution) were considered in the light of polycondensation and depolymerization kinetic reactions involving reactive silicate anions (HO)3SiO– and Si(OH)4) on the surface of the particles and in the composition of soluble fractions. The result is the formation of surface layers of a gel-like structure. With the introduction of alkali metal hydroxides, the initial dissolution rate increases in the range from LiOH to KOH. The existence of peaks in the kinetic dependences of the active silica fractions in highly alkaline environments (M = 50 and M = 3.0) characterizes the secondary polycondensation of silicic acid in the aqueous phase, resulting in the formation of the oligo- and polymeric molecules of a linear structure.

On the existence of a high-temperature polymorph of Na2Ca6Si4O15—implications for the phase equilibria in the system Na2O-CaO-SiO2

Singe crystals of a new high-temperature polymorph of Na2Ca6Si4O15 have been obtained from solid state reactions performed at 1300 °C. The basic crystallographic data of this so-called β-phase at ambient conditions are as follows: space group P1c1, a = 9.0112(5) Å, b = 7.3171(5) Å, c = 10.9723(6) Å, β = 107.720(14)°, V = 689.14(7) Å3, Z = 2. The crystals showed twinning by reticular merohedry (mimicking an orthorhombic C-centred unit cell) which was accounted for during data processing and structure solution. Structure determination was accomplished by direct methods. Least-squares refinements resulted in a residual of R(|F|) = 0.043 for 5811 observed reflections with I > 2σ(I). From a structural point of view β-Na2Ca6Si4O15 can be attributed to the group of mixed-anion silicates containing [Si2O7]-dimers as well as isolated [SiO4]-tetrahedra in the ratio 1:2, i.e. more precisely the formula can be written as Na2Ca6[SiO4]2[Si2O7]. The tetrahedral groups are arranged in layers parallel to (100). Sodium and calcium cations are located between the silicate anions for charge compensation and are coordinated by six to eight nearest oxygen ligands. Alternatively, the structure can be described as a mixed tetrahedral-octahedral framework based on kröhnkite-type [Ca(SiO4)2O2]-chains in which the CaO6-octahedra are corner-linked to bridging SiO4-tetrahedra. The infinite chains are running parallel to [001] and are concentrated in layers parallel to (010). Adjacent layers are shifted relative to each other by an amount of +δ or −δ along a*. Consequently, a …ABABAB… stacking sequence is created. A detailed comparison with related structures such as α-Na2Ca6Si4O15 and other A2B6Si4O15 representatives including topological as well as group theoretical aspects is presented. There are strong indications that monoclinic Na2Ca3Si2O8 mentioned in earlier studies is actually misinterpreted β-Na2Ca6Si4O15. In addition to the detailed crystallographic analysis of the previously unknown compound our results will also help to improve the interpretation of the phase relationships between the compounds in the ternary system Na2O-CaO-SiO2 which are of interest for several applications related to the field of applied mineralogy and materials science.

Structural transformations in sodium silicate liquids under pressure: A molecular dynamics study

We present the results of force-field molecular dynamics simulations of the static and dynamic properties of sodium silicate liquids. We studied the relationship between the structure and properties with varying SiO2 content and pressure. We found that the silicate liquids have at least three types of characteristic structures before the coordination number of Si changes with increasing pressure; these structures are, ionic, network, and coesitic structured liquids. In the ionic liquid like structure, simple diffusion of large silicate anions is dominant in the liquid and their motion is impeded by compression. In contrast, in the network liquid, a small portion of the network of corner-shared SiO4 tetrahedra diffuses via bond exchange and flow globally. Bond exchange is activated by pressure in the entangled network liquid because of distortion of the corner-shared SiO4 tetrahedra network and the tetrahedra themselves. The SiO2 rich liquids soften as a result from of these distortions. The derivative of bulk moduli of the sodium silicate liquids increases significantly at a certain pressure because of changes in densification mechanism. The changes in those densification mechanisms are coherent with the changes in pressure dependence of transport coefficient.

A review of the structural architecture of tellurium oxycompounds

AbstractRelative to its extremely low abundance in the Earth's crust, tellurium is the most mineralogically diverse chemical element, with over 160 mineral species known that contain essential Te, many of them with unique crystal structures. We review the crystal structures of 703 tellurium oxysalts for which good refinements exist, including 55 that are known to occur as minerals. The dataset is restricted to compounds where oxygen is the only ligand that is strongly bound to Te, but most of the Periodic Table is represented in the compounds that are reviewed. The dataset contains 375 structures that contain only Te4+cations and 302 with only Te6+, with 26 of the compounds containing Te in both valence states. Te6+was almost exclusively in rather regular octahedral coordination by oxygen ligands, with only two instances each of 4- and 5-coordination. Conversely, the lone-pair cation Te4+displayed irregular coordination, with a broad range of coordination numbers and bond distances. A threshold was applied for Te4+–O links of ∼2.45 Å or 0.3 valence units with some flexibility, as a criterion to define strongly bound Te–O polymers and larger structural units. Using this criterion, Te4+cations display one-sided 3-, 4- or 5-coordination by oxygen (with rare examples of coordination numbers 2 and 6). For both valence states of Te, examples are known of TemOncomplexes which are monomeric (m= 1; neso), noncyclic finite oligomers (soro), rings (cyclo), infinite chains (ino), layers (phyllo) and frameworks (tecto tellurates). There is a clear analogy to the polymerization classes that are known for silicate anions, but the behaviour of Te is much richer than that of Si for several reasons: (1) the existence of two cationic valence states for Te; (2) the occurrence of multiple coordination numbers; (3) the possibility of edge-sharing by TeOnpolyhedra; (4) the possibility for oxygen ligands to be 3-coordinated by Te; and (5) the occurrence of TemOnpolymers that are cationic, as well as neutral or anionic. While most compounds contain only one or two symmetrically distinct types of Te atom, Pauling's Fifth Rule is frequently violated, and stoichiometrically simple compounds such as CaTeO3can have polymorphs with up to 18 distinct Te sites. There is a tendency for local symmetry features such as the threefold axis of a TeO6octahedron or the acentric symmetry of a Te4+Onpolyhedron to be inherited by the host structure; the latter in particular can lead to useful physical properties such as nonlinear optical behaviour. We develop for the first time a hierarchical taxonomy of Te-oxysalt structures, based upon (1) valence state of Te; (2) polymerization state of TemOncomplexes; (3) polymerization state of larger strongly-bound structural units that include non-Te cations. Structures are readily located and compared within this classification.

Structure Control of Calcium Silicate Hydrate Gels for Dye Removal Applications

The structure of calcium silicate hydrate (C‐S‐H) gels was modified by hydrothermal reaction with aqueous acetic acid solvent, and then the C‐S‐H gels were used for dye removal from aqueous solution. With increasing acetic acid concentration, the Ca:Si molar ratio decreased and the length of the silicate anion chain structure of the C‐S‐H gels increased. The silicate anion chain length affects the number of available silanol groups on the surface of the C‐S‐H gel: the longer the silicate anion chain length, the greater the number of negative charges and the higher the surface potential. C‐S‐H gels with a long silicate anion structure exhibited higher adsorption capacity for methylene blue than gels with a short silicate anion structure. The enhanced adsorption capacity of the C‐S‐H gels is related to the higher number of silanol groups in the bridging silica tetrahedra of the intermediate anion chain structure compared with those in the end units of silica tetrahedra.

Rb2Ca2Si3O9: the first rubidium calcium silicate

Abstract The crystal structure of Rb2Ca2Si3O9 has been characterized by X-ray diffraction techniques and Raman spectroscopy. Crystal growth was performed by the flux method in closed platinum capsules using a polycrystalline ceramic precursor as well as RbCl as a mineralizer. The crystal structure was solved from a single-crystal diffraction data set acquired at 23 °C and refined to a final residual of R(|F|)=0.022 for 1871 independent reflections. Basic crystallographic data are as follows: monoclinic symmetry, space group type P1n1, a=6.5902(3) Å, b=7.3911(3) Å, c=10.5904(4) Å, β=93.782(3)°, V=514.72(3) Å3, Z=2. With respect to the silicate anions the compound can be classified as a sechser single-chain silicate. The undulated chains run parallel to [-101] and are connected by Rb and Ca cations, which are distributed among four crystallographically independent sites. In a first approximation the coordination polyhedra of the two different calcium ions in the asymmetric unit can be described by distorted trigonal prisms and tetragonal pyramids, respectively. The two rubidium sites exhibit more irregular coordination spheres with eight to nine next neighbors. Structural investigations on the new phase are completed by solid state micro-Raman spectroscopy. DFT-calculations were employed for the interpretation of the spectroscopic data including the allocation of the bands to certain vibrational species.

Structural characterization studies on the natural mineral pyrophyllite

ABSTRACTA light-yellow-colored pyrophyllite (Al2O3·SiO2) mineral obtained from Vempalli, Cuddapah district, Andhra Pradesh, India, is investigated in the present work. Chemical analysis carried out using energy-dispersive spectroscopy (EDX) shows that Fe2O3 is present by about 1.56 wt%. Structural characterization was performed using X-ray diffraction (XRD). XRD results suggest that the unit cell is monoclinic with a = 5.16, b = 8.798, c = 9.347 Å and β = 100.46°. The ligands around the metal ion present in the structure are investigated using FTIR spectroscopy. EDX analysis indicates that iron and titanium are only two transition metals present in it. Morphology studied using scanning electron microscopy suggests that the unit cell consists of a dioctahedral layered structure. Fe3+ is present in the location of Al3+ in the unit cell of pyrophyllite. Electron paramagnetic resonance results indicate that the unit cell of the crystal contains Fe(III), and its g values are found to be 4.10 and 2.0. Infrared properties are due to the presence of silicate and hydroxyl anions as ligands. Nonlinear optical measurements carried out using Z-scan reveal the occurrence of strong nonlinear optical limiting in the material, indicating potential applications in laser safety devices.

Effect of formulation of silica‐based solution on corrosion resistance of silicate coating on hot‐dip galvanized steel

Several silica‐based solutions with 50 g/l of SiO2 were prepared from sodium silicate solutions and silica sol; the silicate conversion coatings were obtained by immersing hot‐dip galvanized steel sheets in these solutions. These solutions were characterized using high‐resolution transmission electron microscopy and 29Si nuclear magnetic resonance; the morphology of the coatings was observed by SEM and atomic force microscopy while the corrosion resistance was evaluated by electrochemical measurements as well as neutral salt spray tests. The results show that the coatings obtained from the single silica sol solution had poor adhesion and the coating obtained from the sodium silicate solution with low SiO2/Na2O molar ratio was uneven. By adding the silica sol to the silicate solution with low molar ratio, uniform coatings with better protection property were obtained. According to the results of 29Si nuclear magnetic resonance spectra, the effects of the distribution of silicate anions with various polymerization degrees in the silica‐based solutions on the microstructure and corrosion resistance of the silicate coatings are discussed. Copyright © 2016 John Wiley & Sons, Ltd.

Thermal expansion and structural complexity of Ba silicates with tetrahedrally coordinated Si atoms

Thermal expansion of Ba silicates with tetrahedrally coordinated Si atoms in the temperature range of 25–1100°C had been studied by high-temperature X-ray powder diffraction. The volume thermal expansion coefficients (TECs) are in the range 41–50×10−6°C−1 with an average value of 〈αV〉=45×10−6°C−1. In the structures with chain and layered silicate anions, thermal expansion is anisotropic: the direction of maximal TEC is parallel to the extension of the zweier chains of silicate tetrahedra, which are strained owing to the interactions with Ba2+. The strain is released during thermal expansion due to the increasing effective size of Ba2+ induced by thermal vibrations. Information-theoretic analysis of the structural and topological complexities of Ba silicates indicates that their structural complexity is a function of the topological complexity of their silicate anions. The latter displays a non-linear behaviour with increasing SiO2 content (=the increasing degree of polymerization and increasing dimensionality): it starts from simple topologies, reaches a maximum at topologies of intermediate complexity, and ends up at simple topologies again. The specificity of the interactions of Ba2+ with the silicate anions results in higher complexity of high-temperature α-BaSi2O5 compared to that of low-temperature β-BaSi2O5. This uncommon behaviour may be explained by the vibrational advantages provided by flatter and more complex silicate layers in the α-phase, which overcome negative differences in configurational entropies of the two modifications apparent in the differences of their structural Shannon information.

Composition, silicate anion structure and morphology of calcium silicate hydrates (C-S-H) synthesised by silica-lime reaction and by controlled hydration of tricalcium silicate (C3S)

The main product of Portland cement hydration is C-S-H. Despite constituting more than half of the volume of hydrated pastes and having an important role in strength development, very little is known about the factors that determine its morphology. To investigate the relationship between the chemical composition, silicate anion structure and morphology of C-S-H, samples were synthesised via silica-lime reactions and by the hydration of C3S under controlled lime concentrations and with/without accelerators. The silicate anion structure of the samples was studied by 29Si magic angle spinning nuclear magnetic resonance spectroscopy and the morphology and chemical composition by TEM and SEM. All samples prepared via silica-lime reactions with bulk Ca/Si up to 1.5 were foil-like. The hydration of C3S at fixed lime concentration yielded foil-like C-S-H for [CaO]<22 mmol L− and fibrillar C-S-H for [CaO]>22 mmol L−1. A relationship between the silicate anion structure and the morphology of C-S-H was found for the samples fabricated with accelerators.

Effect of Water Vapor on O2− Content in Ironmaking Slag

In principle, slag basicity can be expressed as the concentration of free oxygen (O2−) in the slag system. This free oxygen content is equilibrated with different silicate anions in addition to other components in the silicate-based slags. X-ray photon spectroscopy (XPS) and scanning electron microscope equipped with energy dispersive spectrometer (SEM-EDS) were used to investigate the effect of water vapor on the free oxygen content in ironmaking slags. It was found that water in the gas atmosphere plays a significant role in the silicate anion equilibria. Water decreases the amount of free oxygen in the studied slags, with the free oxygen expressed as percentage of the total oxygen decreasing in the order of the following gas mixtures: CO+CO2 (44%, pH2O = 0 kPa) > CO+CO2+H2+H2O (41%, pH2O = 10.13 kPa) > H2+H2O (37%, pH2O = 14.19 kPa). The content of free oxygen ion affects the distribution of elements such as sulfur, phosphorus, and manganese. In addition, it affects the iron oxide content in the slag as well as the interaction between slag and furnace lining.

Effects of phosphate and silicate on the transformation of hydroxycarbonate green rust to ferric oxyhydroxides

Hydroxycarbonate green rust (GR1(CO32−)) was prepared by oxidation of aerated aqueous suspensions of Fe(II) hydroxide, and the presence of light promoted the transformation of GR1(CO32−) by dissolved O2 at pH 7.8 and 25°C. Further transformation of GR1(CO32−) in the light was conducted in the presence of orthophosphate (P) or silicate (Si) anions, followed by solution analysis and solid product characterization using X-ray diffraction (XRD), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR). Results show that both P and Si anions significantly affect the transformation of GR1(CO32−) through adsorption on the intermediate products. The time required for complete GR1(CO32−) transformation and the phases, crystallinity and morphology of the transformation products all depend on the Fe/anion molar ratio. When compared to the control, the transformation can be promoted by low Si concentrations but retarded by P. With decreasing Fe/P ratio, the products change from acicular goethite (absence of P) to tabular lepidocrocite (Fe/P: 120-48) and to mixed phases of platelets of ferric GR1(CO32−) (EX-GR1) and minor ferrihydrite (Fe/P: 24-3). In terms of Si, the products are goethites when the Fe/Si ratio of 48-12, and with increasing ratio, the goethite crystallinity and particle size decrease and the morphology changes from acicular (absence of Si) to plate-like or isodimensional particles. The goethite morphology at low Fe/Si ratios is comparable to natural goethite samples commonly found in soils. At Fe/Si=3, the products are EX-GR1 platelets with minor ferrihydrite coexisting. The likely pathway of the oxidative GR1(CO32−) transformation in the control system and in the presence of low concentrations of Si (Fe/Si⩾12) is GR1(CO32−)→amorphous γ-FeOOH-like phase→α-FeOOH via a dissolution–oxidation–precipitation mechanism. In addition, Fe(II) released during dissolution of GR1(CO32−) is adsorbed on the products and the transformation of the γ-FeOOH-like phase to goethite is catalyzed by the adsorbed Fe(II). For the P system, the released Fe(II) forms ternary surface complexes with P on the mineral surfaces without any catalytic role, leading to the formation of lepidocrocite at low P concentrations. Clearly, the oxidative transformation of green rust to various crystalline iron oxyhydroxides depends on the type and concentration (Fe/anion molar ratio) of co-existing anions. This study also suggests that the natural goethite formed by Fe(II) oxidation in the form of plate-like or isodimensional particles is most likely related to the ubiquitous presence of silicates in soil environments.

Effects of manure‐ and lignocellulose‐derived biochars on adsorption and desorption of zinc by acidic types of soil with different properties

SummaryThere is limited information on zinc (Zn2+) adsorption and desorption by different types of soil, especially acidic ones following amendment with manure‐ and lignocellulose‐derived biochars. The novelty of this research is a comparison of the mechanisms of adsorption and desorption of Zn2+ by different types of acidic soil amended with biochar derived from either swine manure or rape straw. Both biochars were prepared at a pyrolysis temperature of 500°C before being incorporated into four types of soil with low pH (pHH2O from 4.61 to 5.91) at an incorporation rate of 3%. Following incubation for 360 days, we carried out batch experiments of Zn2+ adsorption and desorption. The dual‐mode model (DMM) was more suitable for describing the adsorption isotherms of the four soil types following biochar incorporation than the distributed reactivity model (DRM). Swine‐manure biochar enhanced Zn2+ adsorption by soil more than rape‐straw biochar. The increases in Zn2+ adsorption by the different soil types following biochar incorporation were in the order Psammaquent &gt; Plinthudult &gt; Paleudalf &gt; Argiustoll soil, whereas it was most difficult to desorb the Zn2+ already present, in reverse order: Argiustoll &gt; Paleudalf &gt; Plinthudult &gt; Psammaquent soil. This could be attributed to differences in soil pH, organic carbon, cation exchange capacity (CEC) and soil texture. Both non‐electrostatic and electrostatic adsorption mechanisms were identified in the four soil types. The negative surface charge of biochar is responsible for the electrostatic adsorption, whereas functional oxygen groups and mineral components of biochar were responsible for non‐electrostatic adsorption. Non‐electrostatic adsorption was more important for the swine‐manure biochar than for the rape‐straw biochar, which was attributed to the larger content of mineral components (phosphate and silicate anions) of the former. In conclusion, the biochar effects on Zn2+ adsorption depended on the initial Zn2+ concentration, soil pH, soil type and biochar origin.

Effects of Ion Exchange on the Mechanical Properties of Basaltic Glass Fibers

Basaltic glass fibers with different lithium oxide (6–14 mol%) and sodium oxide (2–14 mol%) contents were prepared. The influence of Li2O and Na2O content on the process of fiber manufacturing was investigated. Addition of alkali oxides reduced the forming temperature and substantially expanded the fiber‐forming temperature ranges. The obtained thermal data from differential thermal analysis revealed a decline in glass transition temperature (Tg) of fibers against the compositional changes. The inclusion of Li2O and Na2O in the glass network led to a reduction in its thermal stability. The obtained X‐ray diffraction patterns and IR spectra of Li‐rich and Na‐rich basaltic glass fibers confirmed the formation of highly polymerized structures such as LiAl(Si2O6) and (Na,K)(AlSiO4), respectively, and relatively depolymerized silicate anions. The effects of potassium–lithium and potassium–sodium ion exchange on the mechanical properties of basaltic glass fibers were investigated. As‐received Li‐rich and Na‐rich basaltic glass fibers were ion‐exchanged in potassium nitrate for different exchange times, and their mechanical properties were measured before and after chemical tempering. The measured tensile strength and Young's modulus values of the fibers showed an increase after treatment in molten salt.

Radiotracer Experiments and Monte Carlo Simulations of Sodium Diffusion in Alkali Feldspar: Evidence against the Vacancy Mechanism

Self-diffusion of sodium perpendicular to (001) in a potassium-rich alkali feldspar singlecrystal has been studied by self-diffusion experiments and by Monte Carlo simulations. Sodium diffusivitieswere measured with the radiotracer technique using the 22Na isotope in a temperature intervalfrom 773 K to 1173 K. It was found that self-diffusion coefficients follow a linear Arrhenius relationwith the pre-exponential factor of 1:2 103 cm2/s and an activation enthalpy of 1:3 eV. To study correlationeffects in the monoclinic feldspar structure, a Monte Carlo method was applied assuming thatthe two cation species are randomly distributed on the common sublattice and are not influenced by thefixed sublattice of the silicate and aluminate anions. Correlation factors have been calculated assuminga vacancy mechanism and applying a developed four-frequency model for the nearest-neighborvacancy jumps on the alkali sublattice. Our findings strongly indicate that vacancy diffusion providesonly a minor contribution to sodium self-diffusion in potassium-rich feldspars.

On the ambient pressure polymorph of K2Ca3Si3O10—An unusual mixed-anion silicate and its structural and spectroscopic characterization

An ambient pressure polymorph of K2Ca3Si3O10 has been synthesized via solid state reactions. Single-crystal X-ray diffraction experiments show, that this new modification crystallizes in the triclinic space group P1¯ with the following lattice parameters: a=5.6699(6) Å, b=7.3754(12) Å, c=11.8310(13) Å, α=86.199(11)°, β=80.625(9)°, γ=88.700(11)°. The structure was solved by direct methods and subsequently refined to a residual of R1=0.0261 for 1761 independent observed reflections (I>2σ(I)) and 163 parameters. A special feature of the crystal structure is the coexistence of two different types of silicate anions. Isolated [SiO4]- tetrahedra as well as [Si4O12]- vierer single rings occur in the ratio 2:1, resulting in the crystallochemical formula K4Ca6[SiO4]2[Si4O12]. To the best of our knowledge, this is the first example of an oxo-silicate where insular and cyclic silicate anions appear concomitantly. Charge compensation is provided by Ca and K cations. All calcium atoms are coordinated by 6 oxygen atoms, forming distorted octahedra. By sharing common corners, edges and faces, these [CaO6]-polyhedra build up octahedral layer-like motifs parallel to (010). Potassium ions are located in voids between the silicate anions and [CaO6]-octahedra and are coordinated by 8–9 oxygen atoms. Further characterization of this new compound was carried out by electron microprobe analysis and Raman spectroscopy. DFT calculations were employed (i) to assign Raman bands to certain vibrational modes and (ii) to determine the relative stabilities of the monoclinic high-pressure and the triclinic ambient pressure polymorph of K2Ca3Si3O10.