Si-O Distances Research Articles

A case study of depolymerization in silicates: Melting of quartz and zircon crystallization at high pressure

The experimental study of depolymerization reaction SiO2 + ZrO2 → ZrSiO4 reveals that melting of quartz and crystallization of zircon in the presence of water increases at high pressure. The melting rate of quartz is identified as the rate-determining step of the reaction. In highly polymerized silicates, Si-O-Si bond angles are subject to alteration under the influence of pressure. This phenomenon is particularly noticeable within the three-dimensional lattice structure of quartz, where each corner of the tetrahedra is shared. Theoretical analysis of the Si-O-Si bond angle bending demonstrates a change in oxygen hybridization from sp to sp2, favoring larger Si-O distances. The sp2 hybridization in bridging oxygen promotes acid attack by hydrogen ions of water, resulting in SiO bond heterolysis in quartz. The depolymerization from tectosilicates to nesosilicates leads to an increase in oxygen “size” and polarizability, as well of closer packing of oxygen atoms. This change in behavior and packing of oxygens is further illustrated by the high-pressure-driven transition from tridymite to stishovite. Additionally, the impact of network-modifying elements on the structure of polymerized silicates is investigated; particularly those acting as electron donors, which lengthen Si-O distances in quartz. The findings are discussed in relation to key topics in Earth Sciences, including the crystal chemical basis of the Bowen's reaction series, the “metallization” in rocky planets and the structural controls of hydrous and dry melting in silicates.

Large-scale DFT calculations of multi-component glass systems (SiO2)0.70(Al2O3)0.13(XO)0.17 (X = Mg, Ca, Sr, Ba) : Accuracy of classical force fields

Although molecular dynamics (MD) simulation is a powerful tool for investigating the atomic-scale structures of complex materials, several challenges limit their reliable and accurate application to multi-component glass systems. The available force fields (FFs) that can treat many elements in a multi-component glass are limited, and even if such a FF exists, its accuracy is suspicious due to the large variety and complexity of chemical environments in these materials. First-principles calculations based on the density functional theory (DFT) are reliable, but prohibitively expensive with conventional methods. In this study, we use large-scale DFT techniques and demonstrate that it is possible to perform efficient and accurate DFT calculations of multi-component glass systems, such as (SiO2)0.70(Al2O3)0.13 (XO)0.17 (X= Mg, Ca, Sr, Ba), containing about 1000–5000 atoms. From the results of large-scale DFT calculations, we evaluate the accuracy of some classical FFs, and show that the accuracy for non-bridging oxygen atoms is very low especially when the SiO distance is short. Large differences in the distribution of SiOSi angles observed in the FF-MD and DFT-MD simulations and the unique electronic structure in the case of X= Mg are also discussed.

Correlation between Si-Al disorder and hydrogen-bonding distance variation in ussingite (Na2AlSi3O8OH) revealed by one- and two-dimensional multi-nuclear NMR and first-principles calculation

Abstract Ussingite (Na2AlSi3O8OH) is a mineral with a unique interrupted framework structure and strong hydrogen bonding. It contains 4-, 6-, and 8-membered tetrahedral rings resembling feldspars, but, unlike the latter, is partially depolymerized. There are four crystallographically distinct tetrahedral (T) sites, two of which (T1, T2) are Q4 [i.e., having 4 next nearest neighbor (NNN) T sites], and the other two (T3, T4) are Q3 (i.e., having 3 NNN T sites), each with NNN (in brackets) of T1(1T2, 1T3, 2T4), T2(1T1, 2T3, 1T4), T3(1T1, 2T2), and T4(2T1, 1T2). There is one unique OH site in the T4-O8-H···O2-T3 configuration, where O8 and O2 are nonbridging O atoms (NBO). In the ordered structure, T1 is fully occupied by Al, and the other three T sites by Si. Previous X-ray and neutron diffraction and 1H and 29Si NMR studies gave contradictory conclusions regarding Si-Al disorder. In this study, we were able to unambiguously clarify the issue via comprehensive one- and two-dimensional 1H, 29Si, 27Al, and 23Na NMR and first-principles calculation. It was revealed that there is ~3% Si-Al disorder that occurs between neighboring T1-(O)-T2 sites, such that the formation of Al-O-Al linkage and Al(Q3) are avoided. The disorder was found to result in the development of Si(Q3) sites with various NNN, including 3Al and 3Si, and neighboring OH sites having significantly shorter and longer hydrogen-bonding distances than in the ordered structure, with 1H chemical shifts near 15~16 ppm and 11 ppm, in addition to a main peak near 13.9 ppm. Good correlation was found between 1H chemical shift, hydrogen-bonding (O-H, H···O, and O···O) distances, and Si-O distances in the Si-O-H···O-Si linkage. This suggests that Si-Al disorder is correlated with variation in hydrogen-bonding distances via through-bond transmission of bond valence variations. This could be a universal phenomenon also applicable to other hydrous minerals. The revelation of preferential partition of Al in Q4 over Q3 sites to avoid the formation of Al-OH and Al-NBO provides insight into their behavior in other partially depolymerized hydrous aluminosilicate systems, such as glasses and melts.

Quantitative Structural Description of Zeolites by Machine Learning Analysis of Infrared Spectra

Application of machine learning (ML) algorithms to spectroscopic data has a great potential for obtaining hidden correlations between structural information and spectral features. Here, we apply ML algorithms to theoretically simulated infrared (IR) spectra to establish the structure-spectrum correlations in zeolites. Two hundred thirty different types of zeolite frameworks were considered in the study whose theoretical IR spectra were used as the training ML set. A classification problem was solved to predict the presence or absence of possible tilings and secondary building units (SBUs). Several natural tilings and SBUs were also predicted with an accuracy above 89%. The set of continuous descriptors was also suggested, and the regression problem was also solved using the ExtraTrees algorithm. For the latter problem, additional IR spectra were computed for the structures with artificially modified cell parameters, expanding the database to 470 different spectra of zeolites. The resulting prediction quality above or close to 90% was obtained for the average Si-O distances, Si-O-Si angles, and volume of TO4 tetrahedra. The obtained results provide new possibilities for utilization of infrared spectra as a quantitative tool for characterization of zeolites.

Study of Short-range Ordering in Amorphous and Nanocrystalline Materials from Laboratory based Pair Distribution Function (LPDF)

Determination of local atomic structure at nanoscale for both amorphous and nanocrystalline materials is very difficult and challenging. Local arrangement of atoms needs to be studied for understanding the local or short-range structural information. Pair distribution function (PDF) analysis is a technique used to study the short-range structure of the materials based on local atomic arrangement of atoms using synchrotron and neutron sources. But there is a demand for routine analysis based on laboratory X-ray diffractometer (XRD) using Ag radiation (λ=0.5608 Å) with maximum achievable Q value of 22 Å–1. An attempt has been taken to study the short-range structure in crystalline Ni, silica glass (SiO2) and nano silica using total scattering experiment to show the capabilities and usefulness of PDF technique in laboratory XRD and are compared with the data from synchrotron radiation to achieve good quality scattering data and optimizing technical feasibility of the optics. PDF results of Ni showed the goodness of optical alignment. The first Si-O distance for both silica samples signified that they hold short- range order within the tetrahedral unit while differences are observed at higher radial distances. Laboratory based PDF experiment helped to get local/short- range structural information for better understanding the multifunctional properties of nano and disordered materials.

Neutron Diffraction and Raman Studies of the Incorporation of Sulfate in Silicate Glasses.

The oxidation state, coordination, and local environment of sulfur in alkali silicate (R2O–SiO2; R = Na, Li) and alkali/alkaline-earth silicate (Na2O–MO–SiO2; M = Ca, Ba) glasses have been investigated using neutron diffraction and Raman spectroscopy. With analyses of both the individual total neutron correlation functions and suitable doped–undoped differences, the S–O bonds and (O–O)S correlations were clearly isolated from the other overlapping correlations due to Si–O and (O–O)Si distances in the SiO4 tetrahedra and the modifier–oxygen (R–O and M–O) distances. Clear evidence was obtained that the sulfur is present as SO42– groups, confirmed by the observation in the Raman spectra of the symmetric S–O stretch mode of SO42– groups. The modifier–oxygen bond length distributions were deconvoluted from the neutron correlation functions by fitting. The Na–O and Li–O bond length distributions were clearly asymmetric, whereas no evidence was obtained for asymmetry of the Ca–O and Ba–O distributions. A consideration of the bonding shows that the oxygen atoms in the SO42– groups do not participate in the silicate network and as such constitute a third type of oxygen, “non-network oxygen”, in addition to the bridging and non-bridging oxygens that are bonded to silicon atoms. Thus, each individual sulfate group is surrounded by a shell of modifier and is not connected directly to the silicate network. The addition of SO3 to the glass leads to a conversion of oxygen atoms within the silicate network from non-bridging to bridging so that there is repolymerization of the silicate network. There is evidence that SO3 doping leads to changes in the form of the distribution of Na–O bond lengths with a reduction in the fitted short-bond coordination number and an increase in the fitted long-bond coordination number, and this is consistent with repolymerization of the silicate network. In contrast, there is no evidence that SO3 doping leads to a change in the distribution of Li–O bond lengths with a total Li–O coordination number consistently in excess of 4.

(μ-Di-tert-butyl-silanediolato)bis-[bis-(η5-cyclo-penta-dien-yl)methyl-zirconium

The reaction of t-Bu2Si(OH)2 with two equivalents of Cp2Zr(CH3)2 produces the title t-Bu2SiO2-siloxide bridged dimer, [Zr2(CH3)2(C5H5)4(C8H18O2Si)] or [Cp2Zr(CH3)]2[μ-t-Bu2SiO2] (1), where one methyl group is retained per zirconium atom. The same product is obtained at room temperature even when equimolar ratios of the silanediol and Cp2Zr(CH3)2 are used. Attempts to thermally eliminate methane and produce a bridging methyl-ene complex resulted in decomposition. The crystal structure of 1 displays typical Zr-CH3 and Zr-O distances but the Si-O distance [1.628 (2) Å] and O-Si-O angle [110.86 (15)°] are among the largest observed in this family of compounds suggesting steric crowding between the t-Bu substituents of the silicon atom and the cyclo-penta-dienyl groups. The silicon atom lies on a crystallographic twofold axis and both Cp rings are disordered over two orientations of equal occupancy.

Correlating structural distributions in silica glass with two-dimensional J -resolved spectroscopy

A two-dimensional (2D) $J$-resolved magic-angle spinning nuclear magnetic resonance (NMR) spectrum of silica glass at $^{29}\mathrm{Si}$ natural abundance levels, 4.7%, was measured using the shifted-echo phase-incremented echo train acquisition (SE-PIETA) pulse sequence. At $^{29}\mathrm{Si}$ natural abundance levels the ${J}_{\text{Si-O-Si}}$ coupling splittings appear as overlapping doublet patterns arising from isolated $^{29}\mathrm{Si}\ensuremath{-}\mathrm{O}\ensuremath{-}^{29}\mathrm{Si}$ linkages. The experimental 2D $J$-resolved spectrum is analyzed to obtain a bivariate probability distribution correlating the central Si-O-Si angle of a ${\mathrm{Q}}^{4}\ensuremath{-}{\mathrm{Q}}^{4}$ linkage to its mean Si-O-Si angle (seven angles) using relationships between $^{29}\mathrm{Si}$ isotropic chemical shifts and geminal ${J}_{\text{Si-O-Si}}$ coupling of a ${\mathrm{Q}}^{4}\ensuremath{-}{\mathrm{Q}}^{4}$ to its local structure. To obtain a self-consistent bivariate probability distribution it was necessary to introduce an additional dependence of the $^{29}\mathrm{Si}$ chemical shift of a ${\mathrm{Q}}^{4}$ on mean Si-O distance as well as mean Si-O-Si angle. The implication of this necessary modification is a positive correlation between Si-O-Si angle and Si-O distance in the silica glass, consistent with recent $^{17}\mathrm{O}$ NMR measurements on ambient and densified silica glasses but running opposite to the trend generally found in crystalline silica polymorphs. From the analysis of the $^{29}\mathrm{Si}$ 2D $J$-resolved spectrum we determine a Si-O-Si bond angle distribution in silica glass as having a mean at $147.{8}^{\ensuremath{\circ}}$, a mode at ${147}^{\ensuremath{\circ}}$, and a standard deviation of $10.{7}^{\ensuremath{\circ}}$. Our statistical model for analyzing the experimental $^{29}\mathrm{Si}$ 2D $J$-resolved spectrum also indicates that the individual Si-O-Si bond angle distributions are relatively uncorrelated.

Structural study of Na2O-B2O3-SiO2-La2O3 glasses from molecular simulations using a polarizable force field

The properties of Na2O-B2O3-SiO2+La2O3 glasses (NBS_La) are not well understood. In particular the influence of the lanthanides on the polymerized network structure and organization are not well known. In this work, we use a Polarizable Ion Model (PIM) that was fitted on electronic structure calculations to simulate a series of NBS_La glasses using molecular dynamics. The simulations account well for the main structural characteristics of NBS_La glasses such as the structure factors and the ionic local environments despite minor discrepancies with experiments about the Si-O distance and the B coordination. In particular, we examine the impact of La2O3 addition to Na2O-B2O3-SiO2 glasses in term of structural changes and in the Na migration paths which become more homogeneous.

Study of short range structure of amorphous Silica from PDF using Ag radiation in laboratory XRD system, RAMAN and NEXAFS

At present synchrotron and neutron sources are the preferred choices for the Pair Distribution Function (PDF) analysis, but there is a need to explore PDF in a laboratory XRD system for quick feedback about the short range structure of the amorphous materials. Present work considered both crystalline (quartz) and amorphous silica to study the structural differences in silica by PDF analysis using Ag radiations in laboratory XRD. The structural information about short range ordering of the oxygen (O) atoms around silicon (Si) atoms as obtained by the PDF were compared with the results as obtained by Near Edge X-ray Absorption Fine Structure (NEXAFS) and RAMAN experiments. The PDF studies showed that the amorphous silica possessed short range periodicity within the basic unit of (SiO4)4− tetrahedra with a SiO & OO distance are of about 1.622 Å and 2.713 Å while the short range as well as long range ordered structure present in quartz with SiO & OO distance are 1.562 Å and 2.661 Å respectively. Raman spectra showed some asymmetry in amorphous silica which corresponds to the defects present in the lattice and thus forming the n-fold ring structure with Si and O resulting in the wide variation of bridging bond angle SiOSi in amorphous silica. NEXAFS studies revealed the structure of amorphous silica and quartz in the intermediate range (3–5 Å) at the Si L and O K edges. The structural information about short range ordering of the O around Si atoms as obtained by these methods were found to be in good match with the results as obtained by PDF, suggesting this technique may be used as a screening tool for routine PDF studies of amorphous materials.

Avoidance of a continuous-random-network in vitreous silica: Entropic and energetic consequences

The relative entropic contributions of correlated vs. uncorrelated SiO bond length and SiOSi bond angle distributions in the corner-shared tetrahedral network of vitreous silica are estimated semi-quantitatively using the concept of differential entropy. The results suggest that a positively correlated SiO vs. SiOSi distribution will be entropically stabilized in a glass structure over a negatively correlated distribution, which is typical of crystalline polymorphs of silica. This hypothesis lends support to the recent experimental observation of a positively correlated joint distribution of SiO distances and SiOSi angles in vitreous silica by 17O nuclear magnetic resonance spectroscopy. On the other hand, an estimation of the relative effect of these distributions on the ring strain energy suggests that the glass structure avoids a random or uncorrelated joint distribution of these structural parameters as in a continuous random network, due to the high strain energy associated with such a distribution.

Pressure-induced structural change in MgSiO3 glass at pressures near the Earth’s core–mantle boundary

Knowledge of the structure and properties of silicate magma under extreme pressure plays an important role in understanding the nature and evolution of Earth's deep interior. Here we report the structure of MgSiO3 glass, considered an analog of silicate melts, up to 111 GPa. The first (r1) and second (r2) neighbor distances in the pair distribution function change rapidly, with r1 increasing and r2 decreasing with pressure. At 53-62 GPa, the observed r1 and r2 distances are similar to the Si-O and Si-Si distances, respectively, of crystalline MgSiO3 akimotoite with edge-sharing SiO6 structural motifs. Above 62 GPa, r1 decreases, and r2 remains constant, with increasing pressure until 88 GPa. Above this pressure, r1 remains more or less constant, and r2 begins decreasing again. These observations suggest an ultrahigh-pressure structural change around 88 GPa. The structure above 88 GPa is interpreted as having the closest edge-shared SiO6 structural motifs similar to those of the crystalline postperovskite, with densely packed oxygen atoms. The pressure of the structural change is broadly consistent with or slightly lower than that of the bridgmanite-to-postperovskite transition in crystalline MgSiO3 These results suggest that a structural change may occur in MgSiO3 melt under pressure conditions corresponding to the deep lower mantle.

Structural Evolution of CO2-Filled Pure Silica LTA Zeolite under High-Pressure High-Temperature Conditions

The crystal structure of CO2 filled pure SiO2 LTA zeolite has been studied at high pressures and temperatures using synchrotron based x ray powder diffraction. Its structure consists of 13 CO2 guest molecules, 12 of them accommodated in the large alpha cages and 1 in the beta cages, giving a SiO2:CO2 stoichiometric ratio smaller than 2. The structure remains stable under pressure up to 20 GPa with a slight pressure dependent rhombohedral distortion, indicating that pressure induced amorphization is prevented by the insertion of guest species in this open framework. The ambient-temperature lattice compressibility has been determined. In situ high pressure resistive heating experiments up to 750 K allow us to estimate the thermal expansivity at 5 GPa. Our data confirm that the insertion of CO2 reverses the negative thermal expansion of the empty zeolite structure. No evidence of any chemical reaction was observed. The possibility of synthesizing a silicon carbonate at high temperatures and higher pressures is discussed in terms of the evolution of C-O and Si-O distances between molecular and framework atoms.

Dispersion effects inSiO2polymorphs: Anab initiostudy

The effect of electronic dispersion over a wide variety of SiO2 polymorphs (faujasite, ferrierite, α-cristobalite, α-quartz, coesite, and stishovite) is investigated using state-of-the-art density functional theory. Different functionals and dispersion correction schemes are compared, ranging from the local density approximation to fully nonlocal exchange-correlation functionals. It is shown that both empirical dispersion corrections and fully nonlocal functionals improve the energetics and give correct volumetric data. However, the correct volume results come from error cancellation between an overestimation of the Si-O distance and an underestimation of the Si-O-Si angle. Quantum Monte Carlo is used to compute the quartz-cristobalite energy difference within an accuracy of 0.2 kCal/mol per SiO2 unit. This demonstrates the feasability of achieving subchemical accuracy on extended systems, and confirms the validity of the Slater-Jastrow ansatz for describing SiO2 polymorphs.

Pressure impact on the structure, elasticity, and electron density distribution of CaSi2O5

Ab initio molecular dynamics simulations were used to reveal the mechanism of the fivefold to sixfold transition in Si coordination numbers of CaSi${}_{2}$O${}_{5}$. The longest first-neighbor Si-O distance drops from 2.8 to 1.8 \AA{} upon the triclinic to monoclinic transition. We find significant bulk modulus softening during the structure crossover, which is due to appearance of intermediate Si-O connections in the triclinic phase under slightly nonhydrostatic stress. Nonetheless, no soft phonon modes were found in either structure, indicating that both structures are dynamically stable. Across the transition, ${c}_{33}$ doubles and ${c}_{35}$ increases sixfold in magnitude due to the formation of new Si-O bonds. Chemical bonding analysis reveals distinctions in the electron localization function and bond ellipticity between the regular (1.8 \AA{}) and the dangling Si-O bonds (2.8 \AA{}), both of which suggest an impending disassociation of the dangling Si-O bond.

ChemInform Abstract: Simulating the Properties of Small Pore Silica Zeolites Using Interatomic Potentials

AbstractReview: 66 refs.

Simulating the properties of small pore silicazeolites using interatomic potentials

Despite the sustained use of forcefield methodologies to study SiO(2) polymorphs few reviews on the subject are available in the literature. The present study is an attempt to help fill this gap, focusing on classical forcefields used to reproduce and predict properties of pure silica zeolites (or zeosils) such as cell parameters, SiO distance and especially pore size. Instead of an exhaustive study we have focused on an application where diffusion of hydrocarbons makes important the use of pure silica zeolites. A particular area of interest is small pore zeosils containing 8-rings as the largest window, which are industrially interesting for their ability to perform kinetic separations of mixtures of C3 hydrocarbon molecules whose dimensions are of similar characteristics. A set of forcefields have been selected from the literature to analyze their accuracy and transferability when predicting structural, mechanical and dynamical properties of small pore pure silica zeolites and their performance at selective diffusion of C3 hydrocarbons.

Silica dissolution catalyzed by NaOH: Reaction kinetics and energy barriers simulated by quantum mechanical strategies

Quartz dissolves in Na+ solutions much faster than in pure water though the experimental activation energies are essentially the same. To elucidate the mechanism, silica dissolution was simulated using Gaussian 03 software at B3LYP/3-21G* initially and the B3LYP/6-311G(2d,p) levels, applied to a modeled silica Si4O6(OH)4 in the presence of an NaOH molecule and up to five water molecules. Each water molecule was added one by one to the system and approached the surface silicon in successive geometry optimizations. The atomic positions of the sodium hydroxide, the water molecules and the surface SiO3(OH) moiety were varied to mimic the surface reaction whereas the positions of the remaining Si3O3(OH)3 atoms were frozen to represent the bulk structure throughout the geometry optimization. In the first step a single H2O molecule was added to the system [Si4O6(OH)4 + NaOH]. None of the Si-O bonded interactions were ruptured by the intrusion of the water molecule but the surface Si was stabilized in the Q3Si site (connected to three Si-O-Si bridges), being coordinated by five oxygen atoms. The energy barrier was 63 kJ/mol. In each of the second and third steps one more water molecule was introduced to the system. One Si-O bonded interaction of the Si-O-Si bridges was ruptured to make Q2Si and Q1Si sites in the second and third steps, respectively, and the energy barriers were low (22-29 kJ/mol). In the fourth and fifth steps, the added water molecules were prevented by the sodium ion from reaching the last Si-O-Si bridge, leaving the Si in the Q1Si site. The basis set was raised to the 6-311G(2d,p) level and applied to the 63 kJ/mol barrier found in the first step to determine the maximum energy barrier in the series of reactions. The barrier increased to 88 kJ/mol (82 kJ/mol in enthalpy), which is still in the range of experimental activation energies of 46-96 kJ/mol. In summary sodium works to stabilize the surface silicon in penta-coordination with the energy barrier, 82 kJ/mol, resulting in longer Si-O distances and the weakening of the bonded interactions. This makes the Si-O rupture easier and faster, which gives insight as to how the presence of alkali enhances silica dissolution.

Cs3NdSi8O19 und Cs6Nd2Si21O48: Zwei cäsiumhaltige Oxosilicate des Neodyms im Vergleich / Cs3NdSi8O19 and Cs6Nd2Si21O48: Two Caesium-containing Oxosilicates of Neodymium in Comparison

In the quaternary system Cs / Nd / Si / O, two new representatives, the phyllo-oxosilicate Cs3NdSi8O19 and the tecto-oxosilicate Cs6Nd2Si21O48, were synthesized by CsF-flux-supported solid-state reactions between Nd2O3 and SiO2. The first one, Cs3NdSi8O19 (orthorhombic, Cmcm (no. 63), a = 705.74(5), b = 2712.85(19), c = 1163.72(8) pm, Z = 4), is not isotypic to the related scandium compound Cs3ScSi8O19. The [SiO4]4− tetrahedra (d(Si4+ -O2−) = 156 -163 pm) in the structure of Cs3NdSi8O19 are connected via common corners to form corrugated, loop-branched double layers containing four- and eight-membered rings in the (010) plane and eight-membered rings along [001]. Each of the eight-membered ellipses emerging along [100] is additionally loopbranched by two four-membered chains. The oxosilicate double layers are cross-linked by vertexsharing via otherwise isolated [NdO6]9− octahedra (d(Nd3+-O2-)= 232 - 234 pm) to build up a three-dimensional framework. Also in between the oxosilicate double layers, the (Cs1)+ cations are located on the 8 f site. Each of the octagonal channels along [001] hosts one (Cs3)+ and two (Cs2)+ cations, which both reside at only partially occupied sites (8g and 8 f , respectively) and disorder, because otherwise too short Cs+ ・ ・ ・ Cs+ distances would occur. The second compound, Cs6Nd2Si21O48, crystallizes also in an orthorhombic space group (Pmmn (no. 59), a = 2189.24(15), b = 731.92(5), c = 1593.61(11) pm, Z = 2). Starting from a loop-branched single layer containing five- and eight-membered rings, a three-dimensional framework of vertex-shared [SiO4]4− tetrahedra (d(Si4+-O2−) = 149 - 164 pm) built up, in which the Si-O distances range from 149 to 164 pm within a broad range. In certain cavities, one kind of Nd3+, but four kinds of Cs+ cations (here, all sites with full occupation) are embedded. Also surrounded by only six O2− anions just like in the first case, the Nd3+ cations (d(Nd3+-O2−) = 233 - 237 pm) exhibit an unusually small, but not unknown coordination sphere for this relatively large lanthanoid(III) cation

Structural investigation of low-symmetry vesuvianite collected from Tojyo, Hiroshima, Japan: Implications for hydrogarnet-like substitution

The crystal structure of a unique type of vesuvianite with the space group P4/n [a = 15.576(2), c = 11.835(2) Å] formed by the retrograde alteration of gehlenite in high-temperature skarn at Kushiro (Tojyo, Shobara Hiroshima) southwestern Japan, is determined by single-crystal X-ray diffraction (R1 = 3.63%). Significant vacancies (29%) occur only on one of the two pseudo-equivalent Z1 sites (Z1b). In addition, the Si-O distance at the Z1b site remarkably increases up to 1.707(3) Å, indicating the existence of a hydrogarnet-like substitution, while Z1a, the other pseudo-equivalent Si site, has only 5% vacancies associated with an Si-O distance of 1.636(3) Å. The total refined Si site occupancy of 17.7 atoms per formula unit is in excellent agreement with the results of electron-microprobe analyses.