Crystal-structure refinement of zirconium-rich eudialyte and its place among calcium-poor eudialyte-group minerals

The repeated refinement of the crystal structure of zirconium-rich eudialyte based on the X-ray diffraction data set collected earlier revealed new structural features. The trigonal unit-cell parameters are a = 14.222(3) Å, c = 30.165(5) Å, V = 5283.9 ų. The refinement resulted in the reduction of the R factor from 0.045 (2347F > 4σ(F)) to 0.035 (3124F > 3σ(F)). It was found that the ordering of Ca and Fe in six-membered rings leads to the lowering of the symmetry to R3. An excess amount of zirconium (more than three atoms per symmetrically independent unit) is located in the M2 microregion in square and five-vertex polyhedral positions. However, this amount is insufficient to be dominant, and the deficiency of zirconium is compensated for by sodium atoms. Based on the new data, zirconium-rich eudialyte can be assigned to the oneillite subtype, being a zirconium-rich and aluminum variety of raslakite.     

Modular structure of a potassium-rich analogue of eudialyte with a doubled parameter c

The structure of a new potassium-rich representative of the eudialyte group described by the idealized formula Na₂₇K₈Ca₁₂Fe₃Zr₆[Si₅₂O₁₄₄](O,OH,H₂O)₆Cl₂ was established by the methods of the X-ray diffraction analysis (R = 0.045, 3900 reflections). The unit-cell parameters are a = 14.249(1) Å, c = 60.969(7) Å, V = 10 720.3 ų, sp. gr. R3m. The structure of the mineral is characterized by in-layer order in cation arrangement resulting in doubling of the parameter c and the formation of two different modules, of which one corresponds to composition and structure of eudialyte (with an admixture of kentbrooksite) and the other is the derivative of alluaivite.     

Modular structure of a sodium-rich analogue of eudialyte with the doubled c-parameter and the R3 symmetry

The structure of a new sodium-rich representative of the eudialyte group with the ideal formula (Na,Sr, K)₃₅Ca₁₂Fe₃Zr₆TiSi₅₁O₁₄₄(O,OH,H₂O)₉Cl₃ was established by the X-ray diffraction analysis (R = 0.054 based on 3503 |F|). The unit-cell parameters are a = 14.239(1), c = 60.733(7) Å, V = 10663.9 ų, sp. gr. R3. The mineral structure is characterized by in-layer order of cations resulting in doubling of the c-parameter and the formation of two modules, the composition and the structure of one of which corresponds to eudialyte (with an impurity of kentbrooksite), the prototype of the second module is alluaivite. Lowering of the symmetry is caused by ordering of Ca atoms and the Ca-replacing elements in the alluaivite module and by the displacements of the alkali cations from the m plane.     

Crystal structure of manganese-rich variety of eudialyte from Suchina Hill,India, and manganese ordering in eudialyte-group minerals

The crystal structure of eudialyte of hydrothermal genesis from Sushina Hill, India, where it was found associated with potassium feldspathoid, albite, aegirine, and nepheline, has been solved by X-ray diffraction analysis. One specific feature of the chemical composition of this mineral is a low content of CaO (7–8.5 wt %) and an elevated manganese content (9–10.5 wt % MnO). When compared with eudialytes of magmatic genesis, this eudialyte is enriched in Nb, Sr, Y, and rare earth elements (REEs). It is described within the sp. gr. R3m, with the unit-cell parameters a = 14.2483(3) Å and b = 30.294(1) Å; R = 4.7%. The idealized formula (Z = 3) is (Na,□,Sr)₁₅(Ca,Mn)₆Mn₃(Si,Nb)₂Zr₃[Si₂₄O₇₂](OH,Cl,H₂O)_5.5. It is established that Mn atoms occupy three sites: two subsites in five-vertex polyhedra M2a and M2b, spaced by a distance of 0.87 Å and characterized by occupancies of 0.51 and 0.49, respectively, and the M1 site in the octahedron of six-membered ring, jointly with Ca and REE. With allowance for the established features of chemical composition and structure, the mineral under study can be considered as a strontium-rich variety of kentbrooksite.     

Crystal structure of centrosymmetric 12-layer sodium-rich eudialyte

The structure of a new representative of the eudialyte group with the formula (Na,Sr,K)₁₈Ca₆Zr₃Fe[Si₂₅O₇₂](OH)₂Cl · H₂O from the Lovozero massif (Kola Peninsula) was studied by X-ray diffraction. The trigonal unit-cell parameters are a = 14.226 Å, c = 30.339 Å, sp. gr. R \(\bar 3\) m; the R factor is 0.045 based on 990 reflections. This sample is of interest as a sodium-rich and iron-poor mineral having a rare centrosymmetric structure, in which the M(2) site is occupied predominantly by sodium atoms. The dependence of the formation of centrosymmetric and non-centrosymmetric structures on the composition of eudialyte-group minerals was analyzed.     

Crystal structure of modular sodium-rich and low-iron eudialyte from Lovozero alkaline massif

The structure of the sodium-rich representative of the eudialyte group found by A.P. Khomyakov at the Lovozero massif (Kola Peninsula) is studied by X-ray diffraction. The trigonal cell parameters are: a = 14.2032(1) and c = 60.612(1) Å, V = 10589.13 Å3, space group R3m. The structure is refined to the final R = 5.0% in the anisotropic approximation of atomic displacement parameters using 3742|F| > 3σ(F). The idealized formula (Z = 3) is Na₃₇Ca₁₀Mn₂FeZr₆Si₅₀(Ti, Nb)₂O₁₄₄(OH)₅Cl₃ · H₂O. Like other 24-layer minerals of the eudialyte group, this mineral has a modular structure. Its structure contains two modules, namely, “alluaivite” (with an admixture of “eudialyte”) and “kentbrooksite,” called according to the main structural fragments of alluaivite, eudialyte, and kentbrooksite. The mineral found at the Lovozero alkaline massif shows some chemical and symmetry-structural distinctions from the close-in-composition labyrinthite modular mineral from the Khibiny massif. The difference between the minerals stems from different geochemical conditions of mineral formation in the two regions.     

Crystal structure of ilyukhinite,a new mineral of the eudialyte group

The crystal structure of ilyukhinite, a new mineral of the eudialyte group, is studied by X-ray diffraction. The mineral found in pegmatite bodies of the Kukisvumchorr Mountain (Khibiny alkaline complex) is characterized by low sodium content, high degree of hydration, and predominance of manganese over iron. The trigonal cell has the following parameters: a = 14.1695(6) and c = 31.026(1) Å; space group R3m. The structure is refined to final R = 0.046 in the anisotropic approximation of atomic displacements using 1527F > 3σF. The idealized formula of ilyukhinite (Z = 3) is written as (H₃O,Na)₁₄Ca₆Mn₂Zr₃Si₂₆O₇₂(OH)₂ · 3H₂O. The new mineral differs from other representatives of the eudialyte group by the predominance of both oxonium in the N positions of extra-framework cations and manganese in the М2 position centering the tetragonal pyramid.     

Ikranite: Composition and structure of a new mineral of the eudialyte group

The crystal structure of a new mineral, ikranite, of the eudialyte group discovered in the Lovozero massif (the Kola Peninsula) was established by X-ray diffraction analysis. The crystals belong to the trigonal system and have the unit-cell parameters a = 14.167(2) Å, c = 30.081(2) Å, V = 5228.5 Å 3, sp. gr. R3m. Ikranite is the first purely ring mineral of the eudialyte group (other minerals of this group contain ring platforms of either tetrahedral or mixed types). It is also the first representative of the eudialyte group where Fe³⁺ prevail over Fe²⁺ ions.     

Acidity-dependent mesostructure transformation of highly ordered mesoporous silica materials during a two-step synthesis

Highly ordered mesoporous silica materials have been synthesized under mildly acidic conditions by templating with a nonionic triblock copolymer (Pluronic P104) in a two-step process. It was found that a transformation from the SBA-15 type 2-dimensional (2D) hexagonal channel mesostructure (p6mm symmetry) to the MSU-X type 3-dimensional (3D) worm-like mesostructure could be induced by varying the pH whilst keeping all other conditions constant. The transformation between two types of mesoporous silica materials can be attributed to the effect of varying proton concentration on the interaction between organic micelles and inorganic species. Both types of mesoporous materials have high surface areas, large pore volumes, thick pore walls, large mean pore sizes, and narrow pore size distribution.     

Crystal structure of cation-deficient calciohilairite and possible mechanisms of decationization in mixed-framework minerals

The crystal structure of cation-deficient calciohilairite from the Lovozero massif (the Kola Peninsula) was established (Siemens P4 diffractometer, MoK _α radiation, 409 independent reflections with | F| > 4σ(F), anisotropic refinement, R(F) = 0.037). Like other representatives of the hilairite structure type, calcio-hilairite is described by the space group R32 (a = 10.498(2) Å, c = 7.975(2) Å, Z = 3), whereas its unit-cell parameter c is reduced by a factor of two. Two positions in the cavities of the mixed zirconium-silicon-oxygen framework are occupied by Ca and Na cations in the ratio of 1: 1 (partly occupied A(1) position) and oxonium cations (H₃O) ⁺ and H₂O molecules in the ratio of 1: 2 (A(2) position). Different types of isomorphous replacement accompanying the formation of cation-deficient mixed-framework structures (lovozerite, vinogradovite-lintisite, labuntsovite-nenadkevichite, eudialyte, etc.) are considered. Based on the X-ray diffraction data, the following scheme of isomorphism in the structure of cation-deficient calciohilairite is suggested: 2Na⁺ + H₂O ? 0.5Ca²⁺ + 1.5h□ + (H₃O)⁺, where □ is a vacancy.     

Structure of the new mineral paratsepinite-Na and its place in the labuntsovite group

X-ray diffraction study of high-strontium Ti-enadkevichite showed that it is a new mineral, which was given the name paratsepinite-Na, sp. gr. C2/m, a double value of the parameter c, and a different distribution of the non-framework cations in comparison with tsepinite-Na. In particular, strontium is located not only in the hexagonal window of the small channel, but also in three positions of the large channel. The characteristics of the mosaic blocks constituting the crystal are studied. It is assumed that polysynthetic twinning in these crystals and many other representatives of this family is associated with the phase transformation occurring in this hydrothermal mineral during its cooling after formation. The comparison of all the three structures of the vuoriyarvite subgroups of the labuntsovite group allows us to explain the disability of vuoriyarvite to absorb strontium from the aqueous solution by its smaller unit-cell volume and higher framework charge in comparison with those of tsepinites.     

Optical activity and crystalline structure of crystals of the langasite family

The dependences of the refraction on the structural parameters are considered for the La₃Ga_5.5Nb_0.5O₁₄, La₃Ga_5.5Ta_0.5O₁₄, La₃Ga₅SiO₁₄, Ca₃Ga₂Ge₄O₁₄, and Sr₃Ga₂Ge₄O₁₄ crystals of the langasite family. It is shown that the angle of deviation of the 1a octahedron faces that are normal to the optical axis from 60° is one of the main sources of optical activity of these crystals. The interaction of the cations belonging to the 3e dodecahedron and 1a octahedron, cations of the 3e dodecahedron and 2d tetrahedron, and the repulsion of O²⁻ ions are believed to be the basic interactions affecting the angle value. The dependences of the angle on the crystal-chemical characteristics of the considered crystals are analyzed. The role of the relative sizes of the structural polyhedra is demonstrated.     

Crystal structure of 2,4,6-trinitropyridine and its N-oxide

The structures of 2,4,6-trinitropyridine (TNPy) and its N-oxide were determined by X-ray single crystal diffraction. TNPy and 2,4,6-trinitropyridine-1-oxide (TNPyO) crystallize in space groupPbcn andPnma, respectively. The crystallographic parameters are as follows: TNPy,a = 28.573(6) Å,b = 9.7394(19) Å, andc = 8.7566(18) Å, α = β = γ = 90^°, μ = 0.164 mm^?1,V = 2436.8(8) ų,z = 12,Dx = 1.751 mg/mm³,F(000) = 1296,T = 293(2) K, 1.43^°≤ θ≤ 27.40^°, the finalR factor:R ₁ = 0.0574,wR ₂ = 0.1337. TNPyO,a = 9.6272(19) Å,b = 14.128(3) Å, andc = 5.9943(12) Å, α = β = γ = 90^°, μ = 0.179 mm^?1,V = 815.3(3)ų,z = 4,Dx = 1.875 mg/mm³,F(000) = 464,T = 293(2) K, 2.88^°≤ θ≤ 27.44^°, the finalR factor:R ₁ = 0.0497,wR ₂ = 0.1515.     

Mechanical properties and adhesion of hydrated glass surface layers

This paper reviews results for interfacial adhesion and fracture of silicate glasses that demonstrate the effect of hydrated glass surface layers on the mechanical properties of glass. First, it is shown how the generation of hydrated surface layers formed on alkali borosilicate glasses can control crack propagation rates. Crack growth data, solution analysis and surface stress measurements are used to support a fracture model that involves the generation of surface stress on the crack walls behind the crack tip. A fracture mechanics based model is used to show that stressed layers can contribute to the crack tip stress intensity in a way that either increases or decreases the rate of crack propagation. In the case of alkali containing silicate glasses, tensile stresses formed on the crack walls increase the crack tip stress and contribute to the formation of a low velocity plateau in the stress intensity vs. crack velocity curve. Second, fracture mechanics test techniques are used to examine the adhesive bond formed between hydrated surface layers and bulk silicate glass. The adhesive bonds formed by sol-gel precursors composed of colloidal silica, hydrolyzed organosilanes and alkali silicate solutions are compared to determine the mechanism of interfacial bonding to dense silica substrates. The formation of siloxane bonds across the interface depends upon the nature of the silicate polyanions in solution. For the case of soluble alkali silicate derived films, heat treatments at temperatures as low as 200°C can result interferfacial adhesion energies as large as the fracture energy of silica glass. These results have important implications to the aging and repair of surface damage in glass as well as the adhesion of sol-gel derived thin films.     

Binding of alkaline earth metal ions to nucleotides: crystal structure of a hydrated strontium salt of inosine-5′-monophosphate

Crystals of Sr²⁺ (C₁₀H₁₁PN₄O₈)^2– · 6.5H₂O are orthorhombic, space groupP2₁2₁2₁, witha = 21.925(1),b = 21.503(1),c = 8.600(1) Å, and eight formula weights per unit cell. A trial structure was obtained by Patterson, superposition, and Fourier techniques and was refined by full-matrix least-squares calculations using absorption-corrected, CuK, diffractometer data. The finalR index is 0.032. A strontium ion is coordinated to N(7), the site that is often involved in the binding of transition metals to purine nucleotides. Strontium ions are also directly coordinated to a phosphate-oxygen atom and to ribose-hydroxyl groups. These direct interactions are accompanied by a number of outer-sphere, water-mediated contacts of strontium ions with various acceptor sites on the nucleotides.     

Vapour growth and epitaxy of minerals and synthetic crystals

Surface microtopographs of the following crystals, both natural and synthetic, grown from pure vapour phase (PV), by chemical vapour transport (CVT), from high temperature solution (HTS) and hydrothermal solution (HS) are compared according to the criteria of (1) whether spirals are circular or polygonal and (2) how wide is the step separation of the spiral; SiC (PV, CVT, both synthetic), hematite (CVT, HTS, natural and synthetic), corundum (CVT, HTS, synthetic), mica minerals (PV, CVT, HS, natural and synthetic) and beryl (CVT, HTS, HS, natural and synthetic). Clear differences in morphology and step separation were found between crystals grown from vapour phase and solutions, between PV and CVT, as well as between natural and synthetic crystals. The differences are analysed in conjunction with the recent developments of computer simulations on spiral morphology. The results show interaction between solid and fluid plays very important role in determining the spiral morphology. Oriented intergrowth between different crystals well known among minerals, such as epitaxy, topotaxy, co-axial intergrowth, exsolution etc. are briefly summarized. It is also briefly explained how these relations are used in understand the growth or cooling histories of natural minerals.     

Neutron scattering studies of the structure of a highly tetrahedral form of amorphous carbon

The structure of amorphous carbon prepared by plasma-arc deposition has been determined by neutron scattering and analysed to estimate the fraction of tetrahedrally and trigonally bonded carbon. The mean value of the nearest neighbour C-C distance is close to that of crystalline diamond and a fit to the data for the first interatomic distance gives about 86% tetrahedral bonding. This form of diamond-like carbon therefore appears to be the structural equivalent of a-Si and a-Ge. A somewhat different interpretation is obtained from an analysis of the measured first and second peak coordination numbers and an attempt is made to reconcile the two approaches.     

Preparation and characterization of ordered mesoporous silica membrane

Hexagonal mesoporous silica (MCM-41) membranes were prepared at air-water interface by means of an interfacial silica-surfactant self-assembly process. The free-standing and oriented mesoporous silica membranes with pore size ≈2.9-3.8 nm were synthesized at room temperature in acidic media and were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) observations. Small-angle X-ray diffraction (SAXRD) patterns of membranes clearly indicated that as-synthesized membranes were typical of MCM-41 materials with a periodic hexagonal structure with the channels parallel to the surface. SEM images showed that the as-synthesized membrane was continuous and crack-free. In this paper, some novel findings are reported.     

Molecular structure of diantipyrylphenylmethane and its complex with neodymium nitrate

The structures of diantipyrylphenylmethane (DAPM) and its complex with neodymium nitrate [Nd(NO₃)₃ · DAPM · CH₃OH] · 2CH₃OH are determined by X-ray diffraction. The free ligand adopts a trans conformation with the opposite orientation of the oxygen atoms of the carbonyl groups. In the complex, diantipyrylphenylmethane acts as a bidentate ligand and coordinates the Nd atom through the carbonyl oxygen atoms, thus forming the eight-membered metallocycle. The coordination number of neodymium is nine (six O atoms of the bidentate nitrate groups, two O atoms of diantipyrylphenylmethane, and one O atom of the methanol molecule).