Rings Of Tetrahedra Research Articles

Structural characterization of strontium monoferrite SrFe2O4, a new stuffed framework compound

Single crystals of strontium monoferrite were obtained by growth from a melt at 1550°C. The monoclinic crystals belong to space group P1121/n (a=8.1098(9),b=9.1177(11),c=10.8495(10) Å, γ=91.53(1)°,V=801.96(3) Å3, Z=8,Dcalc=4.36 g cm–3). The structure was solved by direct methods using a single crystal diffraction data set. The main structural features of SrFe2O4 are layers of FeO4-tetrahedra perpendicular to the a-axis. The stacking of the layers parallel to [100] results in a three-dimensional framework containing tunnels, where the strontium cations are located. Individual layers can be described as being built by the condensation of ditrigonally-shaped six-membered tetrahedral rings. The sequence of up (U) and down (D) pointing apices within a single ring is UUUDDD. The structure belongs to the family of the zeolite ABW-type structures and is the first zeolite type compound with exclusively ferric iron occupying the tetrahedral positions of the framework.

ChemInform Abstract: (Ni3‐xMgx) [B3P3O12(OH)6] ×6H2O (x = 1.5): A Novel Borophosphate‐Hydrate Containing Isolated Six‐Membered Rings of Tetrahedra.

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On the Al/Fe substitution in iron doped monocalcium aluminate - the crystal structure of CaAl1.8Fe0.2O4

A calcium ferro-aluminate with composition CaAl 1.8 Fe 0.2 O 4 has been synthesized. The crystal structure of the phase was studied by X-ray single crystal diffraction and refined to an R 1-index value of 0.033, for 1287 observed independent reflections, collected with graphite monochromatized Mo K α-radiation. The compound crystallizes in the monoclinic space group P 12 1 / n l with Z = 12. Cell parameters were a = 8.758(1) A, b = 8.112(1) A, c = 15.311(1) A, β = 90.21(1)°. The crystal structure is characterized by layers of six-membered tetrahedral rings of ditrigonal shape showing two different ring conformations of up (U) and down (D) pointing vertices: UUDUDD and UDUDUD, respectively. The stacking of layers comprises a three-dimensional framework containing channels that are occupied by the calcium cations. The structure is isotypic with CaAl 2 O 4 , the major compound of calcium aluminate cements.

Direct view of the structure of a silica glass fracture surface

The fracture minor surface of a silica glass, prepared at 1×10 −11 mbar in the chamber of an ultra-high-vaccum-atomic-force microscope (UHV–AFM), was imaged at 1×10 −8 mbar with high spatial resolution. The features directly seen were interatomic distances, e.g., O–O and Si–O, groupings of atoms, e.g., SiO 4 tetrahedra and rings of tetrahedra, and network holes. A comparison with a structure calculated by a molecular dynamics simulation gave good coincidence. This direct view of the silica glass surface supports Zachariasen's network structure model of glass.

Structural investigations of celsian glass derived from Ba-LTA zeolite

The structure of stoichiometric celsian glass was investigated by 29Si MAS NMR and IR spectroscopies and reverse Monte Carlo (RMC) simulation of X-ray diffraction data. The glass was prepared by thermally induced phase transformation of Ba2+-exchanged LTA (Linde Type A) zeolite, under annealing conditions prior to hexacelsian crystallization. NMR and IR measurements have shown that the local silicon–aluminium ordering of the starting zeolite framework, as well as the framework fragments in the form of deformed tetrahedral rings, is retained in the glass structure. Interatomic distances and mean coordination numbers were calculated from the three-dimensional RMC derived structure model. The local Ba2+ environment in the glass was compared with the corresponding crystalline polymorphs. Ba–O distances in the glass are longer than the distances in hexacelsian but the coordination number 12.00 and the increase in the Ba–T distance, compared to zeolite, indicate the establishment of hexacelsian-like coordination. Inspection of the three-dimensional RMC model of the glass showed a layered structure, with Ba2+ cations between the layers.

Cordierite I: The coordination of Fe2+

The incorporation of Fe2+ was investigated in four natural cordierite samples. 57Fe Mössbauer, single-crystal UV-VIS optical absorption, and X-ray absorption spectroscopies, as well as X-ray single-crystal diffraction were used. Mössbauer, optical, and XAS spectroscopy show that Fe2+ is incorporated on two different structural sites in two Mg-rich samples. Mössbauer measurements give the best quantitative measure of the amounts of Fe2+, but the optical spectra are the most sensitive for determinations at low concentrations and at high-bulk Fe2+ concentrations in cordierite. The spectroscopic data are most consistent with small amounts of Fe2+ (i.e., 0.02 of Fe2+ per formula unit) being located on a tetrahedral site rather than in the center (or off center) of the six-membered tetrahedral rings or in channel cavities. X-ray single-crystal refinements on two Mg-rich cordierites show a very small excess electron density on T11 and not in the channels. A third refinement on a slightly more iron-rich sample shows, in contrast, no excess electron density on T11. We interpret these data as indicating that small amounts of Fe2+ (0.01 to 0.02 atoms per formula unit) replace tetrahedral Al11 in cordierite, where charge balance is achieved by placing Na in the center of the six-membered rings. This substitution is consistent with the known chemistry of natural cordierites and with simple structural energetics. The identification and assignment of small amounts of Fe2+ on T11 requires spectroscopic determination or careful X-ray single-crystal refinements and cannot be achieved from composition data and structural formula calculations.

Cordierite II: The role of CO2and H2O

Polarized single-crystal Raman spectra at room temperature and 5 K and polarized infrared spectra at room temperature were obtained from four natural cordierites of different compositions in the wave number region of the CO2 symmetric stretching vibration and the H2O stretching vibrations. The CO2 molecules are preferentially aligned parallel to the X axis, consistent with results from X-ray diffraction and optical studies. The CO2 contents of six natural cordierites, previously studied by powder IR methods (Vry et al. 1990), were determined via Raman spectroscopy. A linear relationship was found between CO2 content and the Raman intensity ratio of the normalized CO2 stretching mode against a Si-O stretching mode. This permits a determination of the CO2 contents in cordierite using micro-Raman measurements. The internal stretching modes between 3500 and 3800 cm-1 were assigned to various types of H2O molecules occurring in the channel cavity. Three different orientations of H2O molecules that have no interactions with alkali cations located at 0,0,0 in the sixmembered tetrahedral rings are classified in a static model as Class I H2O molecules. The H···H vector for two of them is parallel to [001], and their molecular planes lie in the XZ and YZ crystal planes. The third type has its H···H vector directed along the X axis and its molecular plane lies in the XZ plane. Two other types of H2O have interactions with the alkali cations located at 0,0,0. They are classified as Class II H2O. They distinguish themselves by the number of H2O molecules bonded to the alkali atoms. The formation of weak hydrogen bonds at low temperatures may explain the appearance of some Raman stretching modes below 200 K. The H2O molecules of Class I-Type I/II are probably dynamically disordered about [001] hopping between orientations in the XY and XZ planes down to 5 K. Class II H2O may also be disordered, but more measurements are required to describe its dynamic behavior.

Dissolution of albite glass and crystal

When normalized by initial surface area, crystalline and amorphous albite release Si and Al at the same rate within error (±40%) as measured at pH 2, 5.6, and 8.4 at 25°C. Differences in density and tetrahedral ring structure between the glass and crystal structures, however, lead to more extensive Na and Al depletion from the glass surface, especially in acid. X-ray photoelectron spectroscopy (XPS) indicates that the chemistry of the altered layers on glass and crystal must be significantly different at a depth of ∼17Å–87Å. Nevertheless, angle-resolved XPS (ARXPS) indicates that the outermost 17Å of the glass and crystal surface are compositionally similar. In neutral and weakly basic conditions, XPS indicates less extensive depletion of Na and Al from reacted glass and crystal surfaces than in acidic conditions. Al enrichment was not observed at any pH on either the crystal or glass surface. At steady state, Al release was stoichiometric for all phases and all pH values, but Na release was always faster than release of Si, especially for the glass. These results are consistent with a model where only the outer surface controls dissolution and the deeper layers of the altered surface do not significantly affect dissolution rate. The similarity in dissolution rate between glass and mineral, if consistent for other phases, may also indicate that some future studies of mineral dissolution could be completed more efficiently by investigation of glass because such studies could reveal the chemical effects in dissolution independent of the microstructure and defects that populate natural mineral samples.

Synthetic tourmaline (olenite) with excess boron replacing silicon in the tetrahedral site: I. Synthesis conditions, chemical and spectroscopic evidence

One selected composition within the system Na2O-Al2O3-B2O3-SiO2-H2O (NABSH) was studied with the aim of synthesizing the new tourmaline end member olenite with the theoretical formula NaAl3Al6 [Si6O18] (BO3)O3(OH). The starting material consisted of a gel with the anhydrous composition 0.625Na2O.4.5Al2O3.6SiO2, but with 100% excess B2O3 added over that of the above formula, and a surplus of water to aid crystallization. Run conditions ranged from 4 to 50 kbar, 400-900°C, but a tourmaline phase could only be obtained at or above 10 kbar, with yields between 80 and 100% when ignoring amorphous quench products from the coexisting fluids. The synthetic olenites exhibit much smaller cell parameters than those reported for natural olenites, and indeed the smallest ones ever measured for any tourmaline phase. Analytical data on an olenite prepared at 25 kbar, 600°C, show excess boron and water relative to the theoretical formula, coupled with deficiencies in Si, Al, and Na. Spectroscopic investigations (MAS NMR, EELS, IR) prove – directly or indirectly – that boron occurs not only in trigonal coordination, but is also located in the tetrahedral ring site. Thus, a provisional structural formula is derived as (Na0.65□0.35) (Al2.72□0.28) (Al5.42Si0.58) [Si3.73B2.27O18] (BO3)3 (OH)3.87 O0.13. In this synthetic olenite the OH-content is close to the maximum of 4.0 p.f.u.; boron replaces tetrahedral Si according to BHSi−1, with this substitution cancelling the proton deficiency of the theoretical olenite formula. Because the sum (B+Si) exceeds 9.0, some Si seems to replace octahedral Al. Nevertheless, octahedral vacancies remain. There are indications that the above tourmaline composition is not unique for the system studied, but that a range of olenite solid solutions exists as a function of starting material and run conditions, possibly extending to the ideal olenite formula. Excess-boron tourmalines are probably confined to very Al-rich (or M3+-rich) compositions which – for stoichiometric reasons – should have proton deficiencies, but these may be compensated by the BHSi−1 substitution.

Spectroscopic and structural properties of synthetic micas on the annite-siderophyllite binary: Synthesis, crystal structure refinement, Mössbauer, and infrared spectroscopy

The effect of the incorporation of Al-Tschermak’s molecule to the trioctahedral potassium mica annite {K}[Fe3]<AlSi3>O10(OH)2 on local and average structure has been investigated by hydrothermal synthesis, structure refinement of X-ray powder diffraction data, Mössbauer and infrared spectroscopy. The various types of brackets indicate different structural sites. Samples with compositions {K}[Fe3-xAlx]<Al1+xSi3-x>O10(OH)2 were prepared by hydrothermal techniques. The maximum solubility of Al3+ is limited to x = 0.92 at 500 °C and to x = 0.82 at 700 °C. The main factor controlling the substitution limits is the ditrigonal distortion of the tetrahedral rings. Lattice parameters decrease linearly with increasing Al3+ content of the mica. A considerable decrease of M2-O and nearly no change of M1-O bond lengths with increasing Al3+ contents is indicative of preferred occupation of the M2 site by [Al3+]. Changes in K-O distances are also very pronounced and reflect the ditrigonal distortion of the tetrahedral sheet. The bimodal ferrous quadrupole splitting distribution (QSD) in annite, extracted from Mössbauer spectra, becomes narrower and more centered around 2.60 mm/s with increasing Al3+ contents, and its evolution suggests an increasing deviation from ideal octahedral coordination of Fe by O, illustrated by the increasing octahedral flattening angle y. The population of individual QSD components proves that it is impossible to resolve cis and trans M-sites in micas by Mössbauer spectroscopy. In the hydroxyl stretching region, up to 7 bands are observed in the infra-red spectra which correspond to OH groups adjacent to 3 Fe2+ (N-bands), to OH groups coordinated by Fe2+, Al3+, and Fe3+ (I-bands) and to configurations having one octahedral vacancy (V-bands). N- and I-type bands are shifted toward lower wavenumbers with increasing Al3+ content because of increasing OH···Otet interactions.

Tensegrities and rotating rings of tetrahedra: a symmetry viewpoint of structural mechanics

Symmetry is a common attribute of both natural and engineering structures. Despite this, the application of symmetry arguments to some of the basic concepts of structural mechanics is still a novelty. This paper shows some of the insights into structural mechanics that can be obtained through careful symmetry arguments, and will show how these can provide a key to understanding the paradoxical behaviour of some symmetric structures.

Crystal chemistry of high-pressure BaSi4O9in the trigonal (P3) barium tetragermanate structure

Crystals of BaSi 4 O 9 synthesized at 4 GPa and 1000 degrees C were determined to be isostructural with barium tetragermanate [trigonal, space group P3, a = 11.2469(11) and c = 4.4851(6) Aa, V = 491.3(1) Aa 3 ]. The structure (R = 2.4%) features a corner-linked framework of three-member silicate tetrahedral rings, which are cross-linked by isolated silicate octahedra. Ba cations occupy tenfold-coordinated sites in channels defined by the silicate framework. This structure is 4.2% denser than the topologically similar benitoite form of high-pressure BaSi 4 O 9 , which was produced by grinding the barium tetragermanate-type crystals described in this report.

Crystal chemistry of Mg-, Fe-bearing muscovites-2M1

Phengitic muscovite-2M 1 crystals [ (super [12]) (K (sub 0.88-0.99) Na (sub 0.01-0.09) Ca (sub 0.00-0.06) Ba (sub 0.00-0.01) ) (super [6]) (Al (sub 1.64-1.88) Fe (super 2+) (sub 0.06-0.29) Fe (super 3+) (sub 0.01-0.16) Mg (sub 0.00-0.16) Mn (sub 0.00-0.07) Ti (sub 0.00-0.06) ) (super [4]) (Si (sub 2.87-3.30) Al (sub 0.70-1.13) )(OH) (sub 1.56-2.07) F (sub 0.00-0.41) O (sub 9.91-10.25) ] from pegmatites and peraluminous granites were refined to investigate the influence of phengitic substitution on the mica structure. Single-crystal X-ray diffraction data were collected for eleven crystals in the C2/c space-group (agreement factor 2.1% < or = R obs , < or =3.9%). Tetrahedral Si and Al cation disorder was found for each sample, with the mean tetrahedral cation-oxygen distances ranging from 1.639 Aa < or ={T1-O}< or =1.647 Aa and 1.640 Aa < or ={T2-O}< or =1.646 Aa. As phengitic substitution increases, the octahedral sheet expands and requires a less distorted (more hexagonal) tetrahedral ring (7.70 degrees < or =alpha < or =11.38 degrees ) and low corrugation of the basal O plane (0.1796 Aa< or =Delta Z< or =0.2296 Aa). The electron density at the M2 site is greater than that required for the ideal muscovite-2M 1 structure, and a small excess of electron density is found in the M1 site. The inner sixfold coordination of the interlayer (A) cation is elongated along c (super *) , which is consistent with the high alpha values and the long A-O11 bond length.

Crystal structure of hexagonal trinepheline; a new synthetic NaAlSiO4modification

The crystal structure of a synthetic NaAlSiO 4 modification has been solved and refined to an R index of 0.020 for 2745 independent reflections. The compound is hexagonal with space group symmetry P6 1 , a = 9.995(2) Aa and c = 24.797(4) Aa. The crystal showed twinning by merohedry according to m 210 , which was accounted for in the calculations. The phase was named trinepheline, following prior studies, because the length of its c lattice parameter is three times the length of the c parameter in nepheline, whereas the a parameter is about 10 Aa in both phases. The crystal structure is characterized by layers of six-membered tetrahedral rings of exclusively oval conformation. The rings are built up by regularly alternating AlO 4 and SiO 4 tetrahedra. The stacking of the layers parallel to the c axis results in a three-dimensional network containing channels that are occupied by the Na cations. Although structural similarities with respect to tridymite derivatives can be found, hexagonal trinepheline represents a new type of stuffed tridymite that is not a simple superstructure of nepheline.

Oligomere Tetraeder-Anionen in Borophosphaten: Darstellung und Kristall­strukturen von NaFe[BP2O7(OH)3] und K2Fe2[B2P4O16(OH)2 ]/Oligomeric Tetrahedral Anions in Borophosphates: Synthesis and Crystal Structures of NaFe[BP2O7(OH)3] and K2Fe2[B2P4O16(OH)2

Abstract Crystals of the title compounds were grown under hydrothermal conditions. The crystal structures were determined by single crystal data. NaFe[BP2O7(OH)3] (monoclinic, C 2/c, a = 1042.0(2), b = 821.5(1), c = 921.7(1) pm, β = 116.60(1)°, Z=4): The crystal structure contains isolated anions [BP2O7(OH)3]4-, which correspond to trinuclear units PO1/2O3-BO2/2(OH)2 -PO1/2O2(OH) of corner sharing tetrahedra. Fe3+ is coordinated octahedrally by oxygen, Na+ is 8 -fold coordinated by oxygen forming cubes. K2Fe2[B2P4O16(OH)2] (triclinic, P1̄, a = 516.7(1), b = 808.9(1), c = 834.0(1) pm, α = 87.06(f), β = 80.21(1), γ = 86.59(1)°, Z = 1): The crystal structure contains isolated anions [B2P4O16(OH)2]8-. Four-membered rings of tetrahedra with two additional terminal phosphate groups are formed by condensation of two trinuclear units P O1/2O3 -BO3/2(OH) -PO2/2O2. Fe3+ is in an octahedral and K+ in an irregularly 10-fold coordination by oxygen.

A rietveld analysis of the crystal structure of ammonioleucite.

The crystal structure of ammonioleucite was refined by a Rietveld analysis using X-ray diffraction data of the type specimen. The refinement with a space group I4l/a was converged at Rwp=12.12, Rp=9.39, RF=4.86, RB=7.71, and Re=2.55% with lattice parameters of a=13.2106(6) and c=13.7210(7) A. Ammonioleucite is isostructural with leucite. The (Si, Al)O4 tetrahedra form 4-, 6-, and 8-membered rings by sharing their apical oxygen atoms. The crystal structure consists of the three-dimensional framework of the rings of the tetrahedra. Ammonium ions are located in cavities in the framework. The (NH4)+ ions in these cavities are larger than the corresponding K+ ions in leucite, causing the cavities to be enlarged relative to those in leucite. Among the 4-, 6-, and 8-membered rings of (Si,Al)O4 tetrahedra, only the 8-membered rings have diameters large enough to allow the migration of exchangeable K+, (NH4)+ and Na+ ions.

Densification of synthetic fused silica under ultraviolet irradiation

Fused silica undergoes densification upon prolonged exposure to high energy radiation and explanations in terms of changes in the silica tetrahedral ring structure have been offered. Under laser irradiation, the densification is often less than 10 parts per million, necessitating sensitive optical techniques for its measurement. This paper reviews some of the key literature, with emphasis on recent studies involving 193 nm excimer laser irradiation, and presents recent evidence that the UV laser-induced densification involves optically-induced weakening of bonds, via a two-photon absorption process, and subsequent structural relaxation of the glass.

A reexamination of the crystal structure of erionite

The crystal structure of a natural erionite from Nizhnyaya Tunguska, Siberia (schematic formula K 2 Ca 3.5 Al 9 Si 27 O 72 · 32H 2 O) has been refined to a residual wR of 0.044. The erionite framework can be described by a sequence AABAAC … of 6-membered rings of tetrahedra. The (Si, Al) distribution (24% of Al in T1 and 31% of Al in T2) is consistent with the strong disorder found in tetrahedral sites for all zeolites describable by a sequence of 6-rings. Erionite, as in the closely related zeolite offretite, shows a slight enrichment of Al in the single 6-ring. K is at the center of the cancrinite cage and coordinates 6 framework oxygen atoms. Ca atoms alternately occupy three sites, quite near each other, on the triad axis parallel to c , at the center of the erionite cage. Their coordination number varies between 6 and 9. The marked differences in T—O distances found between hydrated and dehydrated erionite are explained in terms of interactions between extraframework ions and framework oxygens.

Rb[B2P2O8(OH)] und Cs[B2P2O8(OH )]: Die ersten Borophosphate mit dreidimensional vernetzter Anionenteilstruktur /Rb[B2P2O8(OH)] and Cs[B2P2O8(OH )]: The First Borophosphates with a Threedimensional Anionic Tetrahedral Structure

The isotypic borophosphates Rb[B2P2O8(OH)] and Cs[B2P2O8(OH)] were grown under hydrothermal conditions. The crystal structures were solved by single crystal methods (Rb[B2P2O8(OH)]/ Cs[B2P2O8(OH)]: monoclinic, P21/c, a = 676.2(3)/682.7(3), b = 1217.3(5)/ 1236.8(5), c = 885.0(4)/900.6(5) pm, β = 97.3(1)/ 98.1(1)°. Z = 4). The structures contain a complex framework of three-, four-, six-, nine- and twelvemembered rings of tetrahedra. Along [100] the twelve-membered rings are stacked to give wide channels which contain the Rb+/Cs+-ions in an icosahedral coordination.

Crystal structure refinement of aluminian lizardite-2H2

Other| October 01, 1997 Crystal structure refinement of aluminian lizardite-2H2 Maria Franca Brigatti; Maria Franca Brigatti Universita di Modena, Dipartimento di Scienze della Terra, Modena, Italy Search for other works by this author on: GSW Google Scholar Ermanno Galli; Ermanno Galli Search for other works by this author on: GSW Google Scholar Luca Medici; Luca Medici Search for other works by this author on: GSW Google Scholar Luciano Poppi Luciano Poppi Search for other works by this author on: GSW Google Scholar Author and Article Information Maria Franca Brigatti Universita di Modena, Dipartimento di Scienze della Terra, Modena, Italy Ermanno Galli Luca Medici Luciano Poppi Publisher: Mineralogical Society of America First Online: 02 Mar 2017 Online Issn: 1945-3027 Print Issn: 0003-004X Copyright © 1997 by the Mineralogical Society of America American Mineralogist (1997) 82 (9-10): 931–935. https://doi.org/10.2138/am-1997-9-1010 Article history First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Maria Franca Brigatti, Ermanno Galli, Luca Medici, Luciano Poppi; Crystal structure refinement of aluminian lizardite-2H2. American Mineralogist 1997;; 82 (9-10): 931–935. doi: https://doi.org/10.2138/am-1997-9-1010 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract Well-crystallized euhedral crystals of aluminian lizardite–2H2 (⁠MG2.35Fe0.062+Fe0.073+Al0.52⁠) (Si1.41Al0.59)O5.00(OH)4.00 were found near Schio (Vicenza, Italy). To gain insight into the role of a high Al content lizardite, chemical analyses and single-crystal X-ray data collection were conducted. Structure refinement, completed in space group P63 (agreement factor R = 0.034), gives mean T-O values of 1.654 Å and 1.664 Å for T1 and T2 sites, respectively. The ditrigonal distortion of the six-membered tetrahedral ring is positive (α = +9.7°), as expected for the 2H2 polytype. The octahedral site has a mean bond length similar to that of the Mg-rich octahedra of amesite and distortion parameters similar to those of Al-rich octahedra. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.