Anionic constitution of 1-atmosphere silicate melts: implications for the structure of igneous melts

A structural model is proposed for the polymeric units in silicate melts quenched at 1 atmosphere. The anionic units that have been identified by the use of Raman spectroscopy are SiO(4)(4-) monomers, Si(2)O(7)(6-) dimers, SiO(3)(2-) chains or rings, Si(2)O(5)(2-) sheets, and SiO(2) three-dimensional units. The coexisting anionic species are related to specific ranges of the ratio of nonbridging oxygens to tetrahedrally coordinated cations (NBO/Si). In melts with 2.0 < NBO/Si < approximately 4.0, the equilibrium is of the type [See equation in the PDF file]. In melts with NBO/Si approximately 1.0 to 2.0, the equilibrium anionic species are given by [See equation in the PDF file]. In alkali-silicate melts with NBO/Si <~ 1.3 and in aluminosilicate melts with NBO/T < 1.0, where T is (Si + Al), the anionic species in equilibrium are given by [See equation in the PDF file]. In multicomponent melts with compositions corresponding to those of the major igneous rocks, the anionic species are TO(2), T(2)O(5), T(2)O(6), and TO(4), and the coexisting polymeric units are determined by the second and third of these disproportionation reactions.     

Kornerupine: its crystal structure

Three-dimensional analysis of the crystal structure of kornerupine reveals the crystallochemical formula Mg(VI)(2)Mg(VI)AlVI(6)[Si(2)O(7)] [(Al,Si)(2) SiO(10)]O(4)(OH), with four formula units in the structure cell of a = 16.100 (2) A, b = 13.767(2) A, c = 6.735(2) A; space group, Cmcm. The unusual crystal structure includes walls of Al-O edge and corner-sharing octahedra, and chains of alternating Mg-O and Al-O octahedra fused to the walls by further edge-sharing to form dense slabs. These slabs are held together by [Si(2)O(7)] corner-sharing tetrahedral pairs and [(Al,Si)(2)SiO(10)] corner-sharing tetrahedral triplets.     

Eight-membered cyclosilicate rings in muirite

The determination of the crystal structure of muirite, Ba(10)(Ca,Mn,Ti)(4)Si(8)O(24)(Cl,OH,O)(12) . 4H(2)O, revealed the presence of discrete cyclic silicate anions, (Si(8)O(24))(16-), formed by the condensation of eight silicate tetrahedra. This first reported occurrence of eight-membered rings is of particular interest, because rings with eight tetrahedra are reported to be energetically less stable than rings with six tetrahedra, which have been found in many minerals.     

The magnon pairing mechanism of superconductivity in cuprate ceramics

The magnon pairing mechanism is derived to explain the high-temperature superconductivity of both the La2-xSrxCu(1)O(4) and Y(1)Ba(2)Cu(3)O(7) systems. Critical features include (i) a one- or two-dimensional lattice of linear Cu-O-Cu bonds that contribute to large antiferromagnetic (superexchange) coupling of the Cu(II)(d(9)) orbitals; (ii) holes in the oxygen ppi bands [rather than Cu(III)(d(8))] leading to high mobility hole conduction; and (iii) strong ferromagnetic coupling between oxygen ppi holes and adjacent Cu(II)(d(9)) electrons. The ferromagnetic coupling of the conduction electrons with copper d spins induces the attractive interaction responsible for the superconductivity, leading to triplet-coupled pairs called "tripgems." The disordered Heisenberg lattice of antiferromagnetically coupled copper d spins serves a role analogous to the phonons in a conventional system. This leads to a maximum transition temperature of about 200 K. For La(1.85)Sr(0.15)Cu(1)O(4), the energy gap is in excellent agreement with experiment. For Y(1)Ba(2)Cu(3)O(7), we find that both the CuO sheets and the CuO chains can contribute to the supercurrent.     

Boron-Oxygen Polyanion in the Crystal Structure of Tunellite

The crystal structure of tunellite, SrO.3B(2)O(3).4H(2)O, with infinite sheets of composition n[B(6)O(9)(OH)(2)](2-), has cations and water molecules in the spaces within the sheets. Adjacent sheets are held together by hydrogen bonding through the water molecules. The boron-oxygen polyanions provide the first example in hydrated borate crystals of one oxygen linked to three borons.     

Crystal Structure of Tl2Ba2Ca2Cu3O10, a 125 K Superconductor

There is now a new series of high-temperature superconductors that may be represented as (A(III)O)(2)A(2)(II)Can-1CunO2+2n where A(III) is Bi or Tl, A(II) is Ba or Sr, and n is the number of Cu-O sheets stacked consecutively. There is a general trend toward higher transition temperatures as n increases. The highest n value for a bulk phase is three and is found when A(III) is Tl. This compound, Tl(2)Ba(2)Ca(2)Cu(3)O(10), has the highest transition temperature( approximately 125 K) of any presently known bulk superconductor. The structure of Tl(2)Ba(2)Ca(2)Cu(3)O(10) has been determined from single-crystal x-ray diffraction data and is tetragonal, with a = 3.85 A and c = 35.9 A. No superstructure is observed, and the material is essentially twin-free. Electron microscopy in the Tl/Ba/Ca/Cu/O system has revealed intergrowths where n = 5; such regions may well be responsible for the superconducting onset behavior observed in this system at about 140 K.     

Pentaborate Polyanion in the Crystal Structure of Ulexite, NaCaB5O6(OH)6{middle dot}5H2O

Triclinic ulexite crystals contain isolated borate polyanions [B(5)O(6)(OH)(6)](3-) related to the well known pentaborate polyanion [B(5)O(6)(OH)(4)](-) by addition of two hydroxyl groups to two opposite B-O triangles. The isolated ulexite polyanions form the [B(5)O(7)(OH)(4)](n)(3n-) chains previously found in crystals of the related mineral probertite, NaCaB(5)O(7)(OH)(4).3H(2)O.     

Tuhualite crystal structure

As judged by crystal structure analysis, the crystallochemical formula of tuhualite is (Na,K)(2)(VI) (Fe(+3))(2)(VI) (Fe(+2))(2)(VI) Si(12)O(30) . H(2)O, with four such formulas in the unit cell. The structure is characterized by six-repeat double chains of silicon tetrahedrons and chains of edge-sharing, alternating iron tetrahedrons and octahedrons. Chains of each kind are connected by corner sharing to four chains of the other type. The iron cations appear to be anomalously ordered with Fe(+2) in tetrahedral and Fe(+3) in octahedral sites.     

Electronic structure and valence-bond band structure of cuprate superconducting materials

From ab initio calculations on various clusters representing the La2-xSrxCu(1)O(4) and Y(1)Ba(2)Cu(3)O(7) classes of high-temperature superconductors, it is shown that (i) all copper sites have a Cu(II)(d(9))oxidation state with one unpaired spin that is coupled antiferromagnetically to the spins of adjacent Cu(II) sites; (ii) oxidation beyond the cupric (Cu(II)) state leads not to Cu(III) but rather to oxidized oxygen atoms, with an oxygen ppi hole bridging two Cu(II) sites; (iii) the oxygen ppihole at these oxidized sites is ferromagnetically coupled to the adjacent Cu(II)d electrons despite the fact that this is opposed by the direct dd exchange; and (iv) the hopping of these oxygen ppi holes (in CuO sheets or chains) from site to site is responsible for the conductivity in these systems (N-electron band structures are reported for the migration of these localized charges).     

A New High-Temperature Superconductor: Bi2Sr3-x Cax Cu2O8+y

A new superconductor that displays onset behavior near 120 K has been identified as Bi(2)Sr(3-x)Ca(x)Cu(2)O(8+y), with x ranging from about 0.4 to 0.9. Single crystal x-ray diffraction data were used to determine a pseudo-tetragonal structure based on an A-centered orthorhombic subcell with a = 5.399 A, b= 5.414A, and c = 30.904 A. The structure contains copper-oxygen sheets as in La(2)CuO(4) and YBa(2)Cu(3)O(7), but the copper-oxygen chains present in YBa(2)Cu(3)O(7) do not occur in Bi(2)Sr(3-x)Ca(x)Cu(2)O(8+y). The structure is made up of alternating double copper-oxygen sheets and double bismuth-oxygen sheets. There are Ca(2+) and Sr(2+) cations between the adjacent Cu-O sheets; Sr(2+) cations are also found between the Cu-O and Bi-O sheets. Electron microscopy studies show an incommensurate superstructure along the a axis that can be approximated by an increase of a factor of 5 over the subcell dimension. This superstructure is also observed by x-ray diffraction on single crystals, but twinning can make it appear that the superstructure is along both a and b axes. Flux exclusion begins in our samples at about 116 K and is very strong by 95 K. Electrical measurements on a single crystal of Bi(2)Sr(3-x)Ca(x)Cu(2)O(8+y) show a resistivity drop at about 116 K and apparent zero resistivity at 91 K.     

High-Tc Molecular-Based Magnets: Ferrimagnetic Mixed-Valence Chromium(III)-Chromium(II) Cyanides with Tc at 240 and 190 Kelvin

Molecular-based magnets with high magnetic-ordering temperatures, T(c), can be obtained by mild chemistry methods by focusing on the bimetallic and mixed-valence transition metal micro-cyanide of the Prussian blue family. A simple orbital model was used to predict the electronic structure of the metal ions required to achieve a high ordering temperature. The synthesis and magnetic properties of two compounds, [Cr(5)(CN)(12)].10H(2)O and Cs(0.75) [Cr(2.125)(CN)(6)].5H(2)O, which exhibited magnetic-ordering temperatures of 240 and 190 kelvin, respectively, are reported, together with the strategy for further work.     

Shattuckite and plancheite: a crystal chemical study

The orthorhombic crystal structures of shattuckite, Cu(5)( SiO(3))(4)(OH)(2) and planchétite, Cu(8)(Si(4)0(11))(2)(OH)(4) H(2)O, have been solved. Shattuckite contains silicate chains similar to pyroxene in a complex association with copper atoms, while the closely related planchéite contains silicate chains similar to amphibole.     

三元甘氨酰-L-缬氨酸-铜(Ⅱ)-多吡啶配合物SOD活性及电化学性质

应用改进的氯化硝基四氮唑蓝(NBT)光照还原法研究了甘氨酰-L-缬氨酸-铜(Ⅱ)-多吡啶型配合物:[Cu(Gly-L-Val)(Phen)].3H2O(1)、[Cu(Gly-L-Val)(Tatp)].2H2O(2)、[Cu(Gly-L-Val)(Dppz)].2H2O(3)[其中:Gly-L-Val=甘氨酰-L-缬氨酸;Phen=1,10-邻菲咯啉;Tatp=1,4,8,9-四氮三联苯;Dppz=二吡啶并(3,2-a:2′,3′-c)吩嗪]在水溶液中催化超氧阴离子自由基(O2-.)歧化分解活性,并用循环伏安法研究了配合物的电化学性质.结果表明:3种配合物均具有良好的SOD活性,表观催化速率常数分别为1.03×107、0.96×107和0.79×107L.mol-1.s-1.     

Ammonioborite: new borate polyion and its structure

The mineral ammonioborite is monoclinic with unit cell dimensions (in angstroms): a = 25.27, b = 9.65, and c = 11.56; beta = 94 degrees 17', and the space group is C 2/c. Analysis of the crystal structure revealed the crystallo-chemical formula (NH(4))(3)B(15)O(20)(OH)(8).4H(2)O, with four such formula units in the unit cell. The basic structural unit is the double ring consisting of one BO(4) tetrahedron and four BO(3) triangles: in ammonioborite three of these units are connected to give trimeric ions [B(15)O(20)(OH)(8)](3-).     

Amphibole: first occurrence in a meteorite

This is the first report of an amphibole mineral found in any meteorite. The amphibole richterite (soda tremolite), Na(2)Ca(Mg, Fe)(5)Si(8)O(22)(OH, F)(2), occurs as a primary (preterrestrial) mineral enclosed within graphite nodules in the iron meteorite from Wichita County, Texas.     

Hydroxyl orientation in muscovite as indicated by electrostatic energy calculations

The electrostatic energy of the 2M(1) muscovite structure, KAl(2)(Si(3)Al)- O(10)(OH)(2), has been calculated as a function of the orientation of the hydroxyl group (O-H distance = 0.97 angstrom). The minimum in the electrostatic energy occurs when the OH bond makes an angle of 18 degrees with the cleavage plane and an angle of 31 degrees with the b-axis (in the a-b plane), which is 2.5 degrees away from the orientation of the transition moment as determined from infrared measurements on single crystals. If the K+ ion is excluded from the calculation, the O-H bond makes an angle of 53 degrees with the cleavage plane. This indicates the strong influence that the interlayer cation exerts on the hydroxyl hydrogen in mica structures.     

Crystal Structure of Kernite, Na2B4O6(OH)2 {middle dot} 3H2

Kernite, Na(2)B(4)O(6)(OH)(2). .3H(2)0, contains parallel infinite chains of the borate polyanion[B(4)O(6)(OH)(2)](n)(2n-). The chains are composed of six-membered rings containing one boron-oxygen triangle and two boron-oxygen tetrahedra. The rings are linked through commonly shared boron-oxygen tetrahedra.     

Synthesis and properties of a cuprate superconductor containing double mercury-oxygen layers

A cuprate superconductor containing double mercury layers was synthesized with a high-pressure, high-temperature technique. The compound, with chemical formula Hg(2)Ba(2)-Y1-xCaxCu(2)O(8-delta), contains a double HgO layer with structure similar to that of rock salt. The prototype compound Hg(2)Ba(2)YCu(2)O(8-delta) is an insulator. Superconductivity is induced in the system by partially replacing yttrium with calcium.     

Evidence for a phytotoxic hydroxy-aluminum polymer in organic soil horizons

The toxicity of A1 that has been mobilized in soil, streams, and lakes through acid deposition primarily has been attributed to mononuclear A1 species. Polynuclear A1 species are more toxic than mononuclear species, but they have not been considered to be significant in the environment. Aluminum-27 nuclear magnetic resonance (NMR) spectra of forested spodosol soil horizon samples show the presence of polynuclear A1O(4)A1(12)(OH)(24)(H(2)O)(12)(7+). The AlO(4)A1(12)(OH)(24)(H(2)O)(12)(7+) species accounted for 30 percent of the aqueous A1 observable by NMR, and this could make a significant contribution to the toxicity of the A1 in these soils.     

Spin crossover in a catenane supramolecular system

The compound [Fe(tvp)(2)(NCS)(2)] . CH(3)OH, where tvp is 1,2-di-(4-pyridyl)-ethylene, has been synthesized and characterized by x-ray single-crystal diffraction. It consists of two perpendicular, two-dimensional networks organized in parallel stacks of sheets made up of edge-shared [Fe(II)](4) rhombuses. The fully interlocked networks define large square channels in the [001] direction. Variable-temperature magnetic susceptibility measurements and M?ssbauer studies reveal that this compound shows low-spin to high-spin crossover behavior in the temperature range from 100 to 250 kelvin. The combined structural and magnetic characterization of this kind of compound is fundamental for the interpretation of the mechanism leading to the spin crossover, which is important in the development of electronic devices such as molecular switches.