Si-O Distances Research Articles

Electronegativity of Cr3+ ion and the role of the geometrical feature of the CrO6 octahedron.

Studies on the Si-O distances and the geometrical features of the CrO6 octahedra in several chromium silicates reveal that the electronegativity of the Cr3+ ion depends on the geometrical feature of the CrO6 octahedron. Since the electron density and the screening constant of the Cr3+ ion in the regions directed toward the oxygens decrease with decrease of the repulsion between the 2p orbitals of the oxygen ions and the 3d orbitals in the t2g state of the Cr3+ ion, the effective nuclear charge and the electronegativity of the Cr3+ ion increase with decrease of the repulsion. It seems that the electronegativity of the Cr3+ ion based on the heat of formation corresponds to the value in a regular octahedron.

The crystal structure of NaGaSi2O6 pyroxene.

The crystal structure of NaGaSi2O6 pyroxene synthesized at 1500°C and 60 kb has been refined by the least squares method using 732 reflections measured by an automatic single crystal diffractometer. The unweighted R factor is 0.033 for 689 observed reflections. The cell dimensions are; a=9.557 Å, b=8.679 Å, c=5.260 A, β=107.68°, and V=415.70 Å3. The space group is C2/c and Z=4. Comparison of a present crystal structure refinement of the NaGaSi2O6 pyroxene with refinements of seven NaM3+Si2O6 pyroxenes, previously published, indicates that the Si-O distances correlate with the electronegativity of the M3+ion as two different trends; one is Sc-Cr-V-Ti-Al series and the other is In-Fe-Ga series. The electronegativity of Ga3+ion is found to be 1.9 in Pauling's scale.

エネルギー最小化法による特殊条件下の鉱物結晶構造の予測

The energy minimization method (WMIN, Busing (1981)) has been successfully applied to predicting the crystal structures of minerals of geological interest at high pressure. The repulsive parameters, ionic radius A and ionic compressibility B of some ions have been revised. The changes of the crystal structures with the increase of pressure are summarized as follows : (1) Degrees of shortening of longer bonds are larger than those of shorter bonds; (2) Unshared edges become shorter at the higher rate than the shared edges; In some cases the shortest shared edge remains almost constant; (3) The shape of polyhedron becomes regular, because unshared edges of cation polyhedra tend to approach the length of shared edges. It is often found that the mean Si-O distance increases when the phase transition; however, the mean Si-O distance of a particular phase in the stability field does not differ very much from that of the phase stable at 1 atm. The inter-diffusion coefficients of four transition metals in the forsterite structure previously obtained have been compared with their self-diffusion coefficients in olivine structures estimated on the basis of the potential energy maps by using the WMIN program. We have also applied the WMIN method to predicting the crystal structures of minerals at high temperature and to examining the cation distribution in olivine. Our studies have laid the foundation of the discussion based on the potential energy function about the crystal structures of minerals and about the rate processes applicable to geological phenomena.

Si-O distances in some silicates containing fluor ions.

Si-O distances in some silicates containing fluor ions.

Polymethylated flavonol synthesis is catalysed by distinct O-methyltransferases

T(2) sites while the larger ones prefer T(1); the distortion of T (1) tetrahedron is always smaller than that of T(2); generally divalent cations prefer the completely ordered distribution together with Si cations, while trivalent cations do the partially disordered distribution. The ionic radius of Co ~+ cation (0.57 A) [11] is much larger than that of Si 4+ (0.26 A) [12]. Moreover the distortion [13] of Co tetrahedron (3.00) in T(1) site is smaller than that of Si (7.33) in T(2) site. Co and Si atoms in Ca2CoSi207 are nearly ordered in two tetrahedral sites because of the averaged Si-O distance 1.608 A of a Si tetrahedron. The tetrahedra in CazCoSi207 embody all the features as stated above. Four-coordinated Co 2 + cation and its ordered distribution in Co-gtkermanite contributes to understanding the crystallographic behaviour of Co 2+ cation in the silicates produced at higher temperature.

The crystal structure of NaTiSi2O6 pyroxene.

The crystal structure of NaTiSi2O6 pyroxene synthesized at 1500°C and 50 kb has been refined by the least squares method using 763 reflections measured using an automatic single crystal diffractometer. The unweighted R factor is 0.041 for 687 unrejected reflections. The cell dimensions are; a 9.692 Å, b 8.874 Å, c 5.301 Å, β 106.85°, and V 436.37 Å3; space group C2/c. Comparison of a new crystal structure refinement for the NaTiSi2O6 pyroxene with published refinements for six NaM3+Si2O6 pyroxene indicates that the Si-O distances correlate with the electronegativity of the M3+ ion as two separate trends. Ore is Sc-V-Ti-Al series and the other is Cr-In-Fe series.

Studies on the Si-O distances in NaM3+Si2O6 pyroxenes.

The dependence of the Si-O distances on the electronegativitq, the electronic structure and the size of the non-tetrahedral cations has been re-examined for NaM3+Si2O6 pyroxenes. In the Sc-V-Al series the difference, Δbr-Δmean, decreases with increasing the electronegativity and the diffeaence, Δnbr-Δmean, increases with increasing the electronegativity, where the dmean is the difference between the mean of the four Si-O distances inNaAISi2O6 pyroxene and teat in NaM3+Si2O6 pyroxene, and Δbr-Δmean is the difference between the mean of the Si-O (br) distances and the Δmean. In the Cr-ln-Fe series themean of the Si-O (nbr) distances increases with increasing the electronegativity and thedifference, Δbr-2Δmean, decreases with increasing the electronegativity.

Structure of Co3Al2Si3O12 garnet.

The crystal structure of Co3Al2Si3O12 granet grown by the hydrothermal method has been refined by the least squares method using 635 reflections measured on an automatic single crystal diffractometer. The unweighted R factors is 0.037 for 469 unrejected reflections, of which 259 are non-equivalent. The cell dimensions are: a0=11.455 A, and V=1503.07A3; space group Ia3d. The Si-O distance is 1.627(2)A. Comparison of a new crystal structure refinement for Co3A12Si3O12 garnet with published refinements for some alumino silicate garnets indicates that the Si-O bond lengths correlate with electronegativity of non-tetrahedral cations as two separate trends. One is pyropegrossular series and the other is Co3Al2Si3O12 garnet-almandine-spessartine series. In the former their dodecahedral sites are mainly occupied by M2+cations which have empty 3d orbitals, whereas in the latter the dodecahedral sites are mainly or partially occupied by M2+ cations which have no empty 3d orbitals.

Infrared spectra of M3+2Si2O7 compounds with thortveitite structure.

Infrared spectra of In2Si2O7 and Lu2Si2O7 with thortveitite structure were measured in the frequency range 1300-400cm-1. The frequencies of the anti-symmetrical Si-O-Si stretching in In2Si2O7 and Lu2Si2O7 are 1175cm-1, and 1115cm-1, respectively. The frequency in M3+2Si2O7 compound with thortveitite structure is strongly influenced by the electronegativity and the electronic structure of the M3+ ion. These facts suggest that the Si-O (br) distance in thortveitite structure is strongly influenced by them.

The correlation between the Si-O (bridging) distance and the frequency of the Si-O-Si stretching in clinopyroxenes.

The correlation between the Si-O (bridging) distance and the frequency of the Si-O-Si stretching in clinopyroxenes.

Determination of the crystal structure of cobalt(II) orthosilicate

The crystal structure of Co2SiO4 (olivine structure type) was solved by Patterson and Fourier synthesis and refined by the full matrix least squares method. A final value of the R factor of 0.057 was obtained from 911 independent reflection intensities. The structure symmetry corresponds to space group Pnma with lattice parameters a = 1032.8(3), b = 601.9(2) and c = 478.2(1) pm. The unit cell contains four formula units. The Si-O distance in isolated SiO4 tetrahedra varies from 162.1(3) to 165.9(3) pm. Both cobalt atoms are octahedrally surrounded by oxygen atoms with Co-O distances of 208.0(3)-223.1(2) pm.

The crystal structure of julgoldite

SummaryThe structure of julgoldite, , has been determined and refined to an R value of 4·6 % based on 2417 symmetry-independent reflections, of which 765 are unobserved (weighted R = 4·3 %). The cell dimensions are a 8·922(4), b 6·081(3), c 19·432(9) Å, β 97·60(6)°, the space group is A2/m with Z = 4 and Dcalc 3·56 g cm-3. Julgoldite is isostructural with pumpellyite; the Si-O and Ca-O distances of julgoldite are identical, within experimental accuracy, with corresponding distances in pumpellyite; the Fe-O distances of julgoldite are about 0·09 Å longer than the corresponding (Al, Mg)-O distances in pumpellyite. The four hydroxyl groups and the extent of O-for-OH substitution are identified by valence-sum calculations.

The relationship between Si-O distances and Si-O-Si bond angles in the silica polymorphs

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Zur Kristallchemie des sechsfach koordinierten Siliziums

On the crystal chemistry of six-coordinated silicon. From a comparison of the crystal structures of high-pressure potassium-feldspar K(Si₀,₇₅Al₀,₂₅)₄[vi]O₈, stishovit Si[vi]O₂, thaumasite Ca₃[Si (OH)]₆ (SO₄) (CO₃) • 12H₂O, two modifications of silicon diphosphate Si[vi]P₂O₇ and of pyri-diniumtris (o-phenylenedioxy)-siliconate [C₅H₅NH]₂ [(C₆H₄O₂)₃Si], it follows that the [SiO₄] polyhedron is a slightly distorted octahedron with average values of 1.78 Å for the Si-O distances and 2.52 Å for the O-O edges. The tendency for silicon in phases with composition mSiX[p] • nAR[q] to coordinate itself with six oxygen atoms increases with increasing electronegativity (i. e. increasing acidity) of the elements X and A. In the same direction the pressure, which is necessary for the change [SiO₄] -> [SiO₆] decreases. While the covalency of the Si-O bond increases with increasing pressure up to the change of coordination number of silicon, a suddon drop in this covalency should occur at this change.

Refinement of the structure of pectolite, Ca2NaHSi3O9

The pectolite structure has been refined by least squares using new counterdiffractometer data. Evidence for hydrogen bonding between 0 (3) and 0 (4) is based on the refinement results and examination of infrared spectra. Variations in Si-O distances are discussed in terms of dn-pn bonding between silicon and oxygen.

The crystal structure of dalyite

The crystal structure of dalyite, a potassium (sodium) zirconium silicate K1.7oNao.30ZrSi6015' has been determined and refined by two-dimensional Fourier methods, It is a new type of sheet silicate containing four, six- and eight-membered rings of Si04 tetrahedra, The sheets are joined by Zr06 octa- hedra and by (K,Na)Os polyhedra, The Si-O distances range from 1.54 A to