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  • Author: M. E. Fleet x
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Abstract

Lorandite (Tl2AS2S4) is monoclinic with a = 12.28(1) Å, b = 11.30(1) Å, c = 6.101(6) Å,β = 104°5′(2′), space group P21/a, Z = 4. The crystal structure, consisting of spiral chains of ASS3 pyramids oriented parallel to [010] and connected by Tl atoms, has been confirmed and refined by full-matrix least-squares analysis of three-dimensional diffractometer data to give a value of the conventional residual index of 0.09. The positions of the S and As atoms are markedly different from those reported in the literature. The present study has shown that the bonds from the two non-equivalent As atoms to the non-bridge S are quite short (2.08 Å and 2.20 Å), that each S is tetrahedrally coordinated to As and Tl and that each Tl position is more closely related to one AsS3 pyramid chain than to adjacent chains, the nearest-neighbor S environment of each being a distorted square-pyramidal configuration.

The development of cleavage in lorandite is directly related to the number and type of the interchain bonds. It is suggested that the short As non-bridge S distances in this mineral and in other sulfosalts indicate some degree of π-bonding character in these bonds. Finally, the small difference in the electronegativities of Tl and S, the apparent bonding interaction between adjacent Tl atoms and comparisons with Tl-bearing organic compounds suggest the that Tl atoms are bound by covalent forces to the AsS3-pyramid chains.

Abstract

TX2 phases with the marcasite-type structure may be compressed or extended in [001] by relatively weak forces. The “pair reorientation model” of Brostigen and Kjekshus does not provide unequivocal evidence that class A marcasites have the “normal” structure. It is more probable that class A marcasites are compressed in [001]. The extra cohesive forces result from T–T bonds, through overlap of dxy orbitals, but the c parameter is defined essentially by X–X repulsion. The two 3d electrons per T atom which are promoted to the valence band in dipnictides contribute to both T–T and T–X σ bonds through bond resonance. Variations in the axial ratios c/a and c/b of marcasites with the radius radio R T/R X, irregularities in correlations of the T–X and X–X bond distances and magnetic and electrical properties are all consistent with the present hypothesis.

Abstract

Si K- and L-edge X-ray absorption near-edge structure (XANES) of SiO2-P2O5 and Na2O-SiO2-P2O5 glasses containing P2O5 above 30 mol% were investigated using synchrotron radiation. Both Si K- and L-edge spectra indicate that Si remains fourfold coordinated ([4]Si) with O in these glasses until the content of P2O5 reaches about 32 mol%, at which sixfold coordinated Si ([6]Si) first appears. The Proportion of [6]Si increases qualitatively with increase in the content of P2O5. However, several P2O5-rich glasses contain [4]Si only, possibly pointing to a dependence of [6]Si content on quench rate. These results are consistent with 29Si MAS NMR spectra for silicate-phosphate glasses of similar composition. To estimate further the relative proportions of [4]Si and [6]Si in these glasses using Si K-edge spectra, model composite materials of a-SiO2, containing [4]Si only, and c-SiP2O7, containing [6]Si only, were used to establish the correlation of area ratio for [6]Si and [4]Si edge features with bulk composition. The regression equation may be used for semiquantitative estimation of relative proportions of [6]Si and [4]Si in glasses and other materials of unknown structure with compositions similar to those of the present glass systems.

Abstract

Al K-edge X-ray absorption near-edge structure (XANES) spectra of a range of aluminosilicate and aluminum oxide minerals were collected using synchrotron radiation. The Al K-edge spectra of aluminosilicates containing fourfold-coordinated Al ([4]Al)and sixfold- coordinated AI ([6]Al)are qualitatively interpreted on the basis of a comparison with the Si K-edge spectra of α quartz and stishovite and MO calculations for tetrahedral and octahedral clusters. Some near-edge features are attributed to the multiple scattering (MS) effect from the more distant shell atoms. The Al K-edge (peak C) shifts toward higher energy with an increase in the coordination number (CN) of Al, from 1566.7 eV for [4]Al (averaged for eight samples) to 1567.8 eV for [5]Al and to 1568.3 eV for [6]Al(averaged for 17 samples). For [4]Al and [6]Al aluminosilicates, respectively, the Al K-edge shifts to higher energy with increase in the Al-O bond distance (dAl-O), distortion of the Al polyhedron (ΔAl-O),and decrease in the Al-O bond valence (SAl-O). Also for [4]Al and [6]Al aluminosilicates, the relative intensity of the Al K-edge is correlated with the content (in weight percent) of Al in tetrahedral and octahedral sites, respectively. This correlation therefore establishes Al K-edge spectroscopy as a potential technique for semiquantitatively determining the distribution of Al between fourfold- and sixfold-coordinated sites.

Abstract

High-resolution B K-edge X-ray absorption near-edge structure (XANES) spectra of K2O-SiO2-B2O3-P2O5 glasses are reported using synchrotron radiation. Two prominent features, peak a at about 194.0 eV and peak b at about 198.0 eV, are observed. On the basis of the qualitative MO diagrams of BO3 3- and BO4 3- clusters, peak a is assigned to the transition of B 1s electrons to the unoccupied B 2pz (π*) states for threefold-coordinated B ([3]B),and peak b is assigned to the transition of B 1s electrons to the unoccupied B σ* states for fourfold-coordinated B ([4]B).B K-edge XANES spectroscopy is established as a method for "fingerprinting" [3] B and [4] B in borate and borosilicate minerals, glasses, and melts. Also, the relative proportions of [4JBand [3]Bin the borosilicate glasses are determined from the integrated peak areas for [3]B and [4] B edge peaks and are shown to be generally in good agreement with recent 11B MAS NMR measurements. However, the surface and near-surface structure of powder particles of per boric glasses containing P2O5 appears to have a lower proportion of [3]B entities than the bulk.