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  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.
TX2 phases with the marcasite-type structure may be compressed or extended in  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 . 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 RT/RX, irregularities in correlations of the T–X and X–X bond distances and magnetic and electrical properties are all consistent with the present hypothesis.
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 (Si) with O in these glasses until the content of P2O5 reaches about 32 mol%, at which sixfold coordinated Si (Si) first appears. The Proportion of Si increases qualitatively with increase in the content of P2O5. However, several P2O5-rich glasses contain Si only, possibly pointing to a dependence of 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 Si and Si in these glasses using Si K-edge spectra, model composite materials of a-SiO2, containing Si only, and c-SiP2O7, containing Si only, were used to establish the correlation of area ratio for Si and Si edge features with bulk composition. The regression equation may be used for semiquantitative estimation of relative proportions of Si and Si in glasses and other materials of unknown structure with compositions similar to those of the present glass systems.
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 (Al)and sixfold- coordinated AI (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 Al (averaged for eight samples) to 1567.8 eV for Al and to 1568.3 eV for Al(averaged for 17 samples). For Al and 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 Al and 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.
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 BO33- and BO43- clusters, peak a is assigned to the transition of B 1s electrons to the unoccupied B 2pz (π*) states for threefold-coordinated B (B),and peak b is assigned to the transition of B 1s electrons to the unoccupied B σ* states for fourfold-coordinated B (B).B K-edge XANES spectroscopy is established as a method for "fingerprinting"  B and  B in borate and borosilicate minerals, glasses, and melts. Also, the relative proportions of [4JBand Bin the borosilicate glasses are determined from the integrated peak areas for B and  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 B entities than the bulk.