Short- and long-range structure of crystals as revealed by XAFS spectroscopy

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HONEYDEW SUGARS ELIMINATED BY STIGMACOCCUS SP. NR. ASPER HEMPEL (HEMIPTERA: MARGARODIDAE) FEEDING ON LEGUMINOUS TREES IN BRAZIL.

HONEYDEW SUGARS ELIMINATED BY STIGMACOCCUS SP. NR ASPER HEMPEL (HEMIPTERA: MARGARODIDAE) FEEDING ON LEGUMINOUS TREES IN BRAZIL. The sooty mould coating the trunks of mature trees of Schizolobium excelsum in Brazil was found to be associated with honeydew being eliminated by an undescribed species of margarodid near Stigmacoccus asper Hempel. Analysis of the honeydew sugars by paper chromatography revealed a complex composition. The principal sugar was sucrose, but there were significant amounts of fructose, glucose and three components identified as di-, tri- and tetrasaccharides. The disaccharides were maltose, trehalose, trehalulose and a hexose-hexitol. The other, apparently novel, pair of oligosaccharides were composed of glucose(s) 1,4 linked to the glucose of sucrose. The sugar composition of the tree sap was also determined and found to be glucose and sucrose only. The findings, therefore, imply significant and novel metabolic transformations of sugars by the scale insect and/or its microbial symbionts. Key words: Xylococcinae, sexual reproduction, stigmatriose, stigmatetraose, Amazonia

The local structure of Ca-Na pyroxenes. II. XANES studies at the Mg and Al K edges

X-ray absorption spectra at the Mg and Al K edges have been recorded using synchrotron radiation on synthetic end member diopside (Di) and jadeite (Jd) and on a series of natural Fe-poor Ca-Na clinopyrox- enes compositionally straddling the Jd-Di join. The spectra of C2/c end members and intermediate members of the solid solution series (C-omphacites) are diAerent from those of the intermediate members having P2/n symmetry (P-omphacites). DiAerences can be interpreted and explained by comparing the experimental spectra with theoretical spectra calculated via the full multiple- scattering formalism, starting from the atomic positional parameters determined by single-crystal X-ray diArac- tion structure refinement on the same samples. Atomic clusters with at least 89 atoms, extending to more than 0.60 nm away from the Mg or Al absorbers, are needed to reproduce the experimental spectra. This shows that in the clinopyroxene systems XANES detects medium- rather than short-range order-disorder relationships. Theoretical spectra match the experimental ones well for all features in the regions from 16 to 60 eV above threshold. Experimental near-edge features in the first 16 eV are also reproduced, albeit less accurately. Certain near-edge features of C-omphacites reflect the octahed- ral arrangement of the back-scattering six O atoms nearest neighbours of the probed atom (Mg or Al) lo- cated at site M1 of the crystal structure, thus being in- dicators of short-range order. Others arise again from medium-range order. P-omphacites show more compli- cated spectra than C-omphacites. Their additional fea- tures reflect the increased complexity of the structure and the greater local disorder around the probed atom induced by the two alternative M1, M11 configurations of the six O atoms forming the first coordination shells. Mg and Al are confirmed to be preferentially partitioned in the M1 and M11 site of the P-omphacite crystal structure, however with a certain degree of local disor- der. The relative heights of certain prominent features are directly related to sample composition in terms of Di:Jd ratio in the Al K-edge spectra, whereas they show abrupt variations in the Mg K-edge spectra. They demonstrate that XANES is directly related to compo- sition and may be used to distinguish C- from P-omphacites

Advanced techniques to define intercalation processes

Intercalation is the inclusion or reversible insertion of a guest chemical species (atom, ion, molecule) in a virtually unchanged host-crystal structure. Any type of layer-structured material may give rise to intercalated compounds, the guest species being artificially inserted or naturally included between the host sheets without loss of their planarity. Layer silicates, in particular, may be considered intercalated structures where interlayer guest species and complexes are inserted between the silicate layers. The most common guest species is H2O, which is generally present under natural conditions in intercalated layer silicates such as smectites, vermiculite and halloysite. Past research focused attention on the swelling/shrinking behaviour of intercalated compounds with respect to H2O, and also on the non-stoichiometric, heterogeneous complexes formed from organic liquids such as ethylene glycol and glycerol. The unique combination of layer-silicate features (small crystal size, large surface area) and the small concentrations required to effect a change in the matrix, both coupled with the advanced characterization techniques available, have generated much interest. This interest extends to the special field of nanocomposites, and of graphene, which is also an intercalated layered structure. In general, any guest material inserted into an interlayer space causes a modification in the structure, with spacing-size changes in a particular crystallographic direction (d value). First, a brief introduction on conventional and synchroton-based X-ray techniques used to define crystal size and thickness is given. Then, the peak-broadening approach by conventional X-ray diffraction (XRD) techniques, such as the Scherrer method is presented. Further on, the crystallinity measurements and the Bertaut-Warren-Averbach (BWA) method used in the MudMaster program are described. A short summary is presented of the grazing-incidence diffraction (GIXRD) technique. Finally, additional and complementary information from X-ray absorption spectrometry (XAS), such as short-range order, and detailed local information on atomic positions by angle-resolved X-ray absorption near-edge stucture (AXANES), polarized extended X-ray absorption fine structure (P-EXAFS), and near-edge extended absorption fine structure (NEXAFS) spectroscopies are analysed and discussed. Examples of the applications of these methods to clay minerals, micas and graphene are given

PHASE-RELATIONS OF AENIGMATITE MINERALS IN A SYENITIC EJECTUM, WONCHI VOLCANO, ETHIOPIA

Wonchi volcano (Ethiopia) is the second locality for wilkinsonite, the new mineral related to aenigmatite via the coupled substitution 2Fe3+ = TiFe2+. Wilkinsonite occurs as relics surrounded by rims of riebeckite and biotite in a syenitic ejectum which also contains microphenocrysts of aenigmatite. The two minerals do not represent an equilibrium pair. They both formed in the magma chamber under plutonic to hypabyssal conditions, but during two following stages of crystallisation separated by a major drop in f(o2) conditions