Electron Density distribution and bonding in silicates

This paper summarizes the recent results of the investigation of bonding in silicates obtained by precision X-ray diffraction. The experimental electron density distribution is compared with theoretical electron density maps calculated for model silicate molecules. The characteristic features of the chemical bonds in ortho-, ring-, chain- and framework silicates are discussed.     

Geochemistry and health aspects of F-rich mountainous streams and groundwaters from sierras Pampeanas de Cordoba,Argentina

Symptoms of dental fluorosis have been observed in rural communities located in the Sierras Pampeanas de Córdoba, a mountainous area in Central Argentina. The clinical assessment was performed in the Charbonier Department, where the fluoride (F⁻) intake was determined to be 3.90 ± 0.20 mg day⁻¹ (n = 16). In this community, mild and severe fluorosis reach an incidence of 86.7% (total teeth surface = 636 teeth) among the children population. To determine the origin and distribution of fluorine in natural waters from the Charbonier Department and nearby regions, sampling was performed in the area covering the San Marcos River basin. The obtained results show that F⁻ concentrations vary between ~1 to ~2.5 mg l⁻¹, with an outlier value of 8 mg l⁻¹. The spatial distribution of F⁻ shows that the lowest concentrations are found at the basin’s catchments. Maximum values are located in two sectors of the basin: the Charbonier depression in the eastern part and at the San Marcos village, downstream the main collector, in the western part of the basin. In these two regions, the F⁻ contents in ground- and surface waters are >2.0 mg l⁻¹ and nearly constant. Dissolved F⁻ in natural waters from the study area has its origin in the weathering of F-bearing minerals present in the region’s dominant lithology. The extent of mineral weathering is mostly determined by the residence time of water within the aquatic reservoir. Longer residence times and a major solid–water interaction lead to enhanced release of F⁻. This explains the higher F⁻ concentrations found in basin areas with lower run off. The removal of F⁻ from water appears to occur by neither fluorite precipitation, nor by adsorption. Hence, variations in F⁻ concentrations seem to be more related to regional hydrological conditions.     

Ab initio calculation of H, O, Al and Si NMR parameters, vibrational frequencies and bonding energetics in hydrous silica and Na-aluminosilicate glasses

Ab initio, molecular orbital (MO) calculations were performed on model systems of SiO₂, NaAlSi₃O₈ (albite), H₂O-SiO₂ and H₂O-NaAlSi₃O₈ glasses. Model nuclear magnetic resonance (NMR) isotropic chemical shifts (δ_iso) for ¹H, ¹⁷O, ²⁷Al and ²⁹Si are consistent with experimental data for the SiO₂, NaAlSi₃O₈, H₂O-SiO₂ systems where structural interpretations of the NMR peak assignments are accepted. For H₂O-NaSi₃AlO₈ glass, controversy has surrounded the interpretation of NMR and infrared (IR) spectra. Calculated δ_iso¹H, δ_iso¹⁷O, δ_iso²⁷Al and δ_iso²⁹Si are consistent with the interpretation of Kohn et al. (1992) that Si-(OH)-Al linkages are responsible for the observed peaks in hydrous Na-aluminosilicate glasses. In addition, a theoretical vibrational frequency associated with the Kohn et al. (1992) model agrees well with the observed shoulder near 900 cm⁻¹ in the IR and Raman spectra of hydrous albite glasses. MO calculations suggest that breaking this Si-(OH)-Al linkage requires ∼+56 to +82 kJ/mol which is comparable to the activation energies for viscous flow in hydrous aluminosilicate melts.     

A steady state model of the Somali current

A steady state model of the Somali current including forcing by both the curl and the divergence of the wind stress is discussed. The model equations are linear, but the results presented are for the one nonlinear case. The grid resolution was 12 km in the zonal and 24 km in the meridional direction. The streamfunction and velocity potential of the current are presented for forcing by divergence and compared with a situation when only the curl is present. The results indicate that a two-gyre system appears in July, a representative month for the summer monsoon, only when divergence is included. Computations with available data indicate that the divergence is comparable in magnitude to the curl near the location of the Somali current. The model produces three other important features: (i) strong upwelling off the east coast of Africa, (ii) downwelling over central Arabian sea and (iii) a strong eastward current in the upper layer towards the interior of the Arabian sea near 12°N.     

Composition of arsenopyrite from topaz greisen veins in southeastern Missouri

Arsenopyrite occurs in greisen-sulfide veins hosted by unmetamorphosed Precambrian granite and rhyolite in the Silver Mine district of southeastern Missouri, Greisenization and sulfide mineralization appear to have been a continuous depositional sequence which recorded falling temperature in a near-surface vein environment. Textural criteria imply that equilibrium existed between arsenopyrite and pyrite and that this pair crystallized in an intermediate paragenetic position between the greisen and hydrothermal stages. Thirty-eight electron microprobe spot analyses of 15 arsenopyrite crystals from the Einstein and Gabriel veins failed to disclose chemical zoning of As/S. The compositional range of the analyzed arsenopyrites is 32. 9 to 31. 0 atomic % As. A range of arsenopyrite crystallization temperature from 485°C (±15°) to 455°C (±15°) is indicated for the Gabriel vein. In contrast, arsenopyrites from the Einstein vein record a lower and broader crystallization range of 440°C (±15°) to 368°C (±15°).     

Petrology of some lithic fragments of alkalic high-alumina basalt composition from Apollo 12 coarse fines

Summary The mineralogy and petrology of three lithic fragments of alkalic highalumina basalt (Kreep) composition from the Apollo 12 coarse fines was studied in detail, using an electron microprobe, in order to gain insight into their crystallization histories. Most rocks of this composition are brecciated and our study indicates that a variety of environments of crystallization can be distinguished for mineral fragments and matrices. Mineral fragments are derived from members of the ANT suite (probably troctolites) in fragments 2 and 5, and the alkalic high-alumina basalt suite in fragment 3. The rocks from which they were derived were coarse-grained, recrystallized and equilibrated, as indicated by major, and especially, minor elements. Minor elements in plagioclase, olivine, pyroxene, and zircon are consistently lower in mineral fragments as compared with matrix minerals. The origin of large zircon fragments is problematic but they are probably from the alkalic high-alumina basalt suite. Mineral fragments may have been derived from plutonic rocks (none have yet been recognized from the alkalic high-alumina basalt suite), but possibly also from breccia fragments which were recrystallized in hot, thick ejecta blankets. The matrix of the lithic fragments is of alkalic high-alumina basalt composition and is either igneous or metamorphic, or both. Hence, lithic fragments 2 and 5 are polymict breccias whereas fragment 3 is a monomict breccia. Matrix glasses in fragments 2 and 3 represent melts fractionated along the orthopyroxeneplagioclase cotectic in the olivine-anorthite-silica pseudoternary system. If these liquids could be separated from the residuum and crystallized they would be, as yet, unrecognized members of the alkalic high-alumina basalt suite. The alkalic high-alumina basalt mixing component of fragment 5 (a polymict breccia) has such a composition and may be derived from such a fractionated rock. A mineral fragment of pyroxene intergrown with ilmenite, approximately parallel to (001), is interpreted as decorated shock lamellae rather than as a deep-seated intergrowth, as found in kimberlites. A glass coating on one side of fragment 3 has SiO₂-rich and feldspathic schlieren and appears to be derived, by impact melting, from a rock of granite composition.

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Petrologie einiger Gesteinsfragmente mit alkalic high-alumina basalt Chemismus aus dem Grobanteil von Apollo 12 Bodenproben

Zusammenfassung Drei Gesteinsfragmente mit alkalic high-alumina basalt (Kreep) Chemismus aus der Grobraktion von Apollo 12 Bodenproben wurden mittels einer Elektronenstrahl-Mikrosonde einer detaillierten Studie unterzogen, um Einblick in ihre Genese zu gewinnen. Der überwiegende Teil von Gesteinen dieser Zusammensetzung ist brekziös und unsere Studie zeigt, daß unterschiedliche Kristallisationsbedingungen für die Mineralfragmente und Matrizes herrschten. Die Mineralfragmente in den Fragmenten 2 und 5 stammen von Gesteinen der ANT- (Anorthositisch-Noritisch-Troctolitischen) Reihe (wahrscheinlich von Troctoliten) und in Fragment 3 von Gesteinen der alkalic high-alumina basalt-Reihe.Die Verteilung der Haupt- und Nebenelemente in den Mineralfragmenten zeigt, daß diese von rekristallisierten und equilibrierten, grobkörnigen Gesteinen stammen. Die Konzentrationen der Nebenelemente sind in allen Mineralfragmenten (Plagioklas, Olivin und Zirkon) deutlich geringer als in den Mineralen der Matrix. Die Herkunft der großen Zirkon-Fragmente ist nicht genau zu klären. Sie stammen jedoch wahrscheinlich von Gesteinen der alkalic high-alumina basalt-Reihe. Alle Mineralfragmente könnten von plutonischen Gesteinen stammen (solche sind von der alkalic highalumina basalt-Reihe zur Zeit noch nicht bekannt), sie könnten ihren Ursprung jedoch auch in prä-existenten Brekzien haben, welche in dichten, heißen Auswurfdecken rekristallisierten. Die Matrix der Gesteinsfragmente hat durchwegs eine alkalic high-alumina basalt Zusammensetzung und ist entweder magmatisch oder metamorph, oder beides. Die Fragmente 2 und 5 sind daher als polymikte und das Fragment 3 als monomikte Brekzie zu bezeichnen.Die Matrixgläser in den fragmenten 2 und 3 repräsentieren Rest-schmelzen, welche entlang der Orthopyroxen-Plagioklas-Kotektik im Olivin-Anorthit-SiO₂-System fraktionierten. Diese Schmelzen würden-könnten sie vom System getrennt werden-bisher noch nicht bekannte Glieder der alkalic high-alumina basalt-Reihe darstellen. Eine derartige Zusammensetzung hat jedoch die Mischkomponente im Fragment 5 (eine polymikte Brekzie), welche von einem auf diesem Wege fraktioniertem Gestein stammen könnte.Ilmenit-Lamellen [subparallel zu (001)] in einem Pyroxenfragment stellen eher dekorierte Schocklamellen als Verwachsungen, wie sie aus Kimberliten bekannt sind, dar. Fragment 3 ist einseitig mit einem schlierigen Glas bedeckt, dessen Schlieren angenähert die Zusammensetzung von Alkalifeldspat und reinem SiO₂ haben. Dieses Glas ist offensichtlich eine Impakt-Schmelze eines Gesteines von granitischer Zusammensetzung.

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With 5 Figures     

Reduction of mass loss by the hot Jupiter WASP-12b due to its magnetic field

The influence of the dipolar magnetic field of a “hot Jupiter” with the parameters of the object WASP-12b on the mass-loss rate from its atmosphere is investigated. The results of three-dimensional gas-dynamical and magnetohydrodynamical computations show that the presence of a magnetic moment with a strength of ~0.1 the magnetic moment of Jupiter leads to appreciable variations of the matter flow structure. For example, in the case of the exoplanet WASP-12b with its specified set of atmospheric parameters, the stream from the vicinity of the Lagrange point L₁ is not stopped by the dynamical pressure of the stellar wind, and the envelope remains open. Including the effect of the magnetic field leads to a variation in this picture—the atmosphere becomes quasi-closed, with a characteristic size of order 14 planetary radii, which, in turn, substantially decreases the mass-loss rate by the exoplanet atmosphere (by~70%). This reduction of the mass-loss rate due to the influence of the magnetic fieldmakes it possible for exoplanets to form closed and quasi-closed envelopes in the presence of more strongly overflowing Roche lobes than is possible without a magnetic field.     

Classification of artificial (man-made) ground

The legacy inherited from anthropogenic processes needs to be addressed in order to provide reliable and up-to-date ground information relevant to development and regeneration in the urban environment. The legacy includes voids as well as anthropogenic deposits (artificial ground). Their characteristics derive from former quarrying and mining activities industrial processes creating derelict ground, variably consolidated made ground, and contaminated groundwater and soils. All need to be systematically assessed to inform the planning process and provide the basis for engineering solutions. Site-specific investigation needs to be conducted on the back of good quality geoscientific data. This comes from ‘field’ survey, remotely sensed data interpretation, historical maps, soil geochemical sampling, and geotechnical investigation. Three-dimensional and, in the future, four dimensional, characterization of superficial deposits is required to reach an understanding of the potential spatial lithological variability of artificial ground and the geometry of important surfaces, i.e. the boundary conditions. The classification scheme for artificial ground outlined in this paper and adopted by the British Geological Survey, will help in achieving this understanding.     

Evolution of the northeastern Kaapvaal craton and the central Limpopo belt,South Africa,based on single-zircon ages (extended abstract)

Geologie en Mijnbouw -