Daytime deposition and nighttime dissolution of calcium carbonate controlled by submerged plants in a karst spring-fed pool: insights from high time-resolution monitoring of physico-chemistry of water

Water temperature, dissolved oxygen (DO), pH, and specific conductivity (spc) were measured in a time interval of 15 min in a karst spring and the spring-fed pool with flourishing submerged plants in Guilin, SW China under dry weather for periods of 2 days. Measurements allowed calculation of calcium and bicarbonate concentrations ([Ca²⁺] and [HCO₃ ⁻]), and thus CO₂ partial pressure ( ) and saturation index of calcite (SIc). Results show that there were not any diurnal variations in the physico-chemical parameters of the water for the spring. However, during daytime periods, pool water decreased to far less than the spring water in a few hours, pH and SIc increased to greater than the spring, and [Ca²⁺] and [HCO₃ ⁻] decreased to less than the spring. During nighttime periods, pool water returned to or even increased to greater than the spring, pH and SIc decreased to less than the spring, and [Ca²⁺] and [HCO₃ ⁻] increased to greater than the spring. The decrease in [Ca²⁺] and [HCO₃ ⁻] to less than the spring during daytime periods implies daytime deposition of calcium carbonate, while the increase in [Ca²⁺] and [HCO₃ ⁻] to greater than the spring during nighttime periods implies nighttime dissolution of calcium carbonate. The direction of the observed changes depended essentially on the illumination, indicating that daytime photosynthetic and nighttime respiratory activities in the pool aquatic plant ecosystem, which were further evidenced by the increase and decrease in DO during daytime and nighttime periods respectively, were the main processes involved. The large variations of the components of the carbonate system imply considerable changes of the capacities of CO₂ and O₂ in water. The finding has implications for water sampling strategy in slow-flowing karst streams and other similar environments with stagnant water bodies such as estuaries, lakes, reservoirs, and wetlands, where aquatic plant ecosystem may flourish.     

Granulite facies amphibole and biotite equilibria,and calculated peak-metamorphic water activities

Samples located near the Oregon Dome anorthosite massif in the south-central Adirondack Mountains, New York contain the fluid-buffering mineral assemblages: amphibole + clinopyroxene + orthopyroxene + quartz or biotite + quartz + orthopyroxene + K-feldspar. These rocks were metamorphosed under granulite-facies conditions (T=725°–750°C, P=7.5 kbar) during the Grenville orogeny. The Mg-rich nature of amphiboles, micas, and pyroxenes allow accurate calculation of water activities because corrections for the effects of solid solution are relatively small. The activity of water was low during the peak of granulite-facies metamorphism, with H₂O0.15±0.14. Wollastonite occurrences indicate that the CO₂ was low (<0.3) in nearby rocks, demonstrating that large quantities of CO₂ did not infiltrate in a pervasive manner. The combination of low H₂O with low CO₂ is consistent with the hypothesis that magmatic processes were dominant, generating dry, fluid-absent conditions.Abbreviations fi Fugacity of species i in a fluid - Xi mole fraction of component i in a phase - T temperature - P lithostatic pressure - P _F fluid pressure - _i ^x activity of component i phase X     

Die Kristallstruktur des Voltaits,K2Fe 5 2+ Fe 3 3+ Al[SO4]12·18H2O

Zusammenfassung Die Kristallstruktur von künstlichem Voltait, K₂Fe₅ ²⁺Fe₃ ³⁺Al[SO₄]₁₂· ·18 H₂O, kubisch hexakisoktaedrisch,Fd3c–O _h ⁸,a ₀=27,254 ,Z-16, wurde mittels photographischer Röntgendaten bestimmt. Die Aufklärung der Struktur erfolgte mit Patterson- und Fouriermethoden unter Zuhilfenahme des multiplen isomorphen Ersatzes. Die Verfeinerung nach der Methode der kleinsten Quadrate ergab mit anisotropen Temperaturfaktoren für 726 beobachteteF _hkl R=0,033. Das Hauptmerkmal der Struktur ist ein 3dimensionales Gerüst aus [Fe³⁺O₆]-Oktaedern, [Fe ₅ ^6/2+ Fe ₁ ^6/3+ O₄(H₂O)₂]-Oktaedern und [K⁺O₁₂]-Polyedern, die durch SO₄-Tetraeder verknüpft werden. Hohlräume dieses Gerüstes werden von ungeordnet orientierten [Al(H₂O)₆]-Oktaedern eingenommen. Es wird gezeigt, daß Al als wesentlicher Bestandteil dieses Voltaits angesehen werden muß.

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The crystal structure of voltaite, K₂Fe₅ ²⁺Fe₃ ³⁺Al[SO₄]₁₂·18H₂O

Summary The crystal structure of synthetic voltaite, K₂Fe₅ ²⁺Fe₃ ³⁺Al[SO₄]₁₂· · 18 H₂O, cubic hexakis-octahedral, space groupFd3c–O _h ⁸,a ₀=27.254 ,Z=16, was determined from photographic X-ray data. The structure was solved by Patterson and Fourier-methods with the aid of multiple isomorphic substitution. Least squares refinement with anisotropic temperature factors resulted inR=0.033 for 726 observedF _hkl . The dominant structural feature is a continous framework composed of [Fe³⁺O₆]-octahedra, [Fe ₅ ^6/2+ Fe ₁ ^6/3+ O₄(H₂O)₂]-octahedra and [K⁺O₁₂]-polyhedra linked by SO₄-tetrahedra. The arrangement gives rise to cages occupied by disordered [Al(H₂O)₆]-octahedra. It is shown that Al must be considered to be a essential constituent of such voltaites.

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Mit 2 Abbildungen     

Composite landfill liner design with Ankara clay, Turkey

This study presents an overview of the geotechnical properties of the clayey soils, referred to as Ankara clay, at two sites of the Ankara region in an attempt to design a landfill profile composed of a high density polyethylene (HDPE) geomembrane/clay composite liner through the Hydrologic Evaluation of Landfill Performance (HELP) model and the Water Balance Method. The geotechnical properties of the landfill layers along with the water balance factors (i.e., evapotranspiration, precipitation, temperature, etc.) were assessed to determine the height of the water-saturated zone in the refuse above the composite liner for landfill design. The cumulative expected leakage rates through the composite liner constructed with compacted Ankara clay were related quantitatively to the cumulative average leachate head. The results of this investigation show that the leakage rates through the composite liner are within tolerable limits.     

The effectiveness of “clay” liners as basal isolation of landfills: a case study

A sodium bentonite is being used in the basal-isolation of landfills installed on strongly fractured granitic rocks of N Portugal (NW Iberia). To evaluate the performance of this clay as the ultimate impermeable basal barrier, a detailed study was carried out of the granite fracture network; the Na-bentonite was tested to characterize its mechanical and geotechnical behavior; geophysical surveys were made to search for landfill leakage and the waters found around the landfill were geochemically characterized. Fractures in the granite are usually open and devoid of mineral infillings or clay materials and, thus, permeability of the granite is very high. Dispersal of contaminants can be further enhanced by the position of the landfill on a high steep-sided ridge. Geophysical and geochemical data show evidence for seepage and dispersion of pollutants, which means that the clay failed as an ultimate isolation barrier against seepage. This investigation shows that this can be due to fracturing of the clay under load and/or to its non-homogeneous saturation and extreme shrinking character upon drying, which are accompanied by the formation of extensive cracking. Observations and experimental results suggest that the use of synthetic clays in the safe building of landfill bottom liners needs further research, and extreme care should be taken in preventing that clay water content suffers large variations after saturation, as this process considerably degrades the mechanical behavior and sealing properties of the studied Na-bentonite.     

Transport of stable isotopes: I: Development of a kinetic continuum theory for stable isotope transport

Equations are developed describing migration of stable isotopes via a fluid phase infiltrating porous media. The formalism of continuum fluid mechanics is used to deal with the problem of microscopic inhomogeneity. Provision is made explicitly for local equilibrium exchange of isotopes between minerals and fluids as well as for kinetic control of isotopic exchange. Changing characteristic parameters of transport systems such as porosity, permeability, and changes in modal proportions of minerals due to precipitation or dissolution are taken into account.The kinetic continuum theory (KCIT) is used to show how to deduce the dominant mechanism of mass transport in metasomatic rocks. Determination of the transport mechanism requires data on the spatial distribution of the reaction progress of exchange reactions between minerals and fluids involving at least two stable isotope systems such as ¹³C-¹²C and ¹⁸O-¹⁶O, for example. It is concluded that a combination of field and laboratory measurements of two or more stable isotope systems can be used to place constraints not only on the mechanism of transport but also on the magnitude of fluid fluxes, the identity of fluid sources, and the molecular species composition of fluids.Variables used C number of chemical components - D _i,j hydrodynamic dispersion tensor [m²/s] - D _i ^j diffusion coefficient matrix [m²/s] - D ^* apparent diffusion coefficient, includes sorption, dispersion, porosity and tortuosity [m²/s] - F number of degrees of freedom (variance) - f _i ^j mass or number of isotope j in fluid species i - g acceleration due to gravity [m/s²] - flow [m³/m² s] - j isotope species - j chemical element - k coefficient defined in Eq. 17 - K permeability of porous media [m²], [darcy] - L _ij phenomenological diffusion coefficient matrix [mol²/j m s] - m number of fluid species - n number of isotope exchange vectors - p number of phases - P pressure [Pa] - P ^* hydrological pressure potential [Pa] - R ^j ratio of concentration of rare to common isotope of element j - r number of restrictions imposed on system - s _i ^j mass or number of isotope j in one mole of mineral phase i - t time [s] - V volume [m³] - X _i number of moles of fluid species i in unit fluid volume - X _l number of moles of mineral l in unit volume - X _l ^j mole fraction of isotope j in one mole mineral l - X ^* mole fraction with respect to the whole system - z space coordinate [m] - z transformed space coordinate - z ^* location of an infiltration front [m] - _x–y ^j fractionation factor between two phases, x, y, for isotope j - porosity - fluid viscosity [Ns/m²] - fraction of porosity accessible to a specific mass transport mechanism - chemical potential [j/mole] - stoichiometric reaction coefficient - normalized reaction progress variable - mass, specific mass [gr/cm] - tortuosity - fluid velocity [m/s] - c common isotope - init initial - j isotope species - r rare isotope - tot sum of common and rare isotope - dif diffusive - disp dispersive - eq mineral composition in equilibrium with initial infiltration concentration of the fluid - f fluid - inf infiltrative - r rock, without fluid phase - samp sample - std standard - sys system - tot fluid and rock     

Assessment of the Design of an Experimental Cover with Capillary Barrier Effect Using 4 Years of Field Data

One important step in the design of inclined covers with capillary barrier effect (CCBE) is the determination of the water diversion length (DL). Numerical simulations can predict the DL more precisely than steady-state analytical solutions. Nevertheless, as simplified methods have always been part of engineering design, the application of analytical solutions with conservative boundary conditions, may allow engineers to make reasonable predictions, particularly during the pre-feasibility stage of a project. In this study, a CCBE was designed, constructed and instrumented at the Saint-Tite-des-Caps landfill, Quebec, Canada. This CCBE included a seepage control layer superimposing a sand-gravel capillary barrier. The seepage control layer was made up of deinking by-products (DBP), an industrial by-product that was previously disposed of as waste. The capillary barrier was designed using an adaptation of the Ross analytical solution and the scenario considered was that of steady-state flow during constant seepage flow applied uniformly at the top of the sand-gravel capillary barrier. Although these conditions appear simplistic, they were deemed reasonable because placement of the seepage control layer on the top of the capillary barrier led to very low suctions at the interface, thereby allowing uniform downward seepage rates, limited by the saturated hydraulic conductivity of the DBP. In this paper, a discussion about the behaviour of the cover system based on 4 years of field data from several instruments is presented. The challenge of using DBP, more precisely the settlement of the DBP layer and its impact on k _sat , is also assessed. The DL was reassessed considering the new k _sat . A discussion on the validity of employing analytical solutions to determine DL is also presented. This paper illustrates how certain variables affect the design of inclined CCBEs that include a highly compressible material as seepage control layer.     

An experimental study of the diffusion of oxygen in quartz and albite using an overgrowth technique

Diffusion rates of¹⁸O tracer in quartz ( c, 1 Kb H₂O) and Amelia albite ( 001, 2 Kb H₂O) have been measured, using Secondary Ion Mass Spectrometry (SIMS). A new technique involving hydrothermal deposition of labelled materials has removed the possibility of pressure solution-reprecipitation processes adversely affecting the experiments. Reported diffusion constants are:-quartz ( c), ,Q=98±7 KJ mol^–1 (600–825° C, 1 Kb); Amelia albite ( 001), ,Q=85±7 KJ mol^–1, (400–600° C, 2 Kb). Measured quartz¹⁸O diffusivities decrease discontinuously at the- transition, reflecting strong structural influences. The reported albite data agree with previously recorded studies, but-quartz data indicate significantly lower activation energies. Possible causes of this discrepancy, and some geological consequences, are noted.     

Orthopyroxene-clinopyroxene equilibria in the system CaO-MgO-Al2O3-SiO2 (CMAS): new experimental results and implications for two-pyroxene thermometry

A new set of reversal experiments for coexisting ortho- and clinopyroxenes in the system CMAS at conditions between 1,000–1,570° C and 30 to 50 kb is presented and combined with literature data. Pyroxene behaviour, particularly that of clinopyroxene, is very complicated and different styles of Al incorporation into the pyroxene structure for low and high concentrations of Al are indicated, strongly influencing the exchange of the enstatite component between ortho- and clinopyroxene. Thermodynamic modelling of this exchange is problematic because of the large number of unknown coefficients compared to the number of experiments. Thermometry based on such models becomes very dependent on accuracy of experimental data and analyses of small quantities of elements. Despite this complexity very simple empirical thermometric equations are capable of reproducing experimental conditions in the systems CMS and CMAS over a wide range of P, T conditions. We derived the equation which gives a mean error of estimate of 25° C when applied to CMS and CMAS data.Abbreviations Used in the Text cpx clinopyroxene - di diopside, CaMgSi₂O₆ - en enstatite, Mg₂Si₂O₆ - opx orthopyroxene - px Pyroxene - py pyrope - a _i ^j activity of component i in phase j - activity coefficient - G _P,T (A) molar Gibbs free energy difference of reaction (A) at P, T - X _i ^j mole fraction of component i in phase j     

Temperature dependence of the polarized electronic absorption spectra of olivines. Part II—Cobalt-containing olivines

Polarized electronic absorption spectra of single crystalline Co₂[SiO₄] and (Co_0.64Mg_0.36)₂[SiO₄] (E|| a (|| Z), E || b (|| X), E || c (|| Y)) have been studied in the temperature range 293 T/K 1273. The three polarized spectra show a total of 15 bands. Five bands are caused by spin-allowed transitions in Co²⁺ ions at M1 sites which appear in all polarization directions. Seven polarization-dependent bands can be ascribed to spin-allowed transitions in Co²⁺ ions at M2 sites and three bands may be assigned to spin-forbidden transitions. The assignment of bands due to Co²⁺ ions at M1 and M2 sites has been made on the basis of transition energies and intensity ratios. Further arguments have been derived from the comparison of spectra of crystals with different cobalt content, from the analysis of the polarization dependence of the spectra, and from the evolution of band intensities with temperature.     

Alkaline rocks and carbonatites of Amba Dongar and adjacent areas,Deccan Alkaline Province,Gujarat, India. 2. Complexly zoned clinopyroxene phenocrysts

Summary Clinopyroxene phenocrysts in six samples, representing the compositional range in the alkaline rocks of the region, range overall from aluminian titanian diopside (mg = Mg/[Mg + Fe _t ] = 92) to aegirine (mg = 17), but Ca-pyroxenes are dominant (90% of 851 analyses), Ca-Na pyroxenes minor ( 10%) and Na-pyroxenes rare ( 1%). Pyroxenes in associated subalkaline rocks (dolerites and basalts) are lower-Ca augites of distinct trend. Al and Ti correlate positively in the alkaline rock pyroxenes, but negatively with Si, mirroring decreasing CaTiAl₂O₆ and CaSiAl₂O₆ components in more felsic host-rocks. Although the most evolved pyroxenes in each host-rock show increasingna (Na/[Na + Ca]) and Zr and decreasingmg as host-rock mg decreases, the most primitive pyroxenes in each host-rock do not change, implying they are not at equilibrium. Over 40 paired logitudinal and lateral traverses across 21 phenocrysts reveal very complex zoning, in which up to five growth zones can be recognized in one crystal, separated by either sharp contacts or gradual transitions. These individual zones may show one of seven zoning trends:normal (na asmg moving outwards towards the rim),reverse (na asmg ),inverse (na asmg ),converse (na asmg ),unzoned (no change),symmetric (na andmg display complementary and trends), oroscillatory (superimposed on normal, reverse or unzoned). Ten distinct crystal types can also be recognized, showing particular sequences of zoning trends: for example, Type 1 shows a green, relatively sodic core surrounded by a more primitive rim or overgrowth, and has similar longitudinal and lateral zoning; whereas Type 6 shows extremely erratic variations which differ longitudinally and laterally. Up to four of these types were found in one host-rock, with little pattern in their distribution between different host-rocks. These pyroxenes are believed to record an intricate history of stop-start differentiation, complicated by magma-mixing, entry or disappearance of coprecipitating phases, and inheritance of high-pressure precipitates. Apparently random superimposed variations of Al, Ti and Si within some individual growth zones most probably reflect kinetic (disequilibrium) effects, due to rapid growth and/or supercooling.

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Alkaligesteine und Karbonatite von Amba Dongar und Umgebung Deccan Alkali-Provinz, Gujarat, Indien. 2. Komplex zonierte Klinopyroxenkristalle

Zusammenfassung Klinopyroxenkristalle in sechs Proben, die das ganze Spektrum der Zusammensetzung der Alkaligesteine des Gebietes repräsentieren, schwanken von Aluminium-Titandiopsid (mg = Mg/[Mg + Fe _t ] = 92) bis Aegirin (mg = 17), aber C-Pyroxene dominieren (90% von 851 Analysen). Ca-Na-Pyroxene sind Nebengemengteile (10%) und Na-Pyroxene sind selten ( 1%). Pyroxene in assoziierten subalkalischen Gesteinen (Dolerite und Basalte) sind Ca-arme Augite eines eigenständigen Trendes. Al und Ti korrelieren positiv in den Pyroxenen der Alkaligesteine, aber negativ mit Si, und dies spiegelt abnehmende CaTiAl₂O₆ und CaSiAl₂O₆ Komponenten in den mehr felsischen Wirtsgesteinen wider. Obwohl Pyroxene in jedem Wirtsgestein zunehmende na-Werte (Na/[Na + Ca]) und Zr-Gehalte und mit demmg des Wirtsgesteines auch abnehmendemg-Werte zeigen, lassen die primitivsten Pyroxene in jedem Wirtsgestein keine Änderung erkennen. Dies weist darauf hin, daß sie nicht im Gleichgewicht sind. Über vierzig paarweise angelegte Längs- und Quertraversen über 21 Kristalle zeigen sehr komplexen Zonarbau, in dem bis zu fünf Wachstumszonen in einem Kristall erkannt werden können, die entweder von scharfen Kontakten oder von graduellen Übergangszonen getrennt werden. Diese individuellen Zonen können einen der folgenden sieben Trends erkennen lassen: Normal (na ;mg ; von innen gegen den Rand zu), reverse (na ,mg ), invers (na undmg ), konvers (na ,mg ), unzoniert (keine Änderung), symmetrisch (na undmg zeigen komplementäre und Trends), oder oszillierend (überlagert den normalen, reversen oder unzonierten Trend). Weiters können zehn definierte Kristalltypen erkannt werden, die besondere Abfolgen von Trends der Zonierung erkennen lassen. Zum Beispiel zeigt Typ 1 einen grünen, relativ Na-reichen Kern, umgeben von einem mehr primitiven Rand oder Überwachsungen, wobei die Zonierung in Längs- und Querrichtung ähnlich ist. Demgegenüber zeigt Typ 6 außerordentlich unregelmäßige Zonierungen, die in Längs- und Querschnitten verschieden sind. Bis zu vier dieser Typen wurden in einem Wirtsgestein nachgewiesen, und sie ließen keine deutlichen Verteilungsmuster zwischen verschiedenen Wirtsgesteinen erkennen. Es wird angenommen, daß diese Pyroxene eine komplexe geschichte von stop-start Differentiation wiedergeben, die durch Magmamischung, durch die Zufuhr oder das Verschwinden von gleichzeitig ausfallenden Phasen und durch die Übernahme von Hochdruck-Bildungen weiter kompliziert wird. Unregelmäßig überlagerte Variationen von Al, Ti und Si innerhalb einzelner Wachstumszonen reflektieren sehr wahrscheinlich kinetische (Ungleichgewicht) Effekte, die auf sehr rasches Wachstum und/oder Überkühlung zurückgehen.

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With 11 Figures     

Introduction to pareto optimal mine planning

Summary Recent advances in mathematical optimization have resulted in the development of superior techniques for solving realistic decision making problems. The technique called PARETO OPTIMAL SERIAL DYNAMIC PROGRAMMING is presented here as a tool for rational mine planning. This approach always enables the identification of a set of decision alternatives considered superior to the remaining feasible, usually numerous, decision alternatives, when a number of conflicting, noncommensurable, objectives are simultaneously optimized. It is further noted that the decision makers' truly preferred decision is always one of the members identified as the superior set.Notation [A] Number of initial decision alternatives for production stage 1 - [A ] Accumulated pareto optimal stage objective vector at stage for the remainder of the stagesN – - [A] Accumulated non-pareto optimal stage objective vector at stage for the remainder of the stagesN – - [B] Number of initial decision alternatives for production stage 2 - [C] Number of initial decision alternatives for production stageN - [COG _k,] Cutoff grade for decision,k, at production stage, - [D] Number of initial pareto solutions for production stage 1 - [D ] Decision at stage - [E] Number of initial pareto solutions for production stage 2 - [F] Number of initial pareto solutions for production stageN - [j] A random objective function - [J] Number of objective functions - [LIFE _k,] Minelife for decision,k, at production stage, - [] An arbitrary production stage - [N] Final production stage - [NPV] Expected net present value - [NPV _k,] Project net present value for decision,k, at production stage - [OPR _k,] Ore production rate for decision,k, at production stage, - [S ] State of the system in stage - [ _n] Immediate state objective vector at stage      

Schultenit,PbHAsO4, und PbHPO4: Synthesen und Kristallstrukturen nebst einer Diskussion zur Symmetrie

Zusammenfassung Schultenit, PbHAsO₄ [a=4,859(1) Å,b=6,756(1) Å,c=5,843(1) Å, =95,40(1)°] und PbHPO₄ [a=4,6838(3) Å,b=6,6451(2) Å,c=5,7817(3) Å, =97,138(4)°] sind isotyp und kristallisieren monoklin. Für beide Verbindungen war bei einer Temperatur vonT _c312 K der Übergang von RaumgruppeP _c nachP2/c bekannt. Der Strukturtyp von Schultenit ist durch das Vorliegen einer kurzen Wasserstoffbrückenbindung zwischen zwei in RaumgruppeP2/c über ein Symmetriezentrum ineinander überführbarer XO₄-Tetraeder charakterisiert. Die O–H...O-Bindungslänge beträgt in beiden Verbindungen übereinstimmend 2,46 Å. Mit Hilfe von Röntgen-Einkristallstrukturuntersuchungen konnte gezeigt werden, daß dieser Übergang vonPc nachP2/c offensichtlich nur auf einer Ordnung des H-Atoms beruht, während alle anderen Atome auch bei Zimmertemperatur innerhalb des Fehlers eine zentrosymmetrische Atomanordnung aufweisen.

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Schultenite, PbHAsO₄, and PbHOP₄: Syntheses and crystal structures with a discussion on their symmetry

Summary Schultenite PbHAsO₄ [a=4.859(1) Å,b=6.756(1) Å,c=5,843(1) Å, =95.40(1)°] and PbHPO₄ [a=4,6838(3) Å,b=6,6451(2) Å,c=5.7817(3) Å =97.138(4)°] are isotypic and crystallize monoclinic. For both compounds a transition from space groupPc toP2/c has been described atT _c312 K. The structure type of schultenite is characterized, by a short hydrogen bond between two XO₄ tetrahedra which are combined by a center of symmetry in space groupP2/c. The O–H...O bond length is for both these compounds 2.46 Å. Based on X-ray single crystal structure refinements it has been shown, that the transition fromPc toP2/c is obviously caused only by an ordering of the H atom; all the other atoms are also at room temperature centrosymmetrically arranged within limits of error.

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Hern Prof.Dr.K.Komarek zum 60.Geburtstag gewidmet

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Mit 1 Abbildung     

Kristallchemie natürlicher Telluride I: Verfeinerung der Kristallstruktur des Sylvanits,AuAgTe4

Zusammenfassung Mikrosondenanalysen und die Verfeinerung der Kristallstruktur zeigen, daß Sylvanit, AuAgTe₄, aus Baia de Arie (=Offenbánya), Rumänien, eine stöchiometrische Zusammensetzung und eine geordnete Kristallstruktur besitzt (a=8,95(1) Å,b=4,478(5) Å,c=14,62(2) Å; =145,35(5)°;Z=2; RaumgruppeP2/c–C _2h ⁴ ). Das Au-Atom ist von sechs Te-Atomen in einer für die Oxidationszahl III charakteristischen [4+2]-Koordination umgeben. Um das Ag-Atom (Oxidationszahl I) sind ebenfalls sechs Te-Atome, jedoch in einer [2+2+2]-Koordination, angeordnet. Über gemeinsame Kanten bauen AuTe₆- und AgTe₆-Polyeder Schichten parallel (100) auf. Diese Schichten werden über Te₂-Hanteln (Te–Te=2,82 Å) zu einem Gerüst verknüpft.

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Crystal chemistry of natural tellurides. I: Refinement of the crystal structure of sylvanite, AuAgTe₄

Summary Electron microprobe analyses and the refinement of the crystal structure indicate, that sylvanite, AuAgTe₄, from Baia de Arie (=Offenbánya), Romania, has a stoichiometric composition and an ordered crystal structure (a=8.95(1) Å,b=4.478(5) Å,c=14.62(2) Å; =145.35(5)°;Z=2; space groupP2/c–C _2h ⁴ ). The Au atom is surrounded by six Te atoms in a [4+2] coordination as characteristic for oxidation state III. Around the Ag atom (oxidation state I) are also six Te atoms, but arranged in a [2+2+2] coordination. Via common edges the AuTe₆ and AgTe₆ polyhedra build up sheets parallel to (100). These sheets are combined to a network of Te₂ dumbbells (Te–Te=2.82 Å).

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Mit 2 Abbildungen     

Graphical estimations of magma densities and compositional expansion coefficients

Assuming that the partial molar volume of each chemical component in a magma is constant, the magma density, _m , is expressed as 1/ _m =C _i / _fi , whereC _i is the weight fraction, and _fi is the fractionation density of thei ^th component. Using this linear relationship between 1/ and weight fraction, the density change due to addition or subtraction of any component can be graphically estimated on 1/ vs oxide wt% diagrams. The compositional expansion coefficient of thei ^th component, _fi , is expressed as _i = _m / _fi –1. The compositional expansion coefficient of H₂O has a much larger absolute value than those of any other oxide or mineral components, showing that addition of a small amount of H₂O can significantly decrease magma density. These simple expressions facilitate the estimation of magma densities during fractionation.     

Contrasting serpentinization processes in the eastern Central Alps

Stable isotope compositions have been determined for serpentinites from between Davos (Arosa-Platta nappe, Switzerland) and the Valmalenco (Italy). D and ¹⁸O values (–120 to –60 and 6–10, respectively) in the Arosa-Platta nappe indicate that serpentinization took place on the continent at relatively low temperatures in the presence of limited amounts of metamorphic fluids that contained a component of meteoric water. One sample of chrysotile has a ¹⁸O value of 13 providing evidence of high W/R ratios and low formation temperature of lizardite-chrysotile in this area. In contrast, relatively high D values (–42 to –34) and low ¹⁸O values (4.4–7.4) for serpentine in the eastern part of the Valmalenco suggest a serpentinization process that took place at moderate temperatures in fluids that were dominated by ocean water. The antigorite in the Valmalenco is the first reported example of continental antigorite with an ocean water signature. An amphibole sample from a metasomatically overprinted contact zone to metasediments (D=-36) indicates that the metasomatic event also took place in the presence of ocean water. Lower D values (–93 to –60) of serpentines in the western part of the Valmalenco suggest a different alteration history possibly influenced by fluids associated with contact metamorphism. Low water/rock ratios during regional metamorphism (and metasomatism) have to be assumed for both regions.     

Long-term prediction of intermediate depth earthquakes in the southern Aegean region based on a time-predictable model

Repeat times of strong intermediate depth (60 km h 180 km) earthquakes have been determined by the use of instrumental and historical data for six seismogenic sources in the Benioff zone of the southern Aegean area. For four of these sources, at least two interevent times (three mainshocks) are available for each source. By using the repeat times for these four sources, the following relation has been determined: logT _t = 0.20M _min + 0.19M _p +a, whereT _t is the repeat time (in years),M _min the surface wave magnitude of the smallest earthquake considered,M _p the magnitude of the preceding mainshock and a parameter which varies from source to source. A multilinear correlation coefficient equal to 0.91 was determined for this relation, which indicates that the time predictable model holds to a satisfactory degree for the strong mainshocks of intermediate focal depth in the southern Aegean.By assuming that the ratioT/T _t, whereT is the observed andT _t the calculated repeat time, follows a lognormal distribution, the conditional probabilities for the occurrence of strong (M _s 6.5) and very strong (M _s 7.5) earthquakes during the period 1991–2001 in these four seismogenic sources have been calculated. These probabilities are very high (P > 0.9) for the strong and high (P > 0.5) for the very strong intermediate depth earthquakes which occur in the three sources of the shallower (h < 100 km) part of the Benioff zone where coupling occurs between the front parts of the Mediterranean lithosphere (downgoing) and the Aegean lithosphere.     

The relation between c dimension and exchange components in micas

Summary A number of micas of varying compositions and polytypism have been selected from the literature for multiple linear regression analysis. The c dimension in micas is found to depend on the sizes of the interlayer cation, d_i, and tetrahedral cation, d_t, as well as on the hydroxyl content, [OH]. The regression equation obtained: c_r = 5.415 + 0.071[OH] + 2.098d_i + 2.335d_t with R² = 90.5%, shows that the three variables affect the c-axis dimension in the order d_t > d_i [OH]. Addition of 2- and 3-layer polytypes to the regression analysis reduces R² to 87.2%. Application of the regression analysis to synthetic Al-rich biotites from the literature shows that the amount of [A1^IVA1^VI]_1Y[Fe²⁺, MgSi]_–1y in solid solution is limited and always less than [A1^VIO]_1z[Fe²⁺, MgOH]_–1z (i.e. 0.35 > y z). The maximum value of the vector y in solution is slightly higher than that reported for natural Al-rich biotites.

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Die Beziehung zwischen der Gitterkonstante c und den Austauschkomponenten in Glimmern

Zusammenfassung Eine Anzahl von Glimmern unterschiedlicher Zusammensetzung und Polytypie wurde aus der Literatur für eine multiple lineare Regressionsanalyse ausgewählt. Es stellte sich heraus, dass in Glimmern die Gitterkonstante c von den Grössen des Zwischenschicht-Kations d_i und des tetraedrischen Kations d_t abhängt, ferner vorn Hydroxylgehalt, [OH]. Die erhaltene Regressionsgleichung c_r = 5,415 + 0,071 [OH] + 2,098d_i + 2,335d_t mit R² = 90,5% zeigt, dass die drei Variablen die Grösse der c-Achse in der Reihenfolge d_t > d_i [OH] beeinflussen. Der Einschluss von 2- und 3-Schicht Polytypen in die Regressionsanalysen verkleinert R² auf 87,2%. Die Anwendung der Regressionsanalysen auf synthetische Al-reiche Biotite aus der Literatur zeigt, der Betrag von [A1^IVA1^VI]_1y[Fe²⁺, MgSi]_–1y beschränkt und immer kleiner als [A1^VIO]_1z[Fe²⁺, MgOH]_–1z (mit 0,35 < y z) ist. Der Maximalwert des Vektors y in Lösung ist etwas grösser als jener, der für natürliche Al-reiche Biotite angegeben wurde.

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Fe-Zn exchange reaction between tetrahedrite and sphalerite in natural environments

Microprobe and fluid inclusion analyses of hydrothermal ore deposits containing the subassemblage sphalerite+ tetrahedrite-tennantite [(Cu, Ag)₁₀(Fe, Zn)₂(As,Sb)₄S₁₃] reveal that the Gibbs energies of the reciprocal reaction Cu₁₀Zn₂Sb₄S₁₃ + Cu₁₀Fe₂As₄S₁₃ = Cu₁₀Fe₂Sb₄S₁₃ + Cu₁₀Zn₂As₄S₁₃ and the Fe-Zn exchange reaction 1/2Cu₁₀Fe₂Sb₄S₁₃ + ZnS = 1/2Cu₁₀Zn₂Sb₄S₁₃ + FeS are within the uncertainties of the values established by Sack and Loucks (1985) and Raabe and Sack (1984), 2.59±0.14 and 2.07±0.07 kcal/gfw. However, this study suggests that the Fe-Zn exchange reaction between sphalerite and Sb and Ag-rich tetrahedrites does not obey the simple systematics suggested by Sack and Loucks (1985) wherein tetrahedrite is assumed to behave as an ideal reciprocal solution. Instead these studies show that the configurational Gibbs energy of this exchange reaction,RTln[(X _Fe/X _Zn)^TET(X _ZnS/X _FeS)^SPH], corrected for sphalerite nonideality exhibits both a local maximum and minimum as a function of Ag/(Cu+Ag) ratio at a givenX _FeS ^SPH and temperature. The local maximum forX _FeS ^SPH 0.10 corresponds to the position of the cell edge maximum established for natural tetrahedrites by Riley (1974), Ag/(Ag+Cu)0.4. These studies and the results of structural refinements of Ag-bearing tetrahedrites suggest that in low silver tetrahedrites Ag is preferentially incorporated in trigonal-planar sites but that in tetrahedrites with intermediate and greater Ag/(Ag+Cu) ratio, Ag is preferentially incorporated in tetrahedral sites. A nonconvergent site ordering model for tetrahedrite is developed to quantify and extrapolate these predictions.     

Theoretical phase relations involving cordierite and garnet revisited: the influence of oxygen fugacity on the stability of sapphirine and spinel in the system Mg-Fe-Al-Si-O

The theoreticalP-T grid for stability relations of the phases cordierite (Cd), sapphirine (Sa), hypersthene (Hy), garnet (Ga), spinel (Sp), sillimanite (Si), and quartz (Qz) of Hensen (1971), has proved useful in the interpretation of metamorphic mineral assemblages formed at low oxygen fugacity. Both experimental data and evidence from natural rocks indicate that at high oxygen fugacity compatability relations change as a result of the enlargement of the stability field of spinel, which causes a topological inversion and the stabilisation of the invariant points [Sa], [Ga], and [Cd]. This implies the stable existence of the univariant equilibria (for buffered conditions): Sp+Qz=Ga+Hy+Si+O₂ (Sa, Cd), Cd+Sp+Qz=Hy+Si+O₂ (Sa, Ga) and Sa+Sp+Qz=Hy+Si+O₂ (Ga, Cd) and the divariant reaction: Sp+Qz=Hy+Si+O₂ (Sa, Ga, Cd). These redox equilibria are restricted to conditions of high oxygen fugacity. The proposed theoreticalP-T grids, for both low and high oxygen fugacity, satisfactorily explain all experimental data and metamorphic mineral assemblages so far found in granulites.