Al-O--Al paramagnetic defects in kaolinite

Trapped holes located on Al-O-Al bonds in kaolinite were studied by electron paramagnetic resonance spectroscopy (EPR) at 9.3 and 35 GHz applied to well-crystallized, X-ray irradiated and oriented samples. The Q-band EPR spectrum is characterized by three clearly separated groups of 11 quasi-equidistant superhyperfine lines centered at g_xx=2.040±0.0005, g_yy=2.020±0.0005 and g_zz=2.002±0.001. In each of these groups, the 11 superhyperfine lines exhibit intensities according to the ratios 12345654321. An angular dependence of the Q-band EPR spectrum with respect to the magnetic field is demonstrated by measurements on oriented films of kaolinite. An appropriate numerical treatment of the EPR spectra is described, which allowed extraction of the SuperHyperfine Structures (SHFS). X-and Q-band spectra have also been simulated. It is concluded from these experiments that only one type of center is present. This center, labelled the B-center in the literature, is very probably a hole trapped on oxygen (O^- center) atoms coupled to two octahedral aluminium.     

31P solid-state nuclear magnetic resonance spectroscopy of aluminum phosphate minerals

This study examines the links between ³¹P solidstate NMR studies of aluminum phosphate minerals and their crystallographic structures. We found that ³¹P isotropic chemical shift values, _iso, carry little information about mineral structures. There seems to be no relation between the chemical shift anisotropy, =₃₃–₁₁ (₃₃>₂₂> ₁₁), and indicies of phosphate-tetrahedra distortion. ³¹P¹H heteronuclear magnetic dipole interactions, on the other hand, carry important information about hydrous phosphate mineral structures, information that should prove to be quite valuable in studies of phosphate adsorbed on mineral surfaces. This interaction can be measured through a variety of qualitative and quantitative experiments. It appears that spin diffusion is so rapid that subtle differences in hydrogen-bonding environments cannot be resolved.     

Determination of the principal reflectances of an absorbing uniaxial mineral,ilmenite, from measurements on random sections

We show the steps followed for obtaining the principal reflectances of an absorbing uniaxial mineral, ilmenite, by use of the inference method derived from statistical reflectance parameters of the sample. The values obtained in air, at 580 nm, with this method are: R =19.6±0.3 and R =17.8±0.3.     

Compression of α-MnS (alabandite) and a new high-pressure phase

The compressibility of -Mns (alabandite) was determined by x-ray analysis using a Mao-Bell type diamond anvil cell. The zero pressure bulk modulus (K₀) is 74±2 GPa with the pressure derivative of the bulk modulus (K_o) fixed at four. Allowing (K_o) to vary yielded a statistically better fit with K₀ = 88±6 GPa and k₀ = 2.2±0.6. Our data combined with the data of McCammon (1991) gave K_o = 73±1 GPa with k_o fixed at four. A fit with k_o allowed to vary yielded k_o = 75±2 GPa and k_o = 3.7±0.4. Alabandite transformed from the B1 structure (NaCl-type) to an unknown high-pressure phase at 26 GPa. The high-pressure phase has lower than hexagonal symmetry and it is stable to at least 46±4 GPa.Also affiliated with the James Franck Institute, University of Chicago     

Al-Si ordering in Sr-feldspar SrAl2Si2O8: IR,TEM and single-crystal XRD evidences

Al-Si ordering in Sr-feldspar has been followed by isothermal annealing, starting from a disordered metastable configuration. Ordering could not be followed by changes in the spontaneous strain as cell parameters did not show significant changes with thermal treatment from 0.016 h to 452 h at T=1350° C, while, on the contrary, significant changes in IR spectra are observed. A single crystal obtained from melt (Q _od 0) has been progressively heated up to 678 h at T=1350° C and the relevant structural refinements enabled to monitor changes in degree of Al-Si order up to Q_od = 0.86. In isothermal treatment for Sr-feldspar it is observed a significantly lower Q _od than in anorthite after the same annealing time. TEM observation has shown in Sr-feldspar, also for shortest annealing, b type reflections, while in anorthite, in the same conditions, e type reflections have been observed (Carpenter 1991a). In the first stages of ordering b APDs sized 100 Å (at T=1350° C, 0.33 h) have been observed in Sr-feldspar; APD coarsening occurs with an activation energy of 120±7 kcal mol^-1, not significantly different from anorthite. The ordering process seems to be a slower process in Sr-feldspar than in anorthite, even though data from longer annealing suggest that the Q _od close to the equilibrium is the same in Sr and Ca-feldspar (Q _od = 0.86 at T=1350° C).     

Admontit,ein neues Boratmineral aus der Gipslagerstätte Schildmauer bei Admont in der Steiermark (Österreich)

Zusammenfassung Admontit ist ein neues Magnesiumborat, das in der Gipslagerstätte Schildmauer bei Admont in der Steiermark (Österreich) in Vergesellschaftung mit drei weiteren neuen borhaltigen Mineralien sowie Gips, Anhydrit, Hexahydrit, Löweit, Quarz und Pyrit auftritt.Das Mineral bildet undeutlich ausgebildete farblose Kristalle von monokliner Symmetrie, die zum Teil nachc gestreckt und tafelig nach {100} sind. Keine Spaltbarkeit, Bruch muschelig, Härte wahrscheinlich 2–3,D _gem .=1,82,D _x =1,875g·cm^–3;n =1,442±0,002,n =1,504±0,002, 2V 30°,r. AE(010),n c auf (010) ca. 45°. a ₀=12,68,b ₀=10,07,c ₀=11,32 Å (alle Werte±0,02 Å),=109,68° (±0,1°),Z=2, RaumgruppeP2₁/c. Stärkste Linien des Pulverdiagramms: 12,08(9), 7,60(10), 3,93(8), 2,68(9). Formel: 2 MgO·6 B₂O₃·15 H₂O. In Wasser wird Admontit langsam zersetzt. Erhitzungsversuche zeigten, daß das Gitter zwischen 100 und 200°C zerstört wird. Ein Teil des Wassers entweicht schon unterhalb 100°C, der Rest zwischen 150 und 350°C.

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Admontite, a new borate mineral from the gypsum deposit Schildmauer near Admont in Styria (Austria)

Summary Admontite is a new magnesium borate found in the gypsum deposit of Schildmauer near Admont in Styria (Austria) in association with three other new borium-containing minerals and with gypsum, anhydrite, hexahydrite, löweite, quartz and pyrite.The mineral occurs in poorly developed colourless crystals of monoclinic symmetry, which in part are elongated along thec axis and flattened on {100}. No cleavage, fracture conchoidal, hardness probably 2–3,D _meas .=1.82,D _x =1.875g·cm^–3.n =1.442±0.002,n =1.504±0.002, 2V 30°,r. AE(010),n c on (010) about 45°.a ₀=12.68,b ₀=10.07,c ₀=11.32 Å (all±0.02 Å), =109.68° (±0.1°),Z=2,space groupP2₁/c. Strongest lines of the powder pattern: 12.08(9), 7.60(10), 3.93(8), 2.68(9). Chemical composition: 2 MgO·6 B₂O₃·15 H₂O. Admontite is slowly decomposed in water. Investigations of the thermal behaviour show that the lattice breaks down between 100 and 200°C. Part of the water escapes already under 100°C, the rest between 150 and 350°C.

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Herrn Univ. Prof. Dr.H. Meixner zum 70. Geburtstag gewidmet.     

Thermodynamic properties of strontianite-witherite solid solution (Sr,Ba)CO3

Structural parameters and thermodynamic properties of strontianite — witherite solid solutions have been studied by X-ray powder diffraction, heat flux Calvet calorimetry and cation-exchange equilibria technique. X-ray study of the synthetic samples have shown linear and quadratic (for c-parameter) composition dependencies of the lattice constants in the carbonate solid solution. The thermodynamic energy parameters demonstrate the non-ideal character of strontianite — witherite solid solutions. Enthalpies of solution of the samples have been measured in 2PbO*B₂O₃ at 973 K. The new data on the enthalpy of formation H _f,298.15 ⁰ of SrCO₃ and BaCO₃ were obtained: -1231.4±3.2 and -1209.9±5.8 kJ*mol^-1 respectively. The enthalpy of mixing of the solid solution was found to be positive and asymmetric with maximum at X_Ba (carbonate)=0.35. The composition dependence of the enthalpy of mixing may be described by two — parametric Margules model equation: H _mix=X _BaX _Sr[(4.40±3.91)X _Ba+(28.13±3.91)X _Sr] kJmol^–1 Cation-exchange reactions between carbonates and aqueous SrCl₂-BaCl₂ supercritical solutions (fluids) were carried out at 973 and 1073 K and 2 kbar. Calculated Margules model parameters of the excess free energy are: for orthorhombic carbonate solid solutions W _Sr=W _Ba=11.51±0.40 kJmol^–1 (973 K) and W _Sr=W _Ba=12.09±0.95 kJmol^– (1073 K) for trigonal carbonate solid solutions W _Sr=W _Ba=13.55±0.40 kJmol^– (1073 K).     

Equation of state,structural behaviour and phase diagram of synthetic MgFe<Subscript>2</Subscript>O<Subscript>4</Subscript>, as a function of pressure and temperature

The behaviour of synthetic Mg-ferrite (MgFe₂O₄) has been investigated at high pressure (in situ high-pressure synchrotron radiation powder diffraction at ESRF) and at high temperature (in situ high-temperature X-ray powder diffraction) conditions. The elastic properties determined by the third-order Birch–Murnaghan equation of state result in K₀=181.5(± 1.3) GPa, K=6.32(± 0.14) and K= –0.0638 GPa^–1. The symmetry-independent coordinate of oxygen does not show significant sensitivity to pressure, and the structure shrinking is mainly attributable to the shortening of the cell edge (homogeneous strain). The lattice parameter thermal expansion is described by _a0+_a1*(T–298)+_a2/(T–298)², where _a0=9.1(1) 10^–6 K^–1, _a1=4.9(2) 10^–9 K^–2 and _a2= 5.1(5) 10^–2 K. The high-temperature cation-ordering reaction which MgFe-spinel undergoes has been interpreted by the ONeill model, whose parameters are = 22.2(± 1.8) kJ mol^–1 and =–17.6(± 1.2) kJ mol^–1. The elastic and thermal properties measured have then been used to model the phase diagram of MgFe₂O₄, which shows that the high-pressure transition from spinel to orthorombic CaMn₂O₄-like structure at T < 1700 K is preceded by a decomposition into MgO and Fe₂O₃.     

1Tc-Strontiohilgardit (Ca,Sr)2[B5O8(OH)2Cl] und seine Stellung in der Hilgarditgruppe X2/II[B5O8(OH)2Cl]

Zusammenfassung 1Tc-Strontiohilgardit (Ca, Sr)₂ [B₅O₈(OH)₂,Cl] mit Ca : Sr etwa 1 : 1 ist ein neues Mineral der Hilgarditgruppe. Fundpunkt: Reyersbausen (9° 59,7 E, 51° 36,6 N), Grube Königshall-Hindenburg, Flöz Staßfurt in sylvinitischer Ausbildung.Konstanten : triklin-pedial,a ₀=6,38 Å,b ₀=6,480 Å,c ₀=6,608 Å, =75,4°,=61,2°, =60,5°; tafelige-gestreckte Links- und Re chtskristalle, farblos, wasserunlöslich, piezoelektrisch. Härte 5–7, Dichte 2,99 g cm^–3;n =1,638,n =1,639,n =1,670; 2V =19°.Neue Daten für die Hilgarditgruppe : 2 M (Cc)-Calciumhilgardit (=Hilgardit) =4 Ca₂[B₅O₃(OH)₂Cl], Raumgruppe Cc.3Tc-Calciumhilgardit (=Parahilgardit) = 3 Ca₂[B₅O₃(OH)₂Cl]; trinklin-pedial, ₀=6,31 Å,b =6,484 Å,c ₀=17,50 Å; =84,0°,=79,6°, =60,9°.Die Polymorphiebeziehungen sind geometrisch deutbar durch eine spezielle Art der Polytropie (Stapelung von Links- und Rechtskristallen im Elementarbereich).     

Thermal expansion,heat capacity,and entropy of MgO at mantle pressures

The pressure dependence of the Raman spectrum of forsterite was measured over its entire frequency range to over 200 kbar. The shifts of the Raman modes were used to calculate the pressure dependence of the heat capacity, C _v, and entropy, S, by using statistical thermodynamics of the lattice vibrations. Using the pressure dependence of C _v and other previously measured thermodynamic parameters, the thermal expansion coefficient, , at room temperature was calculated from = K _S (T/P) _S C _V/TVK _T, which yields a constant value of ( ln / ln V)_T= 6.1(5) for forsterite to 10% compression. This value is in agreement with ( ln / ln V)_T for a large variety of materials.At 91 kbar, the compression mechanism of the forsterite lattice abruptly changes causing a strong decrease of the pressure derivative of 6 Raman modes accompanied by large reductions in the intensities of all of the modes. This observation is in agreement with single crystal x-ray diffraction studies to 150 kbar and is interpreted as a second order phase transition.     

Johillerit,Na(Mg,Zn)3 Cu(AsO4)3, ein neues Mineral aus Tsumeb,Namibia

Zusammenfassung Die chemische Analyse des neuen Minerals Johillerit mit der Elektronenmikrosonde ergab: Na₂O 5,4, MgO 18,3, ZnO 5,4, CuO 15,8 und As₂O₅ 55,8, Summe 100.7%. Aus diesem Ergebnis wurde die idealisierte Formel Na(Mg, Zn)₃ Cu(AsO₄)₃ abgeleitet. Johillerit ist monoklin mit der RaumgruppeC2/c. Die Gitterkonstanten sind:a=11,870 (3),b=12,755 (3),c=6,770 (2) , =113,42 (2)°,Z=4. Die stärksten Linien des Pulverdiagramms sind: 4,06 (5) (22 ), 3,50 (4) (310), 3,25 (8) (11 ), 2,75 (10) (330, 240), 2,64 (5) (311, 13 , 40 ), 1,952 (4) (13 , 35 ), 1,682 (4) (20 , 460), 1,660 (5) (40 , 71 , 550, 64 ), 1,522 (4) (442, 153, 13 ). Es bestehen enge strukturelle Beziehungen zwischen Johillerit und O'Danielit, Na(Zn, Mg)₃H₂(AsO₄)₃, sowie einigen synthetischen. Verbindungen.Johillerit ist violett durchscheinend. Die Spaltbarkeit nach {010} ist ausgezeichnet und nach {100} und {001} gut.H (Mohs)3.D=4,15 undD _X =4,21 g·cm^–3. Das Mineral ist optisch zweiachsig positiv, 2V80 (5)°. Die Werte der Lichtbrechung sindn =1,715 (4),n =1,743 (4) undn =1,783 (4). Die Auslöschung istn b und auf (010)n c16°. Johillerit ist stark pleochroitisch mit den AchsenfarbenX=violett-rot,Y = blauviolett undZ = grünblau. Das neue Mineral kommt in radialstrahligen Massen gemeinsam mit kupferhaltigem Adamin und Konichalcit in zersetzem Kupfererz von Tsumeb, Namibia, vor. Die Benennung erfolgte nach Prof. Dr.J.-E. Hiller (1911–1972).

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Johillerite, Na(Mg, Zn) ₃ Cu(AsO ₄ ) ₃ , a new mineral from Tsumeb, Namibia

Summary Electron microprobe analysis of the new mineral johillerite gave Na₂O 5.4, MgO 18.3, ZnO 5.4, CuO 15.8, and As₂O₅ 55.8, total 100.7%. From this result, the ideal formula is given as Na(Mg, Zn)₃ Cu(AsO₄)₃. Johillerite crystallizes monoclinic,C2/c. The unit cell dimensions are:a=11.870(3),b=12.755 (3),c=6.770 (2) , =113.42 (2)°,Z=4. The strongest lines on the X-ray powder diffraction pattern are: 4,06 (5) (22 ), 3,50 (4) (310), 3,25 (8) (11 ), 2,75 (10) (330, 240), 2,64 (5) (311, 13 , 40 ), 1,952 (4) (13 , 35 ), 1,682 (4) (20 , 460), 1,660 (5) (40 , 71 , 550, 64 ), 1,522 (4) (442, 153, 13 ). There is a close relationship between johillerite, o'danielite, Na(Zn, Mg)₃H₂(AsO₄)₃, and some synthetic compounds. Johillerite is violet in colour, transparent. Cleavage is {010} perfect, {100} and {001} good.H (Mohs)3.D=4.15 andD _X =4.21 g·cm^–3. The mineral is optically biaxial positive, 2V80 (5)°. The refractive indices are:n =1.715 (4),n =1.743 (4),n =1.783 (4). The extinction isn b and on (010)n c16°. Strongly pleochroic with axial coloursX=violet-red,Y=bluish violet andZ=greenish blue. The new mineral was found in radiated masses together with cuprian adamite and conichalcite in an oxidized copper ore from Tsumeb, Namibia. It is named in honour of Prof. Dr.J.-E. Hiller (1911–1972).

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An internal variable model for the creep of rocksalt

Summary The creep strain rate of rocksalt, like that of other ductile crystalline materials, can be described by a power law equation of the type ( ) ⁿ , where the active stress is the difference between the total deviatoric applied stress and an internal stress _i . In this paper, the origin and the nature of this internal stress, which develops during inelastic deformation of the material, are discussed. It is shown that this internal stress can serve as an internal (or state) variable in the constitutive model of rocksalt, which reflects the microstructure evolution of the material under the competitive action of hardening and recovery mechanisms.An analysis of experimental data, both our own and those taken from the literature, demonstrates that such a law is able to correctly reproduce rocksalt creep test results in the steady-state domain. The proposed model is in accordance with the macroscopic and microscopic behavior of salt, and with direct measurements of the internal stresses made by others on this material.     

First occurrence of the rare mineral Barylite in Greenland

Summary Barylite has been identified for the first time in Greenland in several specimens from the nepheline syenite pegmatite pockets at Narssârssuk. The crystals are orthorhombic, showing the forms {100}, {210} and {201}, and form thin plates parallel to {100}. Electron microprobe analysis and emission spectrography show a composition close to the ideal formula. The refined unit cell paramters are:a=9.835(2) Å,b=11.654(3) Å andc=4.673(1) Å. The barylite is biaxial negative, 2V =66^o±2^o,n =1.694,n = 1.697 andn _calc.=1.698.

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Das erste grönländische Vorkommen des seltenen Minerales Barylith

Zusammenfassung Der seltene Barylith wurde zum ersten Mal in Grönland in verschiedenen Proben aus Nephelin-Syenit-Pegmatit-Drusen bei Narssârssuk identifiziert. Die Kristalle sind orthorhombisch mit den Formen {100}, {210} und {201}, sie bilden dünne Tafeln nach {100}. Elektronensonden-Analyse und Emissions-Spektrographie ergeben eine chemische Zussammensetzung, die der Idealform nahekommt. Die verfeinerten Zellparameter sind folgende:a=9,835(2) Å,b=11,654(3) Å undc=4,673(1) Å. Barylith ist zweiachsig negativ, 2V =66^o±2^o,n =1,694,n =1.697 undn _calc.= 1,698.

     

Pressure and temperature dependence of the elastic properties of synthetic MnO

The adiabatic elastic stiffness constants of synthetic single-crystal MnO were measured in this study using pulse superposition interferometry. Data were obtained up to 1.0 GPa in pressure and over the temperature range 273 to 473 K. As a result, we were able to determine the complete set of second-order stiffness moduli (C _ij ^s ) and their pressure and temperature derivatives, as well as higher-order properties for selected modes. Relevant results for the adiabatic bulk modulus are: K ^s=155.1±0.8 GPa; (K^s/P)_T=4.70±0.13; and, (K ^s/T)_P= -0.0203±0.0009 GPa/K. Our results for the second-order moduli are generally consistent with the data from previous studies. However, relative to the estimated uncertainties, small and systematic discrepancies appear to characterize the data set. The available evidence indicates that the differences result from microstructural variations (in particular, microcracks and Mn₃O₄ inclusions) between the synthetic MnO specimens used in different investigations. The pure shear mode C ₄₄ exhibits anomalous soft-mode behavior with both temperature (the ambient derivative is positive) and pressure (the ambient derivative is negative). In both cases the C ₄₄ data trends appear to primarily reflect the influence of Mn-Mn magnetoelastic interactions associated with the onset of a paramagnetic-antiferromagnetic (PM AFM) phase transition.     

Röntgenuntersuchung der wasserfreien Sulfate der dreiwertigen Metalle Eisen,Chrom und Gallium

Zusammenfassung Wasserfreies Eisen(III)sulfat, Fe₂(SO₄)₃, existiert in zwei Modifikationen. Nach Einkristallaufnahmen kristallisiert die eine wahrscheinlich in Raumgruppe (a_rh=8,791±0,004 Å, =55°52±2; Z=2), die andere in Raumgruppe C _2h ⁵ –P2₁/n (a=8,296±0,002 Å, b=8,515±0,002 Å, c=11,60±0,002 Å, =90°30; Z=4). Von Cr₂(SO₄)₃ und Ga₂(SO₄)₃ konnten für die rhomboedrischen Modifikationen, die isotyp zu jener von Fe₂(SO₄)₃ sind, aus Pulveraufnahmen die Gitterkonstanten bestimmt werden.

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Summary Anhydrous iron(III)sulfate exists in two modifications. From single-crystal work, one of the modifications crystallizes probably in space group (a_rh=8.791±0.004 Å, =55°52±2; Z=2), whereas the other in space group C _2h ⁵ –P2₁/n (a=8.296±0.002 Å, b=8.515±0.002 Å, c=11.60±0,002 Å, =90°30; Z=4). The lattice constants of the rhombohedral modifications of Cr₂(SO₄)₃ and Ga₂(SO₄)₃, which are isomorphous with rhombohedral Fe₂(SO₄)₃, have been determined from powder photographs.

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Herrn Professor Dr.F. Machatschki zum 70. Geburtstag gewidmet.     

Stable isotope geochemistry and phase equilibria of coesite-bearing whiteschists,Dora Maira Massif,western Alps

Peak metamorphic temperatures for the coesite-pyrope-bearing whiteschists from the Dora Maira Massif, western Alps were determined with oxygen isotope thermometry. The ¹⁸O(smow) values of the quartz (after coesite) (¹⁸O=8.1 to 8.6, n=6), phengite (6.2 to 6.4, n=3), kyanite (6.1, n=2), garnet (5.5 to 5.8, n=9), ellenbergerite (6.3, n=1) and rutile (3.3 to 3.6, n=3) reflect isotopic equilibrium. Temperature estimates based on quartz-garnet-rutile fractionation are 700–750 °C. Minimum pressures are 31–32 kb based on the pressure-sensitive reaction pyrope + coesite = kyanite + enstatite. In order to stabilize pyrope and coesite by the temperature-sensitive dehydration reaction talc+kyanite=pyrope+coesite+H₂O, the a(H₂O) must be reduced to 0.4–0.75 at 700–750 °C. The reduced a(H₂O) cannot be due to dilution by CO₂, as pyrope is not stable at X(CO₂)>0.02 (T=750 °C; P=30 kb). In the absence of a more exotic fluid diluent (e.g. CH₄ or N₂), a melt phase is required. Granite solidus temperatures are 680 °C/30 kb at a(H₂O)=1.0 and are calculated to be 70°C higher at a(H₂O)=0.7, consistent with this hypothesis. Kyanite-jadeite-quartz bands may represent a relict melt phase. Peak P-T-f(H₂O) estimates for the whiteschist are 34±2 kb, 700–750 °C and 0.4–0.75. The oxygen isotope fractionation between quartz (¹⁸O=11.6) and garnet (¹⁸O=8.7) in the surrounding orthognesiss is identical to that in the coesitebearing unit, suggesting that the two units shared a common, final metamorphic history. Hydrogen isotope measurements were made on primary talc and phengite (D(_SMOW)=-27 to-32), on secondary talc and chlorite rite after pyrope (D=-39 to -44) and on the surrounding biotite (D=-64) and phengite (D=-44) gneiss. All phases appear to be in nearequilibrium. The very high D values for the primary hydrous phases is consistent with an initial oceanicderived/connate fluid source. The fluid source for the retrograde talc+chlorite after pyrope may be fluids evolved locally during retrograde melt crystallization. The similar D, but dissimilar ¹⁸O values of the coesite bearing whiteschists and hosting orthogneiss suggest that the two were in hydrogen isotope equilibrium, but not oxygen isotope equilibrium. The unusual hydrogen and oxygen isotope compositions of the coesite-bearing unit can be explained as the result of metasomatism from slab-derived fluids at depth.     

Rooseveltit von San Francisco de los Andes und Cerro Negro de la Aguadita,San Juan,Argentinien

Zusammenfassung Rooseveltit findet sich in der Oxidationszone der Lagerstätten San Francisco de los Andes und Cerro Negro de la Aguadita, in der Provinz San Juan, Argentinien, auf 30°22 S und 69°33 W. Er bildet sehr feinkörnige, weiß-graue, nach Bismuthinit pseudomorphe Aggregate. Die Brechungsindizes liegen zwischenn=2,10 und 2,30. Die Vickershärte beträgt 513 (4–5 der Mohs'schen Härteskala). Mittels Elektronenmikrosonde wurde folgende chemische Zusammensetzung bestimmt: As=21,5±1%, Bi=60,9±2%. Rooseveltit ist monoklin mita ₀=6,878(1)Å, b₀=7,163(1) Å, c₀=6,735(1) Å, =104° 46±1, Z=4, _calc.=6,94 g·cm^–3, RaumgruppeP 2₁/n.Rooseveltit wurde nach drei verschiedenen Methoden synthetisiert. Die Pulverdiagramme der synthetischen Produkte stimmen mit dem des Minerals überein. Die Brechungsindizes wurden mitn =2,13(2) bzw. n=2,25(2) und die Dichte mit _obs.=7,01 g·cm^–3 bestimmt. Zellparameter: a₀-6,882(1) Å, b₀=7,164(1) Å, c₀=6,734(1) Å, =104° 50,5±0,7, _calc.=6,94 g·cm^–3. Das synthetische Material schmilzt um 950°C. Selbst nach mehrstündigem Erhitzen auf 920°C läßt sich keine Veränderung im Pulverdiagramm des Minerals festellen.Es wird versucht, die natürliche Bildung des Rooseveltits zu erklären.

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Rooseveltite from San Francisco de los Andes and Cerro Negro de la Aguadita, San Juan, Argentina

Summary Rooseveltite occurs in the weathering zone of the San Francisco de los Andes and Cerro Negro de la Aguadita mines, located in the San Juan Province, Argentina, at 30° 22S and 69° 33W. It appears in grey, finegrained aggregates pseudomorph after bismuthinite. Refraction index ranges fromn=2.10 to 2.30. The Vickers microhardness is 513 (4–5 of Mohs' scale). Chemical composition from electron micro probe measurements is As 21.5±1% and Bi 60.9±2%. Rooseveltite is monoclinic, with a₀=6.878(1) Å, b₀=7.163(1) Å, c₀=6.735(1) Å, =104° 46±1, Z=4, _calc.=6,94 g·cm^–3, space groupP 2₁/n.The synthetic compound was prepared by three different methods. The powder pattern are the same as that of the mineral. Refraction index n=2.13(2) and n=2.25(2). The measured specific gravity is _pobs.=7,01 g·cm^–3. Cell parameters: a₀=6.882(1) Å, b₀=7.164(1) Å,c ₀=6.734(1) Å, =104° 50.5±0.7, _calc.=6,94 g·cm^–3. The synthetic material melts at about 950°C. After heating to 920°C no variations were observed in the powder diagram of the mineral.It is tried to explain the formation of rooseveltite in natural environment.

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The global Frasnian-Famennian »Kellwasser Event«

During the UppermostP. gigas conodont zone (P. linguiformis zone) the stepwise Frasnian extinction culminates in the global transgressive-regressive Frasnian-Famennian »Kellwasser event«. Most pelagic organisms of the tropical marine environment are more or less affected by this mass extinction. During the early Upper Devonian extinction rates are clearly related to the deposition of black shales and the storage of a great amount of organic carbon in the (shelf) sediments. Contemporaneous positive excursions of the ¹³C recorded from pelagic limestones of Europe and Australia prove a worldwide change in the CO₂ content of the oceans surface water which is in balance with the atmosphere. If the climate is controlled by the CO₂ content of the atmosphere, a cyclic operation process might have developed with the succession: transgression — high organic production — deposition of organic carbon — lowering of CO₂ in the atmosphere — icehouse effect — polar glaciation — regression — erosion of organic carbon and re-supply of CO₂ to the ocean and atmosphere — greenhouse effect — deglaciation — transgression. During the change from a greenhouse to an icehouse effect an overturn of the anoxic oceans probably was one of the causes for the mass killing of low latitude marine organisms.

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Zusammenfassung Während der OberstenP. gigas-Zone (=P. linguiformis-Zone) kulminiert das stufenweise Erlöschen der Frasne Organismen im globalen transgressiv-regressiven »Kellwasser-Ereignis«, das die Frasne-Famenne-Grenze markiert. Die meisten Organismengruppen der tropischen Meere werden mehr oder weniger von dem Massensterben betroffen. Während des tiefen Oberdevons sind die Aussterberaten offensichtlich mit der Ablagerung von Schwarzschiefern verknüpft, in denen eine große Menge organischen Kohlenstoffs auf den Schelfen fixiert wird. Gleichzeitige positive Exkursionen der ¹³C-Kurven, die in pelagischen Kalken aus Europa und Australien gemessen wurden, deuten auf eine weltweite Änderung der CO₂-Gehalte des ozeanischen Oberflächenwassers und der Atmosphäre hin.Sollte das Klima vom CO₂ in der Atmosphäre gesteuert werden, könnte sich folgender Autozyklus entwickelt haben: Transgression — hohe organische Produktion — Ablagerung von organischem Kohlenstoff — Erniedrigung des CO₂-Gehaltes in der Atmosphäre — Abkühlung — polare Vereisung-Regression — Erosion und Oxidation von organischem Kohlenstoff — Erhöhung des CO₂-Gehaltes in der Atmosphäre — Treibhauseffekt — Abschmelzen der polaren Vereisung — Transgression. Aufsteigende anoxische Wässer beim Übergang vom Treibhaus- zum »Kühlhauseffekt« dürften eine der Ursachen für das Massensterben tropischer mariner Organismen gewesen sein.

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Résumé Au cours de la partie supérieure de la zone de conodontes àP. gigas (= zone àP. linguiformis), l'extinction progressive des organismes frasniens a culminé au moment de l'événement de Kellwasser, marqué par une transgression-régression globale à la limite Frasnien-Famennien. La plupart des organismes pélagiques des milieux marins tropicaux ont été affectés à des degrés divers par cette extinction de masse. Pendant le début du Dévonien supérieur, les taux d'extinction sont clairement en relation avec le dépôt de boues noires et le stockage d'une grande quantité de matière organique dans les sédiments de shelf. Simultanément, des variations positives de ¹³C enregistrées dans des calcaires pélagiques d'Europe et d'Australie indiquent un changement modial de la teneur en CO₂ de l'eau de surface des océans et de l'atmosphère.Si on admet que le climat est régi par la teneur en CO₂ de l'atmosphère, le processus cyclique suivant a pu se développer: transgression — forte production organique — dépôt de carbone organique — baisse de la teneur en CO₂ de l'atmosphère — refroidissement — glaciation polaire — régression — érosion et oxydation du carbone organique — élévation de la teneur en CO₂ de l'atmosphère — effet de serre — fusion des glaces polaires — transgression. Lors du passage du stade de l'effet de serre à celui du refroidissement, une remontée des eaux anoxiques océaniques peut être à l'origine de l'extinction de masse des organismes marins tropicaux.

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Structural evidence of orogen-parallel strike slip displacements in the Precordillera of northern Chile

The Chilean Precordillera, situated between the Longitudinal Valley and the Western Cordillera of northern Chile, is made up of several elongate basement ridges following the trend of the Andes. These ridges, which morphologically rise above the Mesozoic-Tertiary cover rocks, are developed as anticlinal cores or as pressure ridges bounded by reverse faults and, thus, show considerable orogennormal shortening. Several nearly vertical faults cut through the Precordillera, parallel or at a very low angle to the mountain ranges. From the following structures it can be inferred that orogen-parallel transcurrent movements took place along the faults: (a) asymmetric en echelon fault arrays, (b) stratigraphic and structural discontinuities across major faults, (c) fabrics in fault rocks, and (d) vertical folds. A dextral displacement of the order of tens of km is probable. The orogen-parallel strikeslip movements as well as the orogen-normal shortening are considered as phenomena of magmatic arc tectonics due to the focussing of central Andean igneous activity on the Precordillera from the Late Cretaceous until the paroxysm of deformation (45-30 Ma). Deformation along the Precordilleran Fault System is related with the development of the large porphyry copper ore deposits of that area. The structural evolution of the Precordillera can be explained by oblique subduction resulting in dextral transpression.

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Zusammenfassung Die nordchilenische Präkordillere, zwischen Längstal und Westkordillere gelegen, ist aus mehreren langgestreckten prämesozoischen Grundgebirgsrücken aufgebaut, die sich als Antiklinalkerne oder als aus diesen hervorgegangene, durch Aufschiebungen begrenzte Rücken über das mesozoisch-tertiäre Deckgebirge erheben. Das Gebiet ist durchzogen von einem System steiler, parallel oder in spitzem Winkel zu diesen Einengungsstrukturen verlaufenden Störungen. Folgende Strukturen zeigen, daß es an diesen Störungen Seitenverschiebungen gegeben hat: (a) asymmetrische en-echelon-Störungsanordnungen, (b) stratigraphische und strukturelle Diskontinuitäten an Störungen, (c) Gefüge der Störungsgesteine und (d) Schlingen. Ein dextraler Versatz von mehreren Zehnern von km ist wahrscheinlich. Orogenparallele Seitenverschiebungen und Orogen-normale Einengung können als Phänomene der Magmatic-Arc-Tektonik gesehen werden, da von der höchsten Oberkreide bis zum Höhepunkt der Deformation (45-30 Ma) die magmatische Aktivität der Anden in der Präkordillere lag. In diesem Zusammenhang stehen auch die Vererzungen der großen porphyry-copper-Lagerstätten des Gebietes. Schiefe, zu dextraler Transpression führende Subduktion wird für die Strukturbildung der Präkordillere verantwortlich gemacht.

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Resumen La Precordillera del norte chileno, situada entre el Valle Longitudinal y la Cordillera Occidental, esta constituida de uno o más dorsales de basamento, los que forman núcleos de grandes anticlinales o pilares en compresión que se elevan morfologicamente sobre las rocas mesozóicas y terciarias de su cobertura. Varias fallas más o menos verticales cortan a la Precordillera paralelamente o con angulo agudo a su rumbo. Las siguientes estructuras permiten deducir que se produjeron movimientos transcurrientes paralelos al orogeno en las fallas: (a) juegos asimétricos escalonados de fallas, (b) discontinuidades estratigráficas y estructurales ligados a la falla, (c) fábricas de las rocas de falla y (d) pliegues con ejes verticales. Un desplazamiento dextral de decinas de kilométras es probable. Las fallas de desplazamiento en el rumbo paralelas al orógeno asi como el acortamiento normal al orógeno se consideran como fenomenos de la tectónica de arco magmático, puesto que desde el Cretácico superior hasta el tiempo de la deformatión (45-30 Ma), la actividad magmática de los Andes Centrales estuvo ubicada en la Precordillera. La deformación en el sistema de fallas de la Precordillera está tambien relacionada con la mineralizatión de los grandes yacimientos de pórfidos cupriferos de la zona. La evolutión tectónica de la Precordillera puede haber sido originada por subductión oblicua produciendo transpresión dextral.

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Die Verbindung Cu2FeSnSe3,8 im Vergleich zum Zinnkies

Zusammenfassung Die Phase Cu₂FeSnSe_3,8 ist tetragonal, Raumgruppe mit ₀=5.69 undc ₀=11,26 Å.Z=2,D _x=5,441. Synthesetemperatur: 410°C.

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The compound Cu₂FeSnSe_3,8 in comparison to stannite

Summary The phase Cu₂FeSnSe_3,8 is tetragonal, space-group with ₀=5.69 andc ₀=11.26 Å.Z=2,D _x=5.441. Temperature of synthesis: 410°C.