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     

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).     

The system: Two alkali feldspars-KCl-NaCl-H2O at moderate to high temperatures and low pressures

Because of frequent discrepancies between the available experimental data and the measured composition of alkali chloride aqueous solutions coexisting with two alkali feldspars in high temperatures-low pressures natural systems, a systematic investigation of the system KAlSi₃O₈-NaAlSi₃O₈-KCl-NaCl-H₂O has been undertaken.Experiments have been carried out at temperatures from 300 °C to 660 °C, pressures from 0.2 to 2 kbar and total chloride concentrations ranging from 0.05 to 14 moles/kg H₂O.No effect of pressure on the feldspars solvus could be detected. Smoothing the experimental data on the basis of the regular assymetric solid solution model yields a critical temperature of 661°C and a critical composition of Or_0.36Ab_0.64.The equilibrium constant C = m _KCl/m _NaCl does not depend on total chloride molality, as long as the aqueous solution is homogeneous. But, in the miscibility gap (liquid+vapour) of the fluid, C is always lower in the vapour than in the liquid. The higher the temperature and the lower the pressure, the more striking this effect. For instance, at 500 ° C C _vaqour/C _liquid = 1 above 1 kb, 0.9 at 600 bars, 0.8 at 500 bars, 0.7 at 400–450 bars.The effect of pressure can be neglected in homogeneous fluids and in the liquid phase of unmixed fluids, but it is very important in the vapour phase (dilute solutions at low pressure).The selected values of C _max are (±0.01) 300 ° C0.083; 400 ° C0.139; 500 ° C0.200; 600 ° C0.264; 650 ° C0.298Such a behaviour of the fluid at low pressures explains the abnormally low values of m _KCl/m _NaCl measured in many natural hydrothermal systems. A new mechanism of alkali metasomatism (especially potassic alterations) is also proposed, taking into account the unmixing of alkali chloride aqueous solutions. This model seems particularly interesting in late magmatic hydrothermal processes, such as those occuring in porphyry type deposits.     

Hochtemperatur-Plagioklas im Meteorit von Oldenburg

Zusammenfassung Der Plagioklas im Meteorit von Oldenburg (10. 9. 1930) zeigt Hochtemperaturoptik, die damit auch für weitere Chondrite wahrscheinlich ist. Zusammensetzung ist An₁₁, 2V = 60 1/2°, = 1,5365, Auslöschungsschiefe 7 1/2°, Winkel zwischen und der 0-Stelle der Auslöschung 24°, Winkel zwischen [001] und = 20°. Anteil 6%, Verzwillingung sehr eng nach dem Albit- und Periklingesetz. Die Indikatrix wird mit den bisherigen Werten für die Hochtemperaturoptik verglichen.     

Eine Differenzmethode zur Erhöhung der Meßgenauigkeit und Erweiterung des Meßbereiches normaler Drehkompensatoren

Zusammenfassung Eine Differenzmethode zur Erhöhung den Genauigkeit der Messung von optischen Gangunterschieden durch Drehkompensatoren und Erweiterung von deren Mcßbereich wind beschrieben und durch Beispiele geprüft und erläutert.Der Vorteil dieses Verfahrens liegt in der erreichbaren Genauigkeit der Bestimmung kleiner und großer , die meist haher liegt als die der direkten Ermittelung durch Kompensation auf . = 0.Bei kleinen und großen , kann durch Messung einer Summe von Gangunterschieden oder eines Restgangunterschiedes anstatt des im Meßpräparat vorliegenden die Messung stets in den besten Meßbereich des jeweils vorliegenden Kompensators gerückt werden. Darüberhinaus sind höhere, durch Kompensatoren mit kleinerem Meßbereich ebenso genau bzw. nosh genauer bestimmbar. Es muß nun durch einen Quarzkeil oder einen underen Kompensator die ungefähre Lage der Kompensationsstelle auf = 0 ermittelt werden.Unter Verwendung monoehromatischen Lichtes können auch in der Additionsstellung von Meßpraparat und Kompensator ebensogut und genau bestimmt werden wie in der bisher traditionell bevorzugten Subtraktionsstellung.     

Thermal expansion of Neapolitan Yellow Tuff

Summary In saturated rocks and soils it is possible to define different coefficients of thermal expansion depending on the drainage conditions. This topic is first examined from the theoretical point of view with regard to an ideal isotropic thermo-elastic porous medium. Some special features of the behaviour of natural soils and rocks during thermal expansion tests are subsequently discussed. An experimental evaluation of some of these coefficients is presented in the second part of the paper. The material investigated is a pyroclastic rock, the so-called Neapolitan Yellow Tuff. Thermal expansion coefficient in drairend conditions has been evaluated, when this material is saturated with water. The e pressure increase induced by heating has been measured in undrained tes temperatures investigated range between room temperature up to 225°C.Different types of apparatus have been used and, when possible, a comparison between the results has been proposed. The results obtained in undrained thermal expansion tests are in agreement with theoretical predictions. This research is part of an on-going study of the complex phenomena known as Bradyseism, which is occurring in a volcanic area a few kilometers from Naples (Italy). Some considerations on this phenomenon are drawn in the last paragraph of the paper.List of Symbols _l linear thermal expansion coefficient - _s volumetric thermal expansion coefficient of the solid phase - _ss volumetric drained thermal expansion coefficient of the solid skeleton - _st volumetric drained thermal expansion coefficient that takes into account variations of porosity induced by heating - _t volumetric drained thermal expansion coefficient of the whole system - _u volumetric undrained thermal expansion coefficient of the whole system - _v volumetric thermal expansion coefficient - _w volumetric thermal expansion coefficient of the water - _l ^* volumetric thermal expansion coefficient of the distilled water - Biot's coefficient - _d dry specific gravity - _s specific gravity of the solid phase - _r ^' uniaxial compressive strength - _r ^' yield stress in isotropic compression - B _T parameter relating undrained thermally generated pore pressure with temperature change - d ₀ maximum uplift during Bradyseismic crises - d _f residual uplift during Bradyseismic crises - K _s compressibility modulus of the solid phase - K _ss compressibility modulus of the solid skeleton - K _w compressibility modulus of the water - n porosity - T temperature - T ₀ initial temperature in thermal expansion, tests performed in thermal expansion cell - T _m temperature at which the volume of waterV _Eexpelled from thermal expansion cell. is measured - V volume of the solid skeleton - V _E volume expelled from thermal expansion cell - V ₀ volume of the expansion cell at temperatureT ₀ - V _s volume of the solid phase     

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.

     

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.     

Jankovićite,Tl5Sb9(As,Sb)4S22, a new TI-sulfosalt from Allchar,Macedonia

Summary A hitherto unknown TI-sulfosalt, containing antimony and arsenic, was discovered by ore microscopy (including quantitative reflectance measurements) and electron microprobe analysis in samples from the Allchar TI-Sb-As sulfide ore deposit (Macedonia). The chemical formula is TI₅Sb₉(As,Sb)₄S₂₂, X-ray single crystal and powder diffration data gave the lattice parameters _o = 7.393(4) Å, b₀ = 8.707(2) Å, c_o = 17.584(3) Å, = 103.81(1)°, = 91.79(1)°, = 109.50(1)°; space group , Z = 1. The new mineral and the new mineral naine jankoviéite have been approved by the IMA/CNMMN.

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Jankoviit, Tl₅Sb₉(As,Sb)₄S₂₂, ein neues TI-Sulfosalz aus Allehar, Macedonien

Zusammenfassung Ein bislang unbekanntes Thallium-Antimon-Arsen-Sulfosalz wurde mittels Erzmikroskopie (inklusive quantitativer Reflexionsmessungen) und Elektronenstrahl-Mikrosonde in Proben aus der TI-Sb-As-Sulfid-Lagerstätte Allchar in Macedonien entdeckt. Die chemische Formel lautet Tl₅Sb₉(As,Sb)₄S₂₂, aus Röntgen-Einkristallaufnahmen und Pulverdiffraktogrammen konnten folgende Gitterkonstanten bestimmt werden: a_o = 7.393(4) Å, b_o = 8.707(2) _Å, c_o = 17.584(3) Å, a = 103.81(1)°, = 91.79(1)°, = 109.50(1)°; Raumgruppe , Z = 1. Das neue Mineral, sowie der neue Mineralname Jankoviit wurden durch die IMA/CNMMN anerkannt.

     

Natural boehmite single crystals from Ceylon

Single crystals of boehmite, up to 0.1 mm in size, were found in open cavities inside a corundum crystal from the Ratnapura area gem gravels in Ceylon. The unit cell parameters are (X-ray powder pattern): a ₀=3.695 Å b ₀=12.212 Å, c ₀=2.867 Å. The crystallographic orientation is based on X-ray single-crystal precession photographs. The crystals show the faces (it010), (001), (101), and (221). Systematic extinctions agree with the space group Amam. Optical orientation: a, b, c. Refractive indices are given. The mineral is optically positive with a large optic axial angle.     

The role of hydrogen bonding in the thermal expansion and dehydration of brushite,di-calcium phosphate dihydrate

The unit-cell and atomic parameters of perdeuterated brushite have been extracted from Rietveld analysis of neutron powder diffraction data within the temperature range 4.2 to 470 K. The thermal expansion of brushite is anisotropic, with the largest expansion along the b axis due principally to the effect of the O(1)···D(4) and O(3)···D(2) hydrogen bonds. Expansion along the c axis, influenced by the Ow1···D(5) interwater hydrogen bond, is also large. The high temperature limits for the expansion coefficients for the unit-cell edges a, b and c are 9.7(5) × 10^–6, 3.82(9) × 10^–5 and 5.54(5) × 10^–5 K^–1, respectively, and for the cell volume it is 9.7(1) × 10^–5 K^–1. The angle displays oscillatory variation, and empirical data analysis results in = 1.28(3) × 10^–6sin(0.0105 T) K^–1, within this temperature range. The evolution of the thermal expansion tensor of brushite has been calculated between 50 T 400 K. At 300 K the magnitudes of the principal axes are ₁₁ = 50(6) × 10^{–6 K–1}, ₂₂ = 26.7(7) × 10^–6 K^–1 and ₃₃ = 7.0(5) × 10^–6 K^–1. The intermediate axis, ₂₂, is parallel to b, and using IRE convention for the tensor orthonormal basis, the axes ₁₁ and ₃₃ have directions equal to (–0.228, 0, –0.974) and (–0.974, 0, 0.228) respectively. Under the conditions of these experiments, the onset of dehydration occurred at temperatures above 400 K. Bond valence analysis combined with assessments of the thermal evolution of the bonding within brushite suggests that dehydration is precipitated through instabilities in the chemical environment of the second water molecule.     

How Cr<Superscript>3+</Superscript> and Fe<Superscript>3+</Superscript> affect Mg–Al order–disorder transformation at high temperature in natural spinels

Three natural Mg(Al_2-yCr_y)O₄ spinels (y 0.07–0.16), highly ordered in terms of Mg–Al, and one Mg(Al_2–yFe³⁺_y)O₄ spinel (y0.08), highly ordered also in terms of Fe³⁺, were studied by means of X-ray single-crystal diffraction. All samples were heated in situ from 25 to 1000 °C in order to follow both thermal expansion and evolution of the structural state of spinel with temperature. Thermal expansion was monitored by means of the variation of cell edge a with temperature, and found to be well represented throughout the temperature range by a regression line a = a₀ (1+T), slightly different at lower and higher temperatures. Thermal expansion coefficient ₁, referring to the lower temperature range (i.e. during pure thermal expansion), was slightly lower than ₂, calculated only over the highest temperatures. The trend showed different slopes for individual crystals. Structural evolution with temperature was studied by means of the variation of oxygen positional parameter u, which is strongly influenced by intersite cation exchange and thus closely correlated with inversion parameter x. In particular, in the three Cr samples, in which Cr resides only in the octahedral site, u parameter variations and hence the order–disorder process, started at about 700 °C. Instead, in the Fe³⁺ sample, this process was triggered at lower temperatures, starting at 550 °C with Fe³⁺–Mg exchange followed at higher temperatures by that of Mg–Al. Cr contents in the Cr samples affected the occupancy of Al in the tetrahedral site at the highest temperatures. In both Mg–Al–Cr and Mg–Al–Fe³⁺ compositions, if CrFe³⁺, parameter u reached the same value only when the Mg–Al exchange was dominant, i.e. at the highest temperatures, but not before. Cation distribution at each temperature was obtained by the bond-length model, applying thermal expansion to pure bond lengths. This method is applied here to complex compositions for the first time.     

A reflected light investigation of ilvaite

Summary The reflectance of oriented crystal faces parallel (100) and (001) of ilvaite was measured in air and in oil at different wavelengths with linearly polarized light. Refractive indices and absorption constants were calculated from the reflectance values. In contrast ton andn ,n has a strong dispersion. For the calculation ofn the absorption constant can be neglected. According to the unit cell ofBelov andMokeeva (1954) with the lattice constantsa ₀=8.82,b ₀=13.07,c ₀=5.86 Å,n vibrates parallel to [001]n parallel to [100] andn parallel to [010]. Ilvaite is optically negative.

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Auflichtuntersuchungen zur Optik des Ilvaits

Zusammenfassung Auf orientiert geschliffenen (100) und (001) Kristallplatten von Ilvait wurde das Reflexionsvermögen in Luft und in Öl mit linear polarisiertem Licht bei verschiedenen Wellenlängen gemessen. Aus den Reflexionswerten wurden die Brechungsindices und Absorptionskonstanten berechnet.n zeigt im Gegensatz zun undn eine auffallend starke Dispersion. Für die Berechnung vonn kann die Absorptionskonstante vernachlässigt werden. Nach der Aufstellung der Elementarzelle vonBelov undMokeeva (1954) mit den Gitterkonstantena ₀=8,82,b ₀=13,07,c ₀=5,86 Å schwingtn parallel [001],n parallel [100] undn parallel [010]. Der Ilvait ist optisch negativ.

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With 2 Figures     

Application of Gibbs Energy Minimization to ModelEarly-Diagenetic Solid-Solution Aqueous-Solution EquilibriaInvolving Authigenic Rhodochrosites in Anoxic BalticSea Sediments

The natural early-diagenetic environment ``anoxicporewater – authigenic mineral phases' has beencharacterized in sediment of the Gotland Deep,Baltic Sea, by a closed-system model. Occurrence ofcarbonate precipitates as thin almost pure whitelaminae was considered as a natural experiment forlong-term equilibration between these phases andporewater. Plots of distribution coefficientsindicate that metastable equilibrium exists betweenporewater and the authigenic Ca-rich rhodochrositephases below 7 cm depth. A thermodynamic model ofporewater geochemistry at in situ P = 25 barand T = 5 °C was developed using the Gibbsenergy minimization (GEM) approach. The values of isobaric-isothermal potentials of Mn, Ca, Fe, Mg,Sr, Ba, C, and O, calculated from the porewatercomposition, were used in a new ``dual thermodynamic'calculation approach to estimate solid activitycoefficients of the end-members in the non-idealsolid solution (Mn, Ca, Mg, Sr, Ba, Fe)CO₃,i.e., at full major and minor multi-componentcomplexity. The regular Margules interactionparameters for the composing binaries estimated bythis model were _Mn-Ca = 1.9 ± 0.5,_Mn-Mg = 0.6,_Ca-Mg = 3.7,_Mn-Fe = 0.2,_Ca-Fe = 2.8,_Mn-Sr = 9.7,_Ca-Sr = 2.15,_Mn-Ba = 4.0,_Ca-Ba = 1.4,validating the theoretical predictions given byLippmann in his pioneering 1980's paper. Thestrictly thermodynamic equilibrium model is not onlyable to match both the measured porewater andcarbonate solid-solution composition, but also topredict that the porewater pH, pe, alkalinity, anddissolved Mn, Fe, and S concentrations arecontrolled by the authigenic mineral bufferingassemblage mackinawite-greigite-rhodochrosite. Ourmodel is only compatible with the idea of ACRformation with typical composition (X_Mnbetween 70–75%) in the topmost sediment layerwhich, however, needs a major source ofMn_aq ^II. This is provided by reduction ofparticulate Mn oxides precipitated in significantamounts in the water column upon major inflow eventsin the Baltic Sea. The model enables also to set upscenarios of changing environmental conditions, e.g.,to predict the non-linear response of the carbonatesolid-solution composition to changes in Mn loading,alkalinity and salinity of the sediment-watersystem. The results suggest that the major andespecially minor element contents (Sr, Mg, Ba) inauthigenic carbonates can be applied as anenvironmental paleoproxy.     

Der Strukturtyp von Emmonsit, {Fe2[TeO3]3·H2O}·xH2O (x=0−1)

Zusammenfassung Emmonsit kristallisiert triklin, RaumgruppeP , Gitterkonstanten:a ₀=7,90 Å,b ₀=8,00 Å,c ₀=7,62 Å, =96^o44, =95^o 0, =84^o 28,Z=2. Der Strukturtyp wurde aus 3-dimensionalen photographischen Röntgendaten ermittelt. Die Eisenatome werden je von 6 Sauerstoffen verzerrt oktaedrisch koordiniert. Jedes Telluratom wird von 3 Sauerstoffen in einem Abstand <2,0 Å umgeben. Ein vierter Sauerstoff hat bezüglich dieser drei einen um etwa 25–35% größeren Abstand, so daß jedes Telluratom im weiteren Sinne eine (3+1)-Koordination aufweist.

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The structure type of emmonsite, {Fe₂[TeO₃]₃·H₂O}·xxH₂O (x=0–1)

Summary Emmonsite is triclinic with space groupP , and lattice constantsa ₀=7.90 Å,b ₀=8.00 Å,c ₀=7.62 Å, =96^o 44, =95^o 0, =84⁰ 28,Z=2. The structure type is derived from 3-dimensional photographic X-ray data. The iron atoms are coordinated by six oxygens in the form of a distorted octahedron. Each tellurium atom is coordinated to 3 oxygens at a distance <2.0 Å. Compared with these 3 Te–O distance the distance of a fourth oxygen is only 25 to 35% greater; therefore each tellurium atom has a (3+1)-coordination of oxygens.

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Mit 2 Abbildungen     

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.     

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.     

Die Struktur des Tief-K2Li[AIF6] und ihre Beziehung zu Elpasolith,K2Na[AlF6] und anderen Strukturen

Zusammenfassung Die Struktur von Kristallen der Tieftemperaturmodifikation des K₂Li[AlF₆] wurden untersucht. Es ergaben sich folgende Daten: Kristallklasse –3m–D _3d: ditrigonal-skalenoedrisch; rhomboedrisches Translationsgitter; RaumgruppeR –3m–D _3d (5). Hexagonale Aufstellung:a ₀=5,574 Åc ₀=13,648 Å ;c/a=2,4485. Inhalt der Zelle 3 · K₂LiAlF₆. Rhomboedrische Aufstellung: _R = 5,573 Å;=60° 01; Inhalt dieser Zelle 1 · K₂LiAlF₆;D _x = 3,066. Die vollständige Struktur wird mitgeteilt und mit der sehr ähnlichen Struktur des kubischen Minerals Elpasolith, K₂Na[AIF₆], verglichen; es besteht zwischen beiden Strukturen Homöotypie, Außerdem werden die optischen Daten und die Spaltbarkeit dieser beiden Kristallarten miteinander verglichen und die Unterschiede qualitativ auf Grund der Kristallstrukturen erklärt. Weitere Homöotypiebeziehungen werden aufgezeigt.     

Über Strontioginorit,eine neue Ginorit-Varietdt aus dem Zechsteinsalz

Zusammenfassung Strontioginorit (Sr, Ca)₂B₁₄O₂₃ 8 H₂O mit Sr: Ca1,3:0,7 tritt in gut ausgebildeten Kristallen im Alteren Steinsalz von Reyershausen bei Göttingen auf. Es ergaben sich folgende Daten : ₀ = 12,85₀ Å,b ₀ = 14,48 Å,c ₀, = 12,85₅, Å, = 101° 35, RaumgruppeP 2_I/a,Z = 4, Dichte=2,25 gcm^–3,n = 1,512,n _/b = 1,524,n //[101] = 1,577.     

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.