Uranium(6+) sorption on montmorillonite: Experimental and surface complexation modeling study

Sorption interactions with montmorillonite and other clay minerals in soils, sediments, and rocks are potentially important mechanisms for attenuating the mobility of U(6+) and other radionuclides through the subsurface environment. Batch experiments were conducted (in equilibrium with atmospheric % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D% aebbfv3ySLgzGueE0jxyaibaiiYdd9qrFfea0dXdf9vqai-hEir8Ve% ea0de9qq-hbrpepeea0db9q8as0-LqLs-Jirpepeea0-as0Fb9pgea% 0lrP0xe9Fve9Fve9qapdbaqaaeGacaGaaiaabeqaamaabaabcaGcba% acbiGaiWiG-bfadaWgaaWcbaacbaGaa43qaiaa+9eadaWgaaqaaiaa% +jdaaWqabaaaleqaaaaa!400D!\[P_{CO_2 } \])to determine the effects of varying pH (2 to 9), solid-mass to solution-volume ratio (M/V = 0.028 to 3.2 g/L), and solution concentration (2 × 10^?7 and 2 × 10^?6 M ²³³U) on U(6+) sorption on SAz-1 montmorillonite. The study focused on U(6+) surface complexation on hydroxylated edge sites as the sorption mechanism of interest because it is expected to be the predominant sorption mechanism at pHs typical of natural waters (pH ≈6 to ≈9). Thus, the experiments were conducted with a 0.1 M NaNO₃ matrix to suppress ion-exchange between U(6+) in solution and interlayer cations. The results show that U(6+) sorption on montmorillonite is a strong function of pH, reaching a maximum at near-neutral pH (≈6 to ≈6.5) and decreasing sharply towards more acidic or more alkaline conditions. A comparison of the pH-dependence of U(6+) sorption with that of U(6+) aqueous speciation indicates a close correspondence between U(6+) sorption and the predominance field of U(6+)-hydroxy complexes. At high pH, sorption is inhibited due to formation of aqueous U(6+)-carbonate complexes. At low pH, the low sorption values indicate that the 0.1 M NaNO₃ matrix was effective in suppressing ion-exchange between the uranyl (UO₂ ²⁺) species and interlayer cations in montmorillonite. At pH and carbonate concentrations typical of natural waters, sorption of U(6+) on montmorillonite can vary by four orders of magnitude and can become negligible at high pH. The experimental results were used to develop a thermodynamic model based on a surface complexation approach to permit predictions of U(6+) sorption at differing physicochemical conditions. A Diffuse-Layer model (DLM) assuming aluminol (>AlOH^?) and silanol (>SiOH^?) edge sites and two U(6+) surface complexation reactions per site effectively simulates the complex sorption behavior observed in the U(6+)-H₂O-CO₂-montmorillonite system at an ionic strength of 0.1 M and pH > 3.5. A comparison of model predictions with data from this study and from published literature shows good agreement and suggests that surface complexation models based on parameters derived from a limited set of data could be useful in extrapolating radionuclide sorption over a range of geochemical conditions. Such an approach could be used to support transport modeling by providing a better alternative to the use of constant K _d s in transport calculations.     

The pressure and temperature stability limits of lawsonite: implications for H2O recycling in subduction zones

The stability relations of lawsonite, CaAl₂Si₂O₇(OH)₂H₂O, have been investigated at pressures of 6 to 14 GPa and temperatures of 740 to 1150°C in a multi-anvil apparatus. Experiments used the bulk composition lawsonite+H₂O to determine the maximum stability of lawsonite. Lawsonite is stable on its own bulk composition to a pressure of 13.5 GPa at 800°C, and between 6.5 and 12 GPa at 1000°C. Its composition does not change with pressure or temperature. All lawsonite reactions have grossular, vapour and two other phases in the system Al₂O₃-SiO₂-H₂O (ASH) on their high-temperature side. A Schreinemakers analysis of the ASH phases was used to relate the reactions to each other. At the lowest pressures studied lawsonite breaks down to grossular+kyanite+coesite+vapour in a reaction passing through 980°C at 6 GPa and 1070°C at 9 GPa. Above 9 GPa the reactions coesite=stishovite and kyanite+vapour=topaz-OH are crossed. The maximum thermal stability of lawsonite is at 1080°C, at 9.4 GPa. At higher pressures the lawsonite breakdown reactions have negative slopes. The reaction lawsonite=grossular+topaz-OH+stishovite+vapour passes through 1070°C at 10 GPa and 1010°C at 12 GPa. At 14 GPa, 740–840°C, lawsonite is unstable relative to the assemblage grossular+diaspore+vapour+a hydrous phase with an Al:Si ratio of 1:1. Oxide totals in electron microprobe analyses suggest that the composition of this phase is AlSiO₃(OH). Two experiments on the bulk composition lawsonite+pyrope [Mg₃Al₂Si₃O₁₂] show that at 10 GPa the reaction lawsonite=Gr-Py_ss+topaz-OH+stishovite+vapour is displaced down temperature from the end-member reaction by 200°C for a garnet composition of Gr₂₀Py₈₀. Calculations suggest similar temperature displacements for reaction between lawsonite and Gr-Py-Alm garnets of compositions likely to occur in high-pressure eclogites. Temperatures in subduction zones remain relatively low to considerable depth, and therefore slab P-T paths can be within the stability field of lawsonite from the conditions of its crystallisation in blueschists and eclogites, up to pressures of at least 10 GPa. Lawsonite contains 11.5 wt% H₂O, which when released may trigger partial melting of the slab or mantle, or be incorporated in hydrous phases such as the aluminosilicates synthesised here. These phases may then transport H₂O to an even greater depth in the mantle.     

Characterization of subsurface geometry and radioactivity distribution in the trench containing Chernobyl clean-up wastes

Several radiometric and geophysical methods were applied to characterize the subsurface geometry and radioactivity distribution (especially ¹³⁷Cs) in a trench with low-level wastes at the Chernobyl site. While surface dose rate measurements and electromagnetic soil conductivity survey produced uninterpretable fields of signals, the ground penetrating radar appeared to be an efficient method for characterization of the subsurface geometry of the waste burial. It was established that the trench had the following dimensions: the length was 70 m; average width 6–8 m, and depth 2–2.5 m. Data on ¹³⁷Cs distribution in the trench were obtained by means of a borehole gamma-logging technique. The total inventory of ¹³⁷Cs was estimated at 600±200 GBq. Geostatistical analysis using the semivariogram function has shown regular spatial correlation patterns for the logarithm-transformed ¹³⁷Cs activity of waste material. The correlation length along the trench was 17 m, while across the trench, and in the vertical direction, it was 4 m. The observed correlation patterns supposedly were caused by the method used to dispose the contaminated topsoil: the bulldozing in the direction perpendicular to a trench axis. Obtained results may be useful for selecting a characterization method and for optimization of sampling strategies for similar waste sites.     

Electrical conductivity,thermopower and <Superscript>57</Superscript>Fe Mössbauer spectroscopy of aegirine (NaFeSi<Subscript>2</Subscript>O<Subscript>6</Subscript>)

DC and AC electrical conductivities were measured on samples of two different crystals of the mineral aegirine (NaFeSi₂O₆) parallel () and perpendicular () to the [001] direction of the clinopyroxene structure between 200 and 600 K. Impedance spectroscopy was applied (20 Hz–1 MHz) and the bulk DC conductivity _DC was determined by extrapolating AC data to zero frequency. In both directions, the log _DC – 1/T curves bend slightly. In the high- and low-temperature limits, differential activation energies were derived for measurements [001] of E_A 0.45 and 0.35 eV, respectively, and the numbers [001] are very similar. The value of _DC [001] with _DC(300 K) 2.0 × 10^{–6 –1}cm^–1 is by a factor of 2–10 above that measured [001], depending on temperature, which means anisotropic charge transport. Below 350 K, the AC conductivity () (/2=frequency) is enhanced relative to _DC for both directions with an increasing difference for rising frequencies on lowering the temperature. An approximate power law for () is noted at higher frequencies and low temperatures with () ^s, which is frequently observed on amorphous and disordered semiconductors. Scaling of () data is possible with reference to _DC, which results in a quasi-universal curve for different temperatures. An attempt was made to discuss DC and AC results in the light of theoretical models of hopping charge transport and of a possible Fe²⁺ Fe³⁺ electron hopping mechanism. The thermopower (Seebeck effect) in the temperature range 360 K < T <770 K is negative in both directions. There is a linear – 1/T relationship above 400 K with activation energy E 0.030 eV [001] and 0.070 eV [001]. ⁵⁷Fe Mössbauer spectroscopy was applied to detect Fe²⁺ in addition to the dominating concentration of Fe³⁺.     

Mg tracer diffusion in synthetic forsterite and San Carlos olivine as a function of P,T and fO2

We present new experimental data on Mg tracer diffusion in oriented single crystals of forsterite (Fo₁₀₀) and San Carlos olivine (Fo₉₂) between 1000–1300° C. The activation energies of diffusion are found to be 400 (±60) kJ/mol (96 kcal/mol) and 275 (±25) kJ/mol (65 kcal/ mol) in forsterite and San Carlos olivine, respectively, along [001] at a fO₂ of 10^–12 bars. There is no change in activation energy of Mg tracer diffusion within this temperature range. Mg tracer diffusion in a nominally pure forsterite is found to be anisotropic (D_c > D_a > D _b) and a function of fO₂. This fO₂ dependence is different from that in olivine containing Fe as a major element, which suggests that the diffusion mechanism of Mg in forsterite is different from that in Fe-bearing olivine at least over some range of fO₂. The diffusion mechanism in nominally pure forsterites may involve impurities present below the limits of detection or alternately, Si or Fe³⁺ interstitial defects, Fe being present as impurity (ppm level) in forsterite. Pressure dependence of Mg tracer diffusivity in forsterite measured to 10 GPa in a multianvil apparatus yields an activation volume of approximately 1–3.5 cm³/ mol. It is found that presence of small amounts of hydrogen bearing species in the atmosphere during diffusion anneal (f_H2 0.2 bars, f_H20 0.24 bars) do not affect Mg tracer diffusion in forsterite within the resolution of our measurement at a total pressure of 1 bar. The observed diffusion process is shown to be extrinsic; hence extrapolation of the diffusion data to lower temperatures should not be plagued by uncertainties related to change of diffusion mechanism from intrinsic to extrinsic.     

Cell parameters of thermally treated anorthite. Al,Si configurations in the average structures of the high temperature calcic plagioclases

Thermal treatments of anorthite carried out at up to 1,547° C show that the unit cell parameter changes as a function of the treatment temperature. The best fit curve found by non-linear least squares analysis is: =91.419-(0.327·10^-6)T ²+(0.199·10^-12)T ⁴-(0.391·10)T ⁶. The results obtained support significant Al,Si disorder (Al0.10, where Al=t ₁(0)-1/3 [t ₁(m)+t ₂(0)+t ₂(m)], Ribbe 1975), in anorthite equilibrated near the melting point and confirm a high temperature series differentiated from the low temperature series for calcic plagioclases in the An₈₅–An₁₀₀ range also. In the plot vs. An-content the high and low temperature curves intersect at An₈₅ composition and progressively diverge in the An₈₅–An₁₀₀ range. The trends of the high and low temperature curves in this range are interpretable on the basis of the degree of Al, Si order in the average structures of calcic plagioclases.     

Stable Isotope Ratios and the Evolution of Acidulous Ground Water

Ground waters in North Hesse (Germany) are conspicuous by high amounts of dissolved inorganic carbon (DIC) at low pH. The DIC is received from the uptake of soil CO₂ and CO₂ of volcanic origin and the subsequent dissolution ofTriassic and Permian limestone and dolomites. The volcanic CO₂ is related to Miocene basaltic magma which has liberated gaseous CO₂ during thebreakthrough to Triassic and Permian sediments. The volcanic CO₂ (-6 < ¹³CCO₂ < -3, PDB) was trapped within pore spacesand intra- and intergranulares of Permian evaporites and Triassic sandstones and was stored within such reservoirs until recent times. The uptake of volcanic CO₂ occurs as ground water migrates through such reservoirs. The ¹³ C/¹² C-signatures of the DIC indicate mixture of soil-CO₂and CO₂ of volcanic origin for the dissolution of marine limestone and dolomites. The obtained two types for CO₂ of volcanic origin with ¹³C_{CO 2}-values of -10 ± 3 and +2 ± 2 can be explained by diffusion of CO₂ through micropores, faults, and interfacesof solids. This mobilisation of CO₂ is accompanied with a kinetic fractionation of -9. ¹³ C-depleted CO₂ is liberated from the reservoir,whereas ¹³ CO₂ is accumulated in the residue     

Lead isotope study of Zn-Pb ore deposits associated with the Basque-Cantabrian basin and Paleozoic basement, Northern Spain

A total of forty-three galena samples from syngenetic and epigenetic Pb-Zn mineralizations emplaced in the Lower Cretaceous Basque-Cantabrian basin and Paleozoic basement of the Cinco Villas massif in the western Pyrenees, have been analyzed for Pb-isotopic composition. Galena from sedex mineralizations hosted in Carboniferous clastic rocks in the Cinco Villas massif display an homogeneous lead isotopic signature (²⁰⁶Pb/²⁰⁴4Pb 18.43, ²⁰⁷Pb/²⁰⁴Pb 15.66, ²⁰⁸Pb/ ²⁰⁴Pb 38.69) suggesting a single lead reservoir. These values are slightly more radiogenic than lead from other European Hercynian deposits, possibly reflecting the influence of a more evolved upper crustal source. Underlying Paleozoic sediments are proposed as lead source for the Cinco Villas massif ores. Analyses from twenty-six galena samples from the four strata-bound ore districts hosted in Mesozoic rocks reveal the existence of two populations regarding their lead isotopic composition. Galena from the western Santander districts (e.g., Reocin) is characterized by more radiogenic isotope values (²⁰⁶Pb/²⁰⁴Pb 18.74, ²⁰⁷Pb/²⁰⁴Pb 15.67, ²⁰⁸Pb/ ²⁰⁴Pb 38.73) than those from the central and eastern districts (Troya-Legorreta, Central and Western Vizcaya, ²⁰⁶Pb/²⁰⁴Pb 18.59, ²⁰⁷Pb/²⁰⁴Pb 15.66, ²⁰⁸Pb/ ²⁰⁴Pb 38.73). In all districts, the most likely source for these mineralizations was the thick sequence of Lower Cretaceous clastic sediments. The existence of two separate lead isotopic populations could be the result of regional difference in the composition of the basement rocks and the clastic sediments derived of it or different evolution histories. In both sub-basins, isotopic ratios indicate an increase in crustal influence as the age of the ores decreases.     

Boron isotopic variations in hydrous rhyolitic melts: a case study from Long Valley,California

In this paper, we present boron isotope analyses of variably degassed rhyolitic glasses from Long Valley, California. The following results indicate that pre-eruptive boron isotopic signatures were preserved in degassed glasses: (1) averaged secondary ionization mass spectrometry (SIMS) measurements of H₂O-rich (~3 wt%) melt inclusions from late erupted Bishop Tuff pumice are indistinguishable from positive thermal ionization mass spectrometry (PTIMS) analysis of vesiculated groundmass glass (¹¹B=+5.0±0.9 and +5.4±5, respectively); (2) SIMS spot-analyses on H₂O-poor obsidian (~0.15 wt% H₂O) from younger Glass Mountain Dome YA (average ¹¹B=+5.2±1.0) overlap with compositionally similar late Bishop Tuff melt inclusions; and (3) four variably degassed obsidian samples from the 0.6 ka Mono Craters (H₂O between 0.74 and 0.10 wt%) are homogeneous with regard to boron (average ¹¹B=+3.2±0.8, MSWD=0.4). Insignificant variations in ¹¹B between early and late Bishop Tuff melt inclusion glasses agree with published experimental data that predict minor ¹¹B depletion in hydrous melts undergoing gas-saturated fractional crystallization. Melt inclusions from two crystal-rich post-caldera lavas (Deer Mountain and South Deadman Dome) are comparatively boron-rich (max. 90 ppm B) and have lower ¹¹B values (average ¹¹B=+2.2±0.8 and –0.4±1.0 ) that are in strong contrast to the boron isotopic composition of post-caldera crystal-poor rhyolites (27 ppm B; ¹¹B=+5.7±0.8). These variations in ¹¹B are too large to be caused by pre-eruptive degassing. Instead, we favor assimilation of ¹¹B depleted low-temperature hydrothermally altered intrusive rocks subsequent to fresh rhyolite recharge.Editorial responsibility: J. HoefsAn erratum to this article can be found at     

Schwefel-Isotopenuntersuchungen an der Blei-Zink-Erzlagerstätte Grund (Westharz,Bundesrepublik Deutschland)

Zusammenfassung Die S-Isotopenverteilung wurde an 67 Sulfid- und 17 Barytproben aus der Blei-Zink-Erzlagerstätte Grund untersucht. Die ³⁴S-Werte der Zinkblende der Mineralisationsphase II liegen im Westfeld-Erzmittel I und in den östlich anschließenden Erzmitteln zwischen +4 und +6, in dem am weitesten westlich liegenden Westfeld-Erzmittel II zwischen +6 und +10. Die Werte für Bleiglanz der Mineralisationsphase II sind +2 bis +4 bzw. +4 bis +7. Die Sulfide der Mineralisationsphase III haben allgemein niedrigere -Werte. Koexistierende Sulfide zeigen eine deutliche Fraktionierung, wobei stets _ZnS > _PbS ist; die Differenz beträgt in der Mineralisationsphase II im Mittel 1,8, in der Phase III 3. Dies deutet auf niedrigere Bildungstemperatur der Minerale der Phase III hin. Zur genetischen Deutung der beobachteten -Abnahme beim Übergang zur Mineralisationsphase II werden vier Modelle diskutiert. Baryte zeigen innerhalb der Lagerstätte recht einheitliche ³⁴S-Werte zwischen +11 und +14,5%. Diese Einheitlichkeit wird durch den Einfluß deszendenter Zechstein-Lösungen erklärt.

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³⁴S-values are given for 67 sulfide and 16 barite specimens from the Pb-Zn-deposit Grund (Harz mountains, W-Germany). In the central part of the deposit the sulfide 's of the first major mineralization (phase II) range from: ZnS +4 to +6 and PbS +2 to +4. The sulfides of the second major mineralization (phase III) are depleted in ³⁴S and range from: ZnS +2 to +4, PbS –1,4 to 3. The sulfides at the western end of the vein system are heavier; the phase II minerals ranging from: ZnS +6 to 10 and PbS +4 to 7. The mean -difference between co-existing ZnS and PbS in phase II is 1,8, in phase III 3. This indicates lowering of temperature of formation for the phase III ore. Four models have been set up in order to explain the observed -variation. Barites with rather uniform 's from +11 to +14,5 are probably affected by descendent solutions from overlying sulfate sediments of Permian age.

     

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.     

Reconstruction of palaeohydrological conditions in a lagoon during the 2nd Zechstein cycle through simultaneous use of δD values of individual n-alkanes and δ<Superscript>18</Superscript>O and δ<Superscript>13</Superscript>C values of carbonates

For the first time ¹⁸O and ¹³C values from carbonates and D values of individual n-alkanes were used to reconstruct palaeohydrological conditions in a lagoon at the southern margin of the Central European Zechstein Basin (CEZB). A 12-m core covering the complete Ca2 interval and adjacent anhydrites (A1 and A2) was analyzed for ¹⁸O and ¹³C values of dolomitized carbonates and D values of individual n-alkanes. ¹⁸O_carb values (+2 to +5 vs. VPDB) were strongly influenced by evaporation and temporal freshwater input into the lagoon. The ¹³C_carb values (–1 to +4 vs. VPDB) were controlled mainly by changes in primary production. Both isotopic ratios show an inverse relationship throughout most of the core, contradicting diagenetic alteration, since ¹³C_carb values are not altered significantly during dolomitization. Assuming a temperature range of 35–40 °C in the lagoon, ¹⁸O_carb values of +2.5 to +8 (vs. VSMOW) for the lagoonal water can be reconstructed. The lagoon may have desiccated twice during the Ca2 interval, as indicated by very high ¹⁸O_carb and low ¹³C_carb values, coinciding with increasing amount of anhydrite in the analyzed sample. These events seem to reflect not just local but a regional intra-Ca2 cyclicity. Measured D values of the short-chain n-alkanes, namely n-C₁₆ and n-C₁₈ which are widely used as indicators for photosynthetic bacterial and algal input, reflect the isotopic composition of the lagoonal water. Assuming constant fractionation during incorporation of hydrogen into lipids of –160, an average D value of +70 (vs. VSMOW) can be reconstructed for the lagoonal water, accounting for very arid conditions. The long-chain n-alkanes n-C₂₇, n-C₂₈, n-C₂₉ and n-C₃₀, thought to be derived from the leaf waxes of terrestrial higher plants, were shown to be depleted in D relative to the short-chain alkanes, therefore indicating a different hydrogen source. Terrestrial plants in arid areas mainly use water supplied by precipitation. By using a smaller fractionation of –120 due to evaporation processes in the leaves, reconstructed values vary between –74 and –9 (vs. VSMOW). These values are not indicating extremely arid conditions, implying that the long-chain n-alkanes were transported trough wind and/or rivers into the lagoon at the Zechstein Sea coast. D_water values, reconstructed using the n-C₁₆ alkane and ¹⁸O _water values, independently reconstructed on the same sample using the temperature dependant fractionation for dolomites are good agreement and suggest high amounts of evaporation affecting the coastal lagoon. Altogether, our results indicate that hydrogen isotopic ratios of n-alkanes give information on their origin and are a useful proxy for palaeoclimatic reconstruction.     

Oxygen isotopes in mantle related and geothermally altered magmatites of the Transhimalayan (Gangdese) ranges

In closed magma systems SiO₂ approximately measures differentiation progress and oxygen isotopes can seem to obey Rayleigh fractionation only as a consequence of the behaviour of SiO₂. The main role of ¹⁸O is as a sensitive indicator of contamination, either at the start of differentiation ( ¹⁸O_init) or as a proportion of fractionation in AFC. Plots of ¹⁸O vs SiO₂-allow to determine initial ¹⁸O values for different sequences for source comparison. For NBS-28=9.60, the ¹⁸O at 48% SiO₂-varies between a high 6.4 for Kiglapait (Kalamarides 1984), 5.9 for Transhimalaya, 5.8 for Hachijo-Jima (Matsuhisa 1979), 5.6 for Koloula (Chivas et al. 1982) and a low 5.3 for the Darran Complex, New Zealand. The Transhimalayan batholiths (Gangdese belt) were emplaced in the Ladakh-Lhasa terrane, between the present-day Banggong-Nujiang, and Indus-Yarlung Tsangbo suture zones, after its accretion to Eurasia. The gradient of the least contaminated continuous ( ¹⁸O vs SiO₂-igneous trend line is similar to that of Koloula, and AFC calculations suggest a low secondary assimilation rate of less than 0.05 times the rate of crystallisation. Outliers enriched in ¹⁸O are frequent in the Lhasa, and apparently rare in the Ladakh transsect. Low- ¹⁸O (5.0–0) granitoids and andesites on the Lhasa-Yangbajain axis are the result of present day or recent near-surface geothermal activity; their quartzes still trace the granitoids to the Transhimalaya ¹⁸O trend line, but the distribution of low total rock or feldspar ¹⁸O values could be a guide to more recent heat flow and thermally marked tectonic lineaments. Two ignimbrites from Maqiang show hardly any ¹⁸O-contamination by crustal material.     

Nickenichite,a new arsenate from the Eifel,Germany

Summary Nickenichite is a new mineral found close to the village of Nickenich at the Nickenicher Sattel, Eifel, Germany. The chemical composition is Na_xCa_yCu_z(Mg, Fe, Al)₃(AsO₄)3, x 0.8, y 0.4, 0.4 and was derived by means of electron microprobe analyses and by a crystal structure investigation. The latter was determined from single-crystal X-ray data:a = 11.882(4)Å,b = 12.760(4)Å,c = 6.647(2)Å, = 112.81(2)°, space group C2/c,Z = 4;R = 0.053 andR _w = 0.033 from 984 observed data and 102 free variables. Nickenichite is structurally related to the minerals o'danielite and johillerite. The two crystallographically different octahedrally coordinated cation positionsMe = (Mg, Fe, Al) have averageMe-O distances of 2.108 Å and 2.056 Å, octahedra share edges to form zig-zag chains in ; the chains are interconnected by AsO₄ tetrahedra. In addition the compound is characterized by partially occupied Na^[4+4], Ca^[6+2] and Cu^[4] positions.

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Nickenichit, ein neues Arsenat aus der Eifel, Deutschland

Zusammenfassung Nickenichit ist ein neues Mineral, das nahe dem Ort Nickenich, am Nickenicher Sattel, Eifel, Deutschland, gefunden wurde. Die chemische Formel ist Na_xCa_yCu_z, (Mg, Fe, Al)₃(AsO₄)₃, x 0,8, y 0,4, z 0,4 und wurde mittels Elektronenstrahl-Mikrosondenanalysen und einer Kristallstrukturuntersuchung ermittelt. Letztere wurde mit Einkristall-Röntgendaten durchgeführt:a = 11,882(4) Å,b = 12,760(4) Å,c = 6,647(2) Å, = 112,81(2)°, Raumgruppe C2/c,Z = 4;R = 0,053 undR _w = 0,033 für 984 beobachtete Daten und 102 freie Variable. Nickenichit zeigt enge strukturelle Beziehungen zu den Mineralen O'Danielit und Johillerit. Die zwei kristallographisch verschiedenen oktaedrisch koordinierten KationpositionenMe = (Mg, Fe, Al) haben mittlereMe-O-Abstände von 2,108 Å und 2,056 Å, die Oktaeder werden über Kanten zu zick-zack-artigen Ketten in verknüpft, diese werden untereinander über AsO₄-Tetraeder vernetzt. Des weiteren ist die Verbindung durch partiell besetzte Na^[4+4]-, Ca^[6+2]- und Cu^[4]-Positionen charakterisiert.

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With 2 Figures     

Accessory minerals of the Cínovec (Zinnwald) granite cupola, Czech Republic: indicators of petrogenetic evolution

Summary A fully cored drillhole was drilled to 1596m by the Czech Geological Survey in 1961–1963 in the central part of the Cínovec (Zinnwald) granite cupola. Two types of granite were intersected: zinnwaldite granite (ZG), observed down to a depth of 730m, and protolithionite granite (PG), occurring to the end of the hole. The core was used to study the distribution and chemistry of: zircon, thorite, xenotime, monazite, bastnäsite, synchysite, REE oxyfluorides and hydroxyfluorides. Zircon occurs throughout the drillcore; it is strongly hydrated and fluorinated with about 18.5wt.% H₂O content in the apical part of the cupola. Its F-content reaches 2.41wt.%. Within the PG, the F concentration in zircon is low. Zircon is poor in Th and U and its HfO₂ contents vary from 1.01 to 5.24wt.%. Thorite is common in the PG, becoming rare in the ZG. It is strongly hydrated (up to 14wt.% H₂O) and fluorinated (up to 2.04wt.% F). Extensive solid solution between ThSiO₄ and YPO₄ was observed. Xenotime is strongly hydrated (up to 16wt.% H₂O), but its F content is low (<0.31wt.%). Two types of monazite were identified: Th-rich (up to 9.3wt.% ThO₂) in the ZG, and Th-poor (<2.5wt.% ThO₂) in the PG. Monazite remained stable during the hydration and fluorination process. Its REE chondrite-normalized distribution patterns show negative anomalies for La and Nd and a pronounced negative anomaly for Eu. Chemical compositions of several REE oxyfluorides and hydroxyfluorides were studied. REE fluorocarbonates are represented by bastnäsite and synchysite. Bastnäsite is abundant in the ZG. Its chondrite-normalized REE patterns are characterized by an important negative Eu anomaly and downward kinks at La and Nd. Synchysite-(Ce) and synchysite-(Y) are particularly well developed in the deeper parts of the cupola, and exhibit REE distribution patterns characterized by a weak negative Eu anomaly (synchysite-(Ce)), or a weak positive Eu anomaly (synchysite-(Y)).The distribution of accessory minerals reveals five major evolution stages: (1) Early magmatic crystallization of albite and orthoclase. (2) A late magmatic stage comprising protolithionite, quartz, accessory zircon, thorite, xenotime and monazite. (3) Interaction of this magmatic association with a fluid phase rich in F, CO₂ and H₂O, leading to the transformation protolithionite zinnwaldite and to the remobilisation of Nb, Ta, Ti, W, Sn. Accessory minerals formed during stage (2) were hydrated and fluorinated, except monazite. (4) The transfer of volatiles into the apical part of the cupola followed by the opening of the magmatic system generated microgranites and hydrolysis-type reactions leading to the appearance of REE oxyfluorides and hydroxyfluorides. (5) A late CO₂- and F-rich fluid phase was responsible for the deposition of REE fluorocarbonates. Monazite and xenotime became unstable in the apical part of the cupola. An influx of fluids with high Ca-activity occurred late during stage (5) and led to the formation of synchysite, and finally to the extensive precipitation of fluorite.     

Tungsten-bearing baotite from Pierrefitte,Pyrenees, France

Summary Baotite occurs in the Garaoulére orebody, at Pierreftte, France, as an accessory mineral, included in alstonite and celsian, and associated with sphalerite, galena, pyrite, siderite and calcite in hydrothermal veins crosscutting calcareous, rutile-bearing, siltstones. Microprobe analyses revealed high W0₃ concentrations (up to 6 wt.%) in baotite. The empirical formula of W-rich baotite is Ba_3.959Ti₄(Ti_3.169W_0.393Fe_0.116Al_0.073 Cr_0.048Nb_0.024)_3.823 Si_4.05O₂₈Cl_1.166. The excess of charges due to the presence of W⁶⁺ and Nb⁵⁺ is compensated by the introduction of M³⁺ (Fe, Al, Cr) into Ti-octahedra, by the appearance of Al in Si-tetrahedra (for W-poor baotite) and by the appearance of vacancies in Ti-octahedra (3Ti⁴ -> 2W⁶⁺ + and in Ba-sites (Ti⁴⁺ Ba²⁺ W⁶⁺, ). The unit-cell parameters of W-rich baotite are: a = 19.92(2), c = 5.930(8) Å. Niobium-rich baotites (Baiyun-Obo,Semenov et al., 1961; Karlstein,Nmec, 1987) are characterized by substitutions: Ti⁴⁺(VI), Si⁴⁺(IV)Nb⁵⁺(VI), Al³⁺(IV) and 2Ti⁴⁺, Ba²⁺ 2Nb⁵⁺, .

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Wolfram führender Baotit von Pierrefitte, Pyrenäen, Frankreich

Zusammenfassung Baotit kommt in dem Garaoulére Erzkórper in Pierrefitte, Frankreich als ein akzessorisches Mineral in Einschlÿussen in Alstonit und Celsian vor. Er ist mit Zinkblende, Bleiglanz, Pyrit, Siderit und Calcit assoziiert. Diese Paragenese ist an hydrothermale Gänge gebunden, die kalkige rutil-führende Siltsteine durchsetzen. Mikrosondenanalysen zeigen hohe W0₃ Gehalte (bis zu 6 Gew.%) in Baotit. Die empirische Formel von wolfram-reichem Baotit ist: Ba_3.959Ti₄(Ti_3.169W_0.393Fe_0.116Al_0.073Cr_0.048Nb_0.024)_3.823 Si_4.05O₂₈Cl_1.66. Der durch die Anwesenheit von W⁶⁺ und Nb⁵⁺ erforderliche Ladungsausgleich ergibt sich durch das Eintreten von M³⁺ (Fe, Al; Cr) in Ti-Oktaeder, und von Al in Si-Tetraeder (für W-armen Baotit) und schließlich durch das Erscheinen von Leerstellen in Ti-Oktaedern (3Ti⁴⁺ 2W6+ + und in Ba-Stellen (Ti⁴⁺, Ba` W⁶⁺, Die Zellparameter von ldW-reichem Baotit sind: a = 19.92(2), c = 5.930(8) Å. Niob-reiche Baotite (Baiyun-Obo, Semenov et al., 1961; Karlstein, Nmec, 1987) sind durch Substitutionen charakterisiert: Ti⁴⁺(VI), Si⁴⁺(IV)Nb⁵⁺(VI), Al³⁺(IV) und 2Ti⁴⁺, Ba²⁺ 2Nb⁵⁺, .

     

Diffusion in fluid-bearing and slightly-melted rocks: experimental and numerical approaches illustrated by iron transport in dunite

Bulk diffusion of iron in synthetic dunites containing 1–6 vol.% fluid or melt at 10 kbar (1 GPa) and 900°–1300° C was examined by encapsulating the samples in platinum, which served as a sink for iron. The rate of iron loss from the dunite was found to depend strongly upon the identity of the fluid, which was varied from CO₂ and H₂O to melts of basaltic and sodium carbonate composition. Carbon dioxide in amounts up to 4 vol.% has no effect upon bulk iron diffusion because it exists in the dunite are isolated pores. The interconnected nature of H₂O, basaltic melt, and carbonate melt, on the other hand, results in marked enhancement of bulk-rock Fe diffusion that is correlated with the diffusivity and solubility of olivine components in the fluid. At 1300° C, 4–5 vol.% of either water or basaltic melt increases the effective bulk diffusivity from the fluidabsent value of 10^-10 cm²/s to 10^-8 cm²/s. A single experiment involving a similar volume fraction of carbonate melt yielded a minimum bulk diffusivity of 10^-7–10^-6 cm²/s. This remarkably high value is attributable to the concurrent high diffusivity and high solubility of olivine components in molten carbonate H₂O has a high diffusivity, estimated at 10^-4 cm²/s in this study, and basaltic melt can dissolve large amounts of olivine, but neither possesses these two qualities in combination. Bulk transport of Fe in dunite containing <2 vol.% of pure H₂O is independent of olivine grain size for samples having an average grain diameter of <10 m to 60 m. This is probably because bulk diffusion specifically in these H₂O-bearing samples is ratelimited by the flux (which is proportional to concentration) of olivine components in the fluid. Given a constant fluid volume fraction, the effect of reducing the grain size is to increase the number of fluid-filled channels, but at the same time to decrease their average aperture, thus keeping constant the cross-sectional area through which the diffusional flux occurs. (Independence of bulk diffusivity from grain size is not anticipated for rocks containing melt, in which the silicate components are much more soluble.) In numerical (finite difference) simulations of selected laboratory experiments, the bulk Fe transport process was modeled as diffusion in fluid-filled tubules of triangular cross-section that are supplied by volume diffusion from contacting olivine grains with which they are in surface equilibrium. Applying a tortuosity factor of 1.7 brings the numerically computed diffusional loss profiles for experiments containing basaltic melt into near-coincidence with the experimentally-determined curves. This success in reproducing the experimental results lends credence to the interpretation of the bulk diffusional loss profiles as composites of gradients due to volume, grain-boundary and fluid-phase diffusion.     

Oxygen isotope studies of potassic volcanic rocks of the Roman Province,Central Italy

The ¹⁸O/¹⁶O ratios of rocks and coexisting minerals were measured for 93 samples of leucite-bearing lavas, pyroclastics, and related volcanic rocks from the Quaternary Roman Co-Magmatic Province, Italy. The ¹⁸O values were found to generally increase northward in the sequence: Ischia (5.8 to 7.0); Somma-Vesuvius and Phlegrean Fields (7.3 to 8.3); Alban Hills (7.3 to 8.7); M. Sabatini (7.3 to 9.7); Vico Volcano (7.4 to 10.2); and M. Vulsini (8.1 to 11.7). The northward increase in ¹⁸O parallels a similar increase in ⁸⁷Sr/⁸⁶Sr, and these data indicate that the Roman magmas have interacted strongly with high-¹⁸O continental crust. A marked increase in ¹⁸O occurs just north of Rome where the Roman Province begins to overlap the calc-alkaline, oversaturated Tuscan Magmatic Province. Therefore, some of the observed ¹⁸O/¹⁶O and ⁸⁷Sr/⁸⁶Sr enrichments in the Roman magmas may have been facilitated by direct mixing with the high-¹⁸O Tuscan magmas or because the high-¹⁸O country rocks underwent widespread heating during a couple of million years of Tuscan igneous activity. Although many of the Roman magmas underwent fractional crystallization without appreciable change in ¹⁸O, contamination has produced a correlation between ¹⁸O and SiO₂ content at several of the volcanic centers; thus the trachytes are typically higher in ¹⁸O than the undersaturated rocks. The major features of the oxygen isotope data can be explained in terms of a simple two-component mixing model in which one end-member was a primary, strongly undersaturated magma derived from the upper mantle, with ¹⁸O+6, ⁸⁷Sr/⁸⁶Sr0.704 to 0.705, and SiO₂<44wt.%. However, none of the analyzed samples have these values, as they have all been contaminated to some extent. The closest approach is found in some of the leucitepyroxenite ejecta from the Alban Hills. The second end-member, derived from the continental crust, had a variable composition with ¹⁸O+12 to +20, ⁸⁷Sr/⁸⁶Sr0.712 to 0.720, and SiO₂65wt.%, and it mixed in much greater proportions in the volcanoes north of Rome than in those of the Alban Hills or the Naples area. The widespread interactions between the Roman magmas and the continental crust are probably due to (1) the fact that such low-SiO₂ magmas always have a very strong tendency to interact with quartz-bearing rocks of the continental crust, and (2) in Italy, these magmas were emplaced into a tectonically very active area containing poorly consolidated sedimentary rocks, and in the northern part of the belt there had been a prior history of extensive calc-alkaline igneous activity.Publication of the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, Contribution Number 2501.     

Synthesis and some properties of iron- and vanadium-bearing kyanites, (Al,Fe3+)2SiO5and (Al,V3+)2SiO5

Iron- and vanadium-bearing kyanites have been synthesized at 900 and 1100° C/20 kb in a piston-cylinder apparatus using Mn₂O₃/Mn₃O₄- and MnO/Mn-mixtures, respectively, as oxygen buffers. Solid solubility on the pseudobinary section Al₂SiO₅-Fe₂SiO₅(-V₂SiO₅) of the system Al₂O₃-Fe₂O₃(V₂O₃)-SiO₂ extends up to 6.5 mole% (14mole %) of the theoretical end member FeSiO₅(V₂SiO₅) at 900°C/20 kb. For bulk compositions with higher Fe₂SiO₅ (V₂SiO₅) contents the corundum type phases M₂O₃(M = Fe³⁺, V³⁺) are found to coexist with the Fe³⁺(V³⁺)-saturated kyanite solid solution plus quartz. The extent of solid solubility on the join Al₂SiO₅-Fe₂SiO₅ at 1 100°C was not found to be significantly higher than at 900° C. Microprobe analyses of iron bearing kyanites gave no significant indication of ternary solid solubility in these mixed crystals. Lattice constants a ₀, b ₀, c ₀, and V₀ of the kyanite solid solutions increase with increasing Fe₂SiO₅- and V₂SiO₅-contents proportionally to the ionic radii of Fe³⁺ and V³⁺, respectively, the triclinic angles ,, remain constant. Iron kyanites are light yellowish-green, vanadium kyanites are light green. Iron kyanites, (Al_1.87 Fe _0.13 ³⁺ )SiO₅, were obtained as crystals up to 700 m in length.     

An ion microprobe study of anhydrous oxygen diffusion in anorthite: a comparison with hydrothermal data and some geological implications

The diffusion rate of ¹⁸O tracer atoms in anorthite (An₉₇Ab₀₃) under anhydrous conditions has been measured using SIMS techniques. The tracer source was ¹⁸O₂ 98.4% gas at 1 bar, in the temperature range 1300° C–850° C. The measured diffusion constants are D ₀=1 _–0.6 ⁺¹ ×10^–9 m²s^–1 Q=236±8 kJ mol^–1 Comparison of these values with published data for ¹⁸O diffusion in anorthite under hydrothermal conditions shows that dry oxygen diffusivities are orders of magnitude lower than equivalent wet values at similar temperatures. The effect of these differences on oxygen isotope equilibration during cooling is discussed.