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1.
‘Invisible gold’ in bismuth chalcogenides   总被引：1，自引：0，他引：1  
Gold concentrations have been determined by LA-ICPMS in bismuth chalcogenides (tellurides and sulfosalts, minerals with modular structures; chalcogen X = Te, Se, and S) from 27 occurrences. Deposit types include epithermal, skarn, intrusion-related and orogenic gold. The samples comprised minerals of the tetradymite group, aleksite series, bismuth sulfosalts (cosalite, lillianite, hodrushite, bismuthinite, and aikinite), and accompanying altaite. Gold concentrations in phases of the tetradymite group range from <0.1 to 2527 ppm. Phases in which Bi > X tend to contain lower gold concentrations than Bi2X3 minerals (tellurobismuthite and tetradymite). Cosalite and lillianite contain Au concentrations ranging up to 574 and 3115 ppm, respectively. Bismuthinite derivatives have lower Au concentrations: <2 ppm in bismuthinite and up to 542 ppm in aikinite. In our samples, Au concentrations in altaite range from <0.2 to 1662 ppm.Smoother parts of the LA-ICPMS profiles suggest lattice-bound gold, whereas irregularities on the profiles are best explained by the presence of gold particles (?1 μm in diameter). Plotting Au vs. Ag for the entire dataset gives a wedge-shaped distribution, suggesting that Ag underpins Au uptake in both bismuth tellurides and sulfosalts. In the tellurides, correlation trends suggest statistical substitution of Ag(Au), together with Pb, into the octahedral site in the layers. In sulfosalts, Au follows coupled substitutions in which M1+ (Ag, Cu) enters the structure. In tellurides, the presence of van der Waals gaps at chalcogen-chalcogen contacts provides for p-type semi-conductive properties critical for gold scavenging from fluids. Such weak bonds may also act as sites for nucleation of Au (nano)particles. In sulfosalts, contacts between different species that replace one another are also highly predictable to act as traps for (nano)particulate gold.Invisible gold in Bi-chalcogenides is useful to (i) identify trends of orefield zonation, (ii) discriminate between ‘melt’ and ‘fluid-driven’ scavenging, and (iii) interpret replacement and remobilisation processes. Bismuth chalcogenides have the potential to be significant Au carriers in sulfide-poor Au systems, e.g., intrusion-related gold, with impact on the overall Au budget if mean Au concentrations are high enough and the minerals are sufficiently abundant.  相似文献   

2.
The crystal structure of Pb6Bi2S9 is investigated at pressures between 0 and 5.6 GPa with X-ray diffraction on single-crystals. The pressure is applied using diamond anvil cells. Heyrovskyite (Bbmm, a = 13.719(4) Å, b = 31.393(9) Å, c = 4.1319(10) Å, Z = 4) is the stable phase of Pb6Bi2S9 at ambient conditions and is built from distorted moduli of PbS-archetype structure with a low stereochemical activity of the Pb2+ and Bi3+ lone electron pairs. Heyrovskyite is stable until at least 3.9 GPa and a first-order phase transition occurs between 3.9 and 4.8 GPa. A single-crystal is retained after the reversible phase transition despite an anisotropic contraction of the unit cell and a volume decrease of 4.2%. The crystal structure of the high pressure phase, β-Pb6Bi2S9, is solved in Pna2 1 (a = 25.302(7) Å, b = 30.819(9) Å, c = 4.0640(13) Å, Z = 8) from synchrotron data at 5.06 GPa. This structure consists of two types of moduli with SnS/TlI-archetype structure in which the Pb and Bi lone pairs are strongly expressed. The mechanism of the phase transition is described in detail and the results are compared to the closely related phase transition in Pb3Bi2S6 (lillianite).  相似文献   

3.
In order to illustrate different applications of the amphibole-albite-chlorite-epidote-quartz geothermobarometer, pressure-temperature-time ( P–T–t ) ± space ( P–T–t–s ) ± deformation ( P–T–t–d ) paths have been established from literature data. They are discussed as a function of the chemical, equilibrium and microstructural data available in each case, and compared with the conclusions already established by other methods. It is clear that it is necessary to know the relative chronology of the events (directions of zoning of minerals in successive microstructural positions) to establish precise P–T paths; this enables reconstruction of complex geodynamic histories. From this point of view, it is necessary to analyse the maximum possible number of minerals in a few well-chosen metabasic rocks showing different generations of blastesis. The rocks should belong to different tectonic units to obtain the best overall picture of a metamorphic complex.  相似文献   

4.
      本期刊登了黄志诚和刘冠邦有关太湖“溅射物”的讨论文章。该文从沉积学观点,对太湖某些沉积物及结核开展了 研究工作,确定了沉积成因菱铁矿结核的存在, 并据此对我们在2009 年提出的太湖“溅射物”冲击成因观点提出质疑。这 些非溅射物结核的新资料,为进一步深入对比研究溅射物提供了某些参考。本文根据溅射物的外形、内部特征以及同位素 资料,与非溅射物结核进行了初步对比,结果显示对太湖“溅射物”的不同结论可能与研究样品差异有关, 相关研究工作 还值得进一步深入下去。  相似文献   

5.
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6.
In the olivine crystal structure, cations are distributed over two inequivalent octahedral sites, M1 and M2. Kinetics of cation exchange between the two octahedral sites in (Co0.1Mg0.9)2SiO4 single crystal have been studied in the temperature range from 600 to 800°C by monitoring the time evolution of the absorbance of Co2+ ions in M1 or M2 sites using optical spectroscopy after rapid temperature jumps. It was found from such temperature-jump induced relaxation experiments that with increasing temperature the absorbance of Co2+ ions in the M1 site decreases while that in the M2 site increases. This indicates a tendency of Co2+ cations to populate the M2 site with increasing temperatures and vice versa. The experimental relaxation data can be modeled using a triple exponential equation based on theoretical analysis. Activation energies of 221 ± 4 and 213 ± 10 kJ/mol were derived from relaxation experiments on the M2 site and M1 site, respectively, for the cation exchange processes in (Co0.1Mg0.9)2SiO4 olivine. Implications for cation diffusion at low temperatures are discussed.  相似文献   

7.
KAlSi3O8 sanidine dissociates into a mixture of K2Si4O9 wadeite, Al2SiO5 kyanite and SiO2 coesite, which further recombine into KAlSi3O8 hollandite with increasing pressure. Enthalpies of KAlSi3O8 sanidine and hollandite, K2Si4O9 wadeite and Al2SiO5 kyanite were measured by high-temperature solution calorimetry. Using the data, enthalpies of transitions at 298 K were obtained as 65.1 ± 7.4 kJ mol–1 for sanidine wadeite + kyanite + coesite and 99.3 ± 3.6 kJ mol–1 for wadeite + kyanite + coesite hollandite. The isobaric heat capacity of KAlSi3O8 hollandite was measured at 160–700 K by differential scanning calorimetry, and was also calculated using the Kieffer model. Combination of both the results yielded a heat-capacity equation of KAlSi3O8 hollandite above 298 K as Cp=3.896 × 102–1.823 × 103T–0.5–1.293 × 107T–2+1.631 × 109T–3 (Cp in J mol–1 K–1, T in K). The equilibrium transition boundaries were calculated using these new data on the transition enthalpies and heat capacity. The calculated transition boundaries are in general agreement with the phase relations experimentally determined previously. The calculated boundary for wadeite + kyanite + coesite hollandite intersects with the coesite–stishovite transition boundary, resulting in a stability field of the assemblage of wadeite + kyanite + stishovite below about 1273 K at about 8 GPa. Some phase–equilibrium experiments in the present study confirmed that sanidine transforms directly to wadeite + kyanite + coesite at 1373 K at about 6.3 GPa, without an intervening stability field of KAlSiO4 kalsilite + coesite which was previously suggested. The transition boundaries in KAlSi3O8 determined in this study put some constraints on the stability range of KAlSi3O8 hollandite in the mantle and that of sanidine inclusions in kimberlitic diamonds.  相似文献   

8.
Saraswati Raju 《Geoforum》2006,37(2):155-158
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9.
Ruth Fincher 《Geoforum》2011,42(5):539-549
In creating separate and distinct spaces and forms of socialising for themselves, ‘international’ and ‘local’ university students in central Melbourne are influenced by three key, spatially-linked processes. The first is institutional - it includes the allocation of international students to a certain type of housing which differs from that chosen by local students, and the practice of universities, student clubs and local churches to gather international and local students into separate groupings. The second is locational. It is the process by which international students find themselves living principally in and around the edge of the central city, an entertainment district with rowdy characteristics that these students often find distasteful. Local students, in contrast, locate in the inner suburbs for the most part, at a distance from the city centre hotspot. The third process sees socialising habits that are enacted in and by virtue of the public places in which they occur. This self-enacting socialisation further separates local students from international students. All three processes demonstrate how certain characteristics of the built environment can be invested with particular meanings and become complicit in shaping racialised social interactions.  相似文献   

10.
Barium carbonate (BaCO3) was examined in a diamond anvil cell up to a pressure of 73 GPa using an in situ angle-dispersive X-ray diffraction technique. Three new phases of BaCO3 were observed at pressures >10 GPa. From 10 to 24 GPa, BaCO3-IV had a post-aragonite structure with space group Pmmn. There are two molecules in a single unit cell (Z = 2) of the orthorhombic phase, which is same as the high-pressure phases of CaCO3 and SrCO3. The isothermal bulk modulus of BaCO3-IV is K 0 = 84(4) GPa, with V 0 = 129.0(7) Å3 when K 0′ = 4. The c axis of the unit cell parameter is less compressible than the a and b axes. The relative change in volume that accompanies the transformation between BaCO3-III and BaCO3-IV is ～6%. BaCO3-V, which has an orthorhombic symmetry, was synthesized at 50 GPa. As the pressure increases, BaCO3-V is transformed into tetragonal BaCO3-VI. This transformation is likely to be second order, because the diffraction pattern of BaCO3-V is similar to that of BaCO3-VI, and some single peaks in BaCO3-VI become doublets in BaCO3-V. After decompression, the new high-pressure phases transform into BaCO3-II. Our findings resolve a dispute regarding the stable high-pressure phases of BaCO3.  相似文献   

11.
We have used density functional theory to investigate the stability of MgAl2O4 polymorphs under pressure. Our results can reasonably explain the transition sequence of MgAl2O4 polymorphs observed in previous experiments. The spinel phase (stable at ambient conditions) dissociates into periclase and corundum at 14 GPa. With increasing pressure, a phase change from the two oxides to a calcium-ferrite phase occurs, and finally transforms to a calcium-titanate phase at 68 GPa. The calcium-titanate phase is stable up to at least 150 GPa, and we did not observe a stability field for a hexagonal phase or periclase + Rh2O3(II)-type Al2O3. The bulk moduli of the phases calculated in this study are in good agreement with those measured in high-pressure experiments. Our results differ from those of a previous study using similar methods. We attribute this inconsistency to an incomplete optimization of a cell shape and ionic positions at high pressures in the previous calculations.  相似文献   

12.
Methodological debates about interviewing ‘elites’ have recently received significant attention within human geography. Many of the contributors to this debate have suggested that there is something intrinsically different about interviewing ‘up’, which geography’s methodological literature needs to make space to consider. This paper argues that, in fact, the distinction between ‘elite’ interviewees and other types of interviewees is based on inadequate and widely critiqued conceptions of power. If, instead, geographers employ a poststructural understanding of power, we may be able to achieve a more sophisticated analysis of power relations within the interview space.  相似文献   

13.
The photoluminescence and excitation spectra of sodalites from Greenland, Canada and Xinjiang (China) are observed at 300 and 10 K in detail. The features of the emission and excitation spectra of the orange-yellow fluorescence of these sodalites are independent of the locality. The emission spectra at 300 and 10 K consist of a broad band with a series of peaks and a maximum peak at 648 and 645.9 nm, respectively. The excitation spectra obtained by monitoring the orange-yellow fluorescence at 300 and 10 K consist of a main band with a peak at 392 nm. The luminescence efficiency of the heat-treated sodalite from Xinjiang is about seven times as high as that of untreated natural sodalite. The emission spectrum of the S2 − center in sodalite at 10 K consists of a band with a clearly resolved structure with a series of maxima spaced about 560 cm−1 (20–25 nm) apart. Each narrow band at 10 K shows a fine structure consisting of a small peak due to the stretching vibration of the isotopic species of 32S34S−, a main peak due to that of the isotopic species of 32S2 − and five peaks due to phonon sidebands of the main peak.  相似文献   

14.
The Cretaceous blueschist belt, Tavşanlı Zone, representing the subducted and exhumed northern continental margin of the Anatolide–Tauride platform is exposed in Western Anatolia. The Sivrihisar area east of Tavşanlı is made up of tectonic units consisting of i) metaclastics and conformably overlying massive marbles (coherent blueschist unit), ii) blueschist-eclogite unit, iii) marble–calcschist intercalation and iv) metaperidotite slab. The metaclastics are composed of jadeite–lawsonite–glaucophane and jadeite–glaucophane–chloritoid schists, phengite phyllites, and calcschists with glaucophane–lawsonite metabasite layers. The blueschist-eclogite unit representing strongly sheared, deeply buried and imbricated tectonic slices of accreted uppermost levels of the oceanic crust with minor metamorphosed serpentinite bodies consists of lawsonite-bearing eclogitic metabasites (approximately 90% of the field), lawsonite eclogites, metagabbros, serpentinites, pelagic marbles, omphacite–glaucophane–lawsonite metapelites and metacherts. The mineral assemblage of the lawsonite eclogite (garnet + omphacite > 70%) is omphacite, garnet, lawsonite, glaucophane, phengite and rutile. Lawsonite eclogite lenses are enclosed by garnet–lawsonite blueschist envelopes.Textural evidence from lawsonite eclogites and country rocks reveals that they did not leave the stability field of lawsonite during subduction and exhumation. The widespread preservation of lawsonite in eclogitic metabasites and eclogites can be attributed to rapid subduction and subsequent exhumation in a low geothermal gradient of the oceanic crust material without experiencing a thermal relaxation. Peak P–T conditions of lawsonite eclogites are estimated at 24 ± 1 kbar and 460 ± 25 °C. These P–T conditions indicate a remarkably low geotherm of 6.2 °C/km corresponding to a burial depth of 74 km.  相似文献   

15.
In the present paper, the parameters affecting the uncertainties on the estimation of M max have been investigated by exploring different methodologies being used in the analysis of seismicity catalogue and estimation of seismicity parameters. A critical issue to be addressed before any scientific analysis is to assess the quality, consistency, and homogeneity of the data. The empirical relationships between different magnitude scales have been used for conversions for homogenization of seismicity catalogues to be used for further seismic hazard assessment studies. An endeavour has been made to quantify the uncertainties due to magnitude conversions and the seismic hazard parameters are then estimated using different methods to consider the epistemic uncertainty in the process. The study area chosen is around Delhi. The b value and the magnitude of completeness for the four seismogenic sources considered around Delhi varied more than 40% using the three catalogues compiled based on different magnitude conversion relationships. The effect of the uncertainties has been then shown on the estimation of M max and the probabilities of occurrence of different magnitudes. It has been emphasized to consider the uncertainties and their quantification to carry out seismic hazard assessment and in turn the seismic microzonation.  相似文献   

16.
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17.
The proposed geothermobarometer is based on an empirical calibration which takes account of two equilibria involving the tremolite, edenite, pargasite and hastingsite components in amphiboles. It has applications to assemblages found in metabasic rocks of widely different chemical compositions (magnesian to Fe-rich metabasalts), and for metamorphism ranging from lower greenschist to highest amphibolite facies. Knowing the Si(T1), Aliv, Alvi, Fe3+, Fe2+, Mg, Ca, NaM4, NaA and A vacancy in an amphibole, and the Al3+ and X Mg in coexisting epidote and chlorite, it is possible to calculate two values of In K d for this assemblage. These equilibria involve edenite-tremolite and (pargasite/hastingsite)-tremolite end-members in amphibole (the calculation program is given). For these equilibria, the isopleths (iso-values of K d) have been calculated for 0.27 < X Mg < 0.75 and 0 < X Fe3+= Fe3+/(Fe3++ Alvi) < 0.8. It is then possible to determine pressure and temperature directly when X Mg, X Fe3+, In K d for tremoliteedenite and In K d for (pargasite/hastingsite)-tremolite are known. Application of this geothermobarometer is limited to Ca-free plagioclase assemblages, and complete P–T paths can be drawn only if all the minerals are considered together. Phase relations at successive stages of crystallization can be constrained by studying the relationships between the coexisting minerals, their zoning and the metamorphic fabrics.  相似文献   

18.
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19.
Seagrass beds form an important part of the coastal ecosystem in many parts of the world but are very sensitive to anthropogenic nutrient increases. In the last decades, stable isotopes have been used as tracers of anthropogenic nutrient sources and to distinguish these impacts from natural environmental change, as well as in the identification of food sources in isotopic food web reconstruction. Thus, it is important to establish the extent of natural variations on the stable isotope composition of seagrass, validating their ability to act as both tracers of nutrients and food sources. Around the world, depending on the seagrass species and ecosystem, values of seagrass N normally vary from 0 to 8?‰ δ15N. In this study, highly unusual seagrass N isotope values were observed on the east coast of Qatar, with significant spatial variation over a scale of a few metres, and with δ15N values ranging from +2.95 to ?12.39?‰ within a single bay during March 2012. This pattern of variation was consistent over a period of a year although there was a seasonal effect on the seagrass δ15N values. Seagrass, water column and sediment nutrient profiles were not correlated with seagrass δ15N values and neither were longer-term indicators of nutrient limitation such as seagrass biomass and height. Sediment δ15N values were correlated with Halodule uninervis δ15N values and this, together with the small spatial scale of variation, suggest that localised sediment processes may be responsible for the extreme isotopic values. Consistent differences in sediment to plant 15N discrimination between seagrass species also suggest that species-specific nutrient uptake mechanisms contribute to the observed δ15N values. This study reports some of the most extreme, negative δ15N values ever noted for seagrass (as low as ?12.4?‰) and some of the most highly spatially variable (values varied over 15.4?‰ in a relatively small area of only 655 ha). These results are widely relevant, as they demonstrate the need for adequate spatial and temporal sampling when working with N stable isotopes to identify food sources in food web studies or as tracers of anthropogenic nutrients.  相似文献   

20.
The present analysis adjusts previous estimates of global ocean CaCO3 production rates substantially upward, to 133 × 1012 mol yr?1 plankton production and 42 × 1012 mol yr?1 shelf benthos production. The plankton adjustment is consistent with recent satellite-based estimates; the benthos adjustment includes primarily an upward adjustment of CaCO3 production on so-called carbonate-poor sedimentary shelves and secondarily pays greater attention to high CaCO3 mass (calcimass) and turnover of shelf communities on temperate and polar shelves. Estimated CaCO3 sediment accumulation rates remain about the same as they have been for some years: ~20 × 1012 mol yr?1 on shelves and 11 × 1012 mol yr?1 in the deep ocean. The differences between production and accumulation of calcareous materials call for dissolution of ~22 × 1012 mol yr?1 (~50 %) of shelf benthonic carbonate production and 122 × 1012 mol yr?1 (>90 %) of planktonic production. Most CaCO3 production, whether planktonic or benthonic, is assumed to take place in water depths of <100 m, while most dissolution is assumed to occur below this depth. The molar ratio of CO2 release to CaCO3 precipitation (CO2↑/CaCO3↓) is <1.0 and varies with depth. This ratio, Ψ, is presently about 0.66 in surface seawater and 0.85 in ocean waters deeper than about 1000 m. The net flux of CO2 associated with CaCO3 reactions in the global ocean in late preindustrial time is estimated to be an apparent influx from the atmosphere to the ocean, of +7 × 1012 mol C yr?1, at a time scale of 102–103 years. The CaCO3-mediated influx of CO2 is approximately offset by CO2 release from organic C oxidation in the water column. Continuing ocean acidification will have effects on CaCO3 and organic C metabolic responses to the oceanic inorganic C cycle, although those responses remain poorly quantified.  相似文献   

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