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1.
The influence of structural defects on the polymorphic transformations in quartz was studied by electron paramagnetic resonance and electron microscopy. It was established that the kinetics of the accumulation of the clusters of the β-phase depends on isomorphic impurities in quartz. It was confirmed that germanium ions and vacancies in quartz participate in the formation of the clusters. Obtained results allow the assumption that the amount of germanium required to preserve clusters increases with increasing temperature and vice versa. An explanation is presented for experimentally observed decomposition of clusters during artificial heating of some quartz samples. It was found that the clusters of the β-phase are autonomous units, i.e. may exist independently of host crystalline structure of quartz. The possible mechanism of influence of Ge impurity on the temperature of α-β-transition in quarts is discussed.  相似文献   

2.

The distribution of substitutional Al, Ti, and Ge impurities in quartz samples from the Darasun, Teremkinskoe, and Talatui gold deposits, located in the Darasun ore field, were studied by electron paramagnetic resonance. The relationship between the isomorphous substitution and dynamic recrystallization of quartz was studied by optical and scanning electron microscopy. It was found that analysis of the plots of interdependence between the concentrations of various substitutional impurities in quartz (isogens) can detect development trends of isomorphous substitution. Two isomorphous substitution stages were recognized, one associated with quartz crystallization, and the other, with its subsequent dynamic recrystallization. The first stage is characterized by incorporation of Al impurity into the quartz crystal lattice, and the second, by incorporation Ti impurity. A Ge impurity is a catalyst for isomorphous substitution, and its concentrations vary widely. It is noted that the second stage plays a decisive role, because it accounts for the incorporation of the larger part of substitutional impurities. This process is facilitated by the dynamic recrystallization of quartz. Four genetic quartz groups, described by individual isogens, have been recognized in the Darasun ore field. Two of them correspond to quartz crystallized directly from a magmatogenic fluid or redeposited with the melt’s participation, and the other two groups, to quartz crystallized from an altered fluid. It was found that substitutional Al concentrations are retained in quartz after redeposition, whereas substitutional Ti concentrations decrease dramatically Mineral formation processes at each gold deposit are reviewed. Two types of temperature zoning, normal and reverse, have been recognized at the Darasun deposit. Each is characterized by an individual genetic quartz group and the degree of closedness of the mineral formation system. The genetically similar magmatogenic quartz samples found at the Darasun and Talatui deposits indicate the uniformity of the mineralization process in the Darasun ore field. The established trends of isomorphous substitution in quartz are useful in studies of the ore formation histories of gold and other ore deposits.

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3.
Summary Investigations of natural and synthetic quartz specimens by cathodoluminescence (CL) microscopy and spectroscopy, electron paramagnetic resonance (EPR) and trace-element analysis showed that various luminescence colours and emission bands can be ascribed to different intrinsic and extrinsic defects. The perceived visible luminescence colours in quartz depend on the relative intensities of the dominant emission bands between 380 and 700 nm. Some of the CL emissions of quartz from the UV to the yellow spectral region (175 nm, 290 nm, 340 nm, 420 nm, 450 nm, 580 nm) can be related to intrinsic lattice defects. Extrinsic defects such as the alkali (or hydrogen)-compensated [AlO4/M+] centre have been suggested as being responsible for the transient emission band at 380–390 nm and the short-lived blue-green CL centered around 500 nm. CL emissions between 620 and 650 nm in the red spectral region are attributed to the nonbridging oxygen hole centre (NBOHC) with several precursors. The weak but highly variable CL colours and emission spectra of quartz can be related to genetic conditions of quartz formation. Hence, both luminescence microscopy and spectroscopy can be used widely in various applications in geosciences and techniques. One of the most important fields of application of quartz CL is the ability to reveal internal structures, growth zoning and lattice defects in quartz crystals not discernible by means of other analytical techniques. Other fields of investigations are the modal analysis of rocks, the provenance evaluation of clastic sediments, diagenetic studies, the reconstruction of alteration processes and fluid flow, the detection of radiation damage or investigations of ultra-pure quartz and silica glass in technical applications. Zusammenfassung Ursachen, spektrale Charakteristika und praktische Anwendungen der Kathodolumineszenz (KL) von Quarz – eine Revision Untersuchungen von natürlichen und synthetischen Quarzproben mittels Kathodolumineszenz (KL) Mikroskopie und -spektroskopie, Elektron Paramagnetischer Resonanz (EPR) und Spurenelementanalysen zeigen verschiedene Lumineszenzfarben und Emissionsbanden, die unterschiedlichen intrinsischen und extrinsischen Defekten zugeordnet werden k?nnen. Die sichtbaren Lumineszenzfarben von Quarz werden durch unterschiedliche Intensit?tsverh?ltnisse der dominierenden Emissionsbanden zwischen 380 und 700 nm verursacht. Einige der KL Emissionen vom UV bis zum gelben Spektralbereich (175 nm, 290 nm, 340 nm, 420 nm, 450 nm, 580 nm) stehen im Zusammenhang mit intrinsischen Defekten. Die kurzlebigen Lumineszenzemissionen bei 380–390 nm sowie 500 nm werden mit kompensierten [AlO4/M+]-Zentren in Verbindung gebracht. Die KL-Emissionen im roten Spektralbereich bei 620 bis 650 nm haben ihre Ursache im “nonbridging oxygen hole centre” (NBOHC) mit verschiedenen Vorl?uferzentren. Die unterschiedlichen KL-Farben und Emissionsspektren von Quarz k?nnen oft bestimmten genetischen Bildungsbedingungen zugeordnet werden und erm?glichen deshalb vielf?ltige Anwendungen in den Geowissenschaften und in der Technik. Eine der gravierendsten Einsatzm?glichkeiten ist die Sichtbarmachung von Internstrukturen, Wachstumszonierungen und Defekten im Quarz, die mit anderen Analysenmethoden nicht oder nur schwer nachweisbar sind. Weitere wesentliche Untersuchungsschwerpunkte sind die Modalanalyse von Gesteinen, die Eduktanalyse klastischer Sedimente, Diageneseuntersuchungen, die Rekonstruktion von Alterationsprozessen und Fluidmigrationen, der Nachweis von Strahlungssch?den oder die Untersuchung von ultrareinem Quarz und Silikaglas für technische Anwendungen. Received March 29, 2000 Accepted October 27, 2000  相似文献   

4.
A study has been made of thermoluminescence from synthetic quartz with varying hydroxyl impurity concentrations up to approximately 300 H/106 Si which are associated with a “broad-band” IR absorption in the range 2600–3700 cm−1. These hydroxyl defects are known to be important in the hydrolytic weakening of quartz. We have found only minor differences in the glow curves of unheated crystals but significant intensity increases when “wet” crystals are heated sufficiently to cause bubble formation. It would seem that the electron traps are unaffected by the bubble formation, but the electron/luminescence centre radiative recombination probability is increased.  相似文献   

5.
Impurity inhomogeneities and other structural defects have been studied by means of transmission electron microscopy (TEM), X-ray microanalysis and electron paramagnetic resonance (EPR) in untreated and heat-treated quartz samples of three genetic types: hydrothermal, pegmatitic and magmatic. The impurities present are Al, Na and H2O, which occupy tetrahedral (Al3+) or interstitial (Na+, H2O) positions in the quartz lattice. Impurities form imperfections of various degrees of segregation: from point defects to micropores with a gas-liquid content. Their size, form, density and distribution in the lattice depend on the formation conditions of the quartz, the presence of dislocations and plane defects serving as sinks for the impurity atoms, and the heat treatment regime. Experimental data indicate that gas-liquid inclusions of dimensions up to some microns are the result of impurity segregation during postcrystallizational cooling. Crystalline quartz amorphizes upon electron irradiation. A model of structural water explaining experimentally observed features of this phenomenon is proposed whereby the water molecule, represented as a dipole, enters microregions of the silica lattice with a high impurity content and there forms a bond between ‘defective’ [SiO3]2? and [AlO4]5? tetrahedra. On irradiation, the Si---O donor-acceptor bonds trap nonelastically scattered electrons and are ruptured as a result. The water released by this lattice discontinuity forms microbubbles that diffuse along sinks into the larger micropores thus further increasing their volume.  相似文献   

6.
The thermal stabilities and decay kinetics of three peroxy radicals (Centers #1, B and B′) and three other radiation-induced defects (#3, C′ and E1′) in natural quartz from the high-grade McArthur River uranium deposit (Athabasca basin, Canada) have been investigated by isochronal and isothermal annealing experiments and electron paramagnetic resonance (EPR) spectroscopy. Single-crystal EPR spectra of isochronally (2 h) annealed quartz show that these centers all grow in intensity to 280°C and then decay with further increase in temperature, but their disappearance temperatures differ markedly and depend on the initial concentrations (e.g., Center #1 in a dark smoky quartz is annealed out at 380°C, B and B′ at 420°C and #3 and C′ at 580°C). The isothermal decay processes of these centers are all of the second order type. The calculated activation energies for the peroxy radicals [#1 and B + B′ at 0.36 (9) and 0.83 (8) eV, respectively] are smaller than those of Centers #3, C′ and E1′ [1.09 (8), 1.24 (8) and 1.45 (7) eV, respectively]. Gamma-ray irradiations of thermally bleached quartz restore a fraction of the peroxy radicals, suggesting that their diamagnetic precursors are stable up to at least 800°C. The unusual decay characteristics of “peroxy radicals” in quartz reported in the literature are shown to most likely arise from multiple radiation-induced defects. These results have implications for not only applications of peroxy radicals in quartz for EPR dating but also better understanding of thermoluminescence and cathodoluminescence spectra of this mineral.  相似文献   

7.
In this study we use two dimensional chemical patterns and numerical modeling to estimate the relative rates of chemical transport along interphase boundaries (ib) and through grain (s) interiors during retrograde Fe–Mg exchange between garnet and biotite at a garnet–biotite–quartz triple junction. We demonstrate that systematic variations in garnet–rim compositions, which are frequently observed along garnet–quartz interfaces, and deviations from concentric retrograde zoning patterns start to develop when chemical transport along the interphase boundaries becomes slow during cooling. The capacities for chemical transport along an interphase boundary depend on the product D ib K ib/s a, where D ib is the diffusion coefficient of the exchangeable components within the interphase boundary medium, K ib/s is the equilibrium partitioning coefficient between the cation exchange partners and the interphase boundary medium and a is the interphase boundary width. The model is applied to analyze the retrograde zoning patterns in garnets from the Mozambique belt (SE-Kenya), which cooled from 820°C at a rate of ca. 2°C/my. It is found that non-equilibrated compositions in garnet along garnet/quartz interphase boundaries started to develop below 700°C due to insufficient rates of chemical transport along these boundaries. The transport capacities of garnet/quartz interphase boundaries was estimated to have been between about 1E-23 cm3/s (575°C) and 1E-20 cm3/s (700°C) from modeling the observed X Fe pattern in garnet close to a garnet–quartz–biotite triple junction and relying on published data on the diffusivity of Fe2+ in garnet. Similar transport capacities are obtained; when the interphase boundary is assumed to be filled with a material that has the transport properties and chemical composition of a free melt in equilibrium with garnet, biotite and quartz at the respective conditions. In contrast, if the transport properties of the interphase boundary medium are related to the diffusivities and solubility of Fe2+ and FeOH+ within a free aqueous solution, chemical transport along the interphase boundaries would be much more efficient, and exchange equilibrium would have been maintained during the entire cooling history of the rocks. The observation of systematic deviations from local equilibrium along the garnet–quartz interphase boundaries leads us to exclude the presence of an aqueous fluid along the interphase boundary at any time during cooling.  相似文献   

8.
The nature of the solubility of water as [4H]Si defects in quartz, and their role in providing a source of molecular water on heating, is investigated. Existing ab inito energy calculations on the incorporation of water in quartz are used to show that energetically 4H for Si substitution is likely to constitute the most prevalent mode of water uptake on the atomic scale in quartz under equilibrium conditions, and that the planar defects previously observed by a number of different authors by electron microscopy in wet quartz are likely to be planar rafts of aggregated [4H]Si defects which are formed on supersaturation. These new conclusions call into question the previous identification of the planar defects as high pressure water clusters and require that their role in the production of molecular water in the context of recent theories of hydrolytic weakening be re-assessed. Accordingly the existing ab initio results have been used to establish the characteristics of the phase diagram for the system quartz-water in the temperature and pressure range of interest in hydrolytic weakening. Additional electron-optical experiments on wet quartz show that, on annealing at temperature in the electron microscope, similar planar defects develop in wet quartz by a diffusion process. In the context of existing theories of hydrolytic weakening it is now proposed that the conversion of [4H]Si defects to molecular water, where this is dictated by the equilibrium phase diagram, leads to a relatively large increase in volume and to the appearance of the bubbles of free water and the nucleation of associated prismatic dislocation loops of Burgers vector b=1/3 a $\langle 11\bar 20\rangle $ as previously observed. Ultimately the development of these loops leads to dislocation-induced plasticity.  相似文献   

9.
Mechanisms of the incorporation of isomorphic impurities of Al, Ti, and Ge into quartz were studied by EPR techniques. For this purpose, laboratory experiments were carried out on quartz annealing, which allowed us to model the processes of impurity atom introduction into the quartz lattice. The investigation of the kinetics of these processes showed that they are described by diffusion-controlled reactions. In many samples, a proportional relationship was observed between the concentrations of Al and Ti impurities incorporated into the quartz structure during laboratory annealing. A comparison of the experimental results with the character of the natural distribution of isomorphic impurities in quartz revealed their similarity. Based on the analysis of the results of our investigations, two main mechanisms were proposed for isomorphic substitutions in quartz. One of them is referred to as the capture mechanism and corresponds to the incorporation of isomorphic impurities during mineral formation. The second, diffusion mechanism operates after the crystallization of quartz. The isomorphic impurities incorporated into the quartz structure by this mechanism are either formed through the decomposition and transformation of composite complexes or as a result of diffusion from crystalline and gas-liquid inclusions. It was suggested that both mechanisms are responsible for the incorporation of Al and Ge impurities into the mineral lattice, whereas Ti is introduced mainly by the diffusion mechanism. The accounting for the mechanisms of isomorphic substitutions provides a means to significantly increase the reliability of the interpretation of genetic information recorded in the distribution of structural impurities in quartz. Original Russian Text ? L.T. Rakov, 2006, published in Geokhimiya, 2006, No. 10, pp. 1085–1096.  相似文献   

10.
An analysis is presented of equilibrium in six specimens ofgarnet—biotite—sillimanite—plagioclase—potashfeldspar—quartz ... gneiss from a metamorphic terrainin south-western Quebec. A nearly uniform Ti content of biotite may be accounted forby an equilibrium (a) involving biotite, sillimanite, quartz,garnet, potash feldspar, and H2O. The nature of the distributionof Fe and Mg between garnet and biotite may be accounted forby another equilibrium (b) involving the same mineral suite,or by a simple exchange equilibrium (c) involving only garnetand biotite. The distribution of Mn between garnet and biotiteis accounted for by an exchange equilibrium (d). A nearly uniformvalue of the ratio Ca content of plagioclase/Ca content of garnetmay be accounted for by an equilibrium (e) involving plagioclase,garnet, sillimanite, and quartz. A proposed equilibrium (f)involving biotite, quartz, ilmenite, potash feldspar, sillimanite,and H2O conflicts with equilibrium (a) and was evidently notestablished in the gneisses. The factors governing the Ca contentof biotite remain largely unknown. Some of these equilibria form potential indicators of relativegeologic temperature, pressure, and chemical potential of H2O.  相似文献   

11.
The incorporation of OH defects in quartz as a function of Li content in the bulk system and pressures was investigated. Quartz crystals were grown in water-saturated granitic systems, containing various amounts Li, B and P, supplied as accessory phases such as spodumene, tourmaline or apatite in the starting mixtures. High pressure experiments were performed at temperatures between 900 and 1100 °C, and pressures between 5 and 20 kbar with a piston cylinder apparatus, and the synthesized quartz crystals were analyzed by IR spectroscopy, electron microprobe and LA-ICP-MS spectroscopy. All IR absorption spectra revealed absorption features that can be assigned to AlOH (3313, 3379 and 3431 cm?1) and (4H)Si defects (3585 cm?1), whereas quartz grown in the Li and B systems exhibited two additional bands related, respectively, to LiOH (3483 cm?1) and BOH defects (3596 cm?1). It was further observed that LiOH incorporation increases with higher spodumene content in the starting material and decreases with pressure, until no LiOH defects are observed at pressure higher than 15 kbar. Specifically, the most pronounced reduction of LiOH defects occurs in a rather narrow pressure interval (10–15 kbar) close to the high-quartz/low-quartz transition. However, the link between the transition and the defect incorporation remains unclear. Li total concentrations always exceed the Li-coupled LiOH defects, suggesting the simultaneous presence of dry AlLi defects. Results of this study suggest that LiOH defects are detectable only in quartz crystals grown from middle and upper crustal sections (such as hydrothermal quartz) and not in quartz from deep roots of orogenic granitoids.  相似文献   

12.
The paper presents pioneering data on the composition, texture, and crystal structure of molybdenite from various types of molybdenum mineralization at the Bystrinsky Cu–Au–Fe porphyry–skarn deposit in the eastern Transbaikal region, Russia. The data were obtained using electron microprobe analysis (EMPA), laser ablation–inductively coupled plasma mass spectrometry (LA-ICP-MS), and high-resolution transmission electron microscopy (HRTEM). Molybdenite found at the deposit in skarn, sulfide-poor quartz veins, and quartz–feldspar alteration markedly differs in the concentrations of trace elements determined by their species in the mineral, as well as in its structural features. Molybdenite-2H from skarn associated with phyllosilicates occurs as ultrafine crystals with uniform shape and texture; no dislocations or inclusions were found but amorphous silica was. The molybdenite composition is highly contrasting in the content and distribution of both structure-related (Re, W, and Se) and other (Mn, Co, Ni, Cu, Zn, As, Ag, Cd, Sb, Te, Ag, Pd, Au, Hg, Pb, and Bi) metals. In the sulfide-poor quartz veins, highly structurally heterogeneous (2H + 3R) molybdenite microcrystals with abundant defects (dislocations and volumetrically distributed inclusions) are associated with illite, goethite, and barite. Some single crystals are unique three-phase (2H + 3R polytypes + amorphous MoS2). The mineral has a low concentration of all trace elements, which are uniformly distributed. However, individual domains with uniquely high Pd, Te, Ni, Hg, and W concentrations caused by mineral inclusions are found in some grains. Molybdenite from quartz–feldspar alteration is characterized by low concentrations of all trace elements except for Re and Se, which enrich some domains of the grains. Our data indicate that the compositional and structural heterogeneity of molybdenite from the Bystrinsky deposit are its crucial features, which obviously correlate with the types of Mo mineralization.  相似文献   

13.
We present an approach developed to compute chemical equilibrium and its corresponding reactive chemical transport when dominating precipitated species (DPS) exist. In computing chemical equilibrium, most models take the concentrations or activities of component species and precipitated species as the master variables. However, when the amount of a precipitated species is much larger than those of other species, small computational errors on this DPS concentration might introduce large errors on the concentrations of other species and would cause non‐mass‐conserved numerical results. To deal with the existence of DPS, we pick as master variables the concentration change, rather than the concentration, of DPS to compute chemical equilibrium. Since the concentration changes of DPS will no longer be much larger than the concentrations of other species in determining equilibrium, our approach is able to provide correct numerical results. We also employ the modified total analytical concentrations, rather than the total analytical concentrations, of aqueous components as the dependent variables in presenting and solving corresponding transport equations. Several examples are given to reveal the numerical problems associated with DPS and to demonstrate the success of our approach. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

14.
The revised titanium-in-zircon geothermometer was applied to Paleoproterozoic ultrahigh-temperature (UHT) granulites at Tuguiwula, Inner Mongolia, North China Craton. The Tuguiwula granulites contain diagnostic UHT mineral assemblages such as sapphirine + quartz and high alumina orthopyroxene + sillimanite + quartz, suggesting formation under temperatures of ca. 1,000°C and pressures of up to 10 kbar. Here, we report detailed petrographic studies and ICP-MS data on titanium concentration in zircons associated with the UHT assemblages. The zircons associated with sapphirine–spinel–sillimanite–magnetite assemblages have the highest Ti concentration of up to 57 ppm, yielding a temperature of 941°C, and suggesting that the growth of zircons occurred under ultrahigh-temperature conditions. The maximum temperatures obtained by the revised Ti-in-zircon geothermometer is lower than the equilibrium temperature of sapphirine + quartz, indicating an interval of cooling history of the granulites from UHT condition to ca. 940°C. Many of the zircons have Ti concentrations ranging from 10 to 33 ppm, indicating their growth or recrystallization under lower temperatures of ca. 745–870°C. These zircons are interpreted to have recrystallized during the retrograde stage indicated by microstructures such as cordierite rim or corona between spinel and quartz, and orthopyroxene–cordierite symplectite around garnet. Previous geochronological study on the zircons of the Tuguiwula UHT granulites gave a mean U–Pb SHRIMP age of 1.92 Ga. However, based on the Ti-in-zircon geothermometer results reported in this work, and considering the relatively slow thermal relaxation of these rocks, we infer that the timing of peak UHT metamorphism in the Tuguiwula area could be slightly older than 1.92 Ga.  相似文献   

15.
The area-perimeter fractal dimension (D) of quartz grains has earlier been proposed as a strain-rate gauge based on experimental deformation of quartz aggregates. To test the application in naturally deformed rocks, D is calculated in (a) three quartzites belonging to the Lunavada Group of rocks (Aravalli Mountain Belt, NW India) that developed textures between 420–600°C and (b) one quartz reef sample from the Malanjkhand Granite (Central India), which underwent dynamic recrystallization between 250–400°C. Using the above T ranges and calculated D values, strain-rates are calculated for the two sets of samples. A 10−12.7 s−1 strain rate at 250°C is calculated for the quartz reef sample. However, at higher temperatures the calculated strain-rate is >10−10 s−1 for the quartz reef and the quartzite samples. The quartzites show evidence of dynamic recrystallization by grain boundary migration (GBM) and subgrain rotation (SGR), while the quartz reef is replete with evidence of bulging (BLG) recrystallization. T and calculated strainrates are plotted on available recrystallization map of quartz. It is demonstrated that whilst the T/strain-rate of the quartzites does not fall in the region of GBM and SGR, the T/strain-rate of the quartz reef falls in the BLG region. The problems with strain-rate calculations using area perimeter fractal dimension are discussed. It is concluded that the method of strain-rate calculation can be used only for lower T.  相似文献   

16.
The mid-Jurassic calcalkaline Russian Peak intrusive complex,located in the Klamath Mountains of northern California, consistsof an elliptical peridotite-to-quartz diorite suite intrudedby two plutons of granodiorite. Several techniques were usedto decipher the crystallization conditions for ultramafic rocks,quartz diorite, and granodiorite, including comparison of parageneseswith crystallization experiments, application of geothermometersand barometers, and evaluation of phase equilibria. Contactmetamorphic assemblages, hornblende barometry, and amphibolesubstitution schemes indicate that pressures of intrusion were{small tilde}3 kbar. Plagioclase and pyroxene thermometry indicateintrusion temperatures of {small tilde}1000C for quartz dioriteand 900C for granodiorite. Phase equilibrium analysis for thereaction phlogopite+quartz=K-feldspar+enstatite+H2O, coupledwith an estimate of the water-saturated quartz diorite solidus,suggests that the solidus of two-pyroxene quartz diorite wasat {small tilde}780C with a mole fraction of water of {smalltilde}0•55. The composition of granodiorite is very similarto that used in several crystallization experiments and indicatesa solidus of 70025C. Estimates of oxygen fugacity, obtainedfrom equilibrium relations of olivine, orthopyroxene, and spinelin ultramafic rocks, magnetite and ilmenite in quartz diorite,and magnetite, K-feldspar, and biotite in quartz diorite andgranodiorite are 2•1–2•5 and 1•0–1•3log units above the quartz-fayalite-magnetite (QFM) buffer forgranodiorite and quartz diorite at their respective solidustemperatures; and 1•0–4•0 log units above QFMfor ultramafic rocks and quartz diorite at subsolidus temperatures.Thus, the quartz diorite magma was hotter, drier, and slightlyreduced relative to the grandiorite magma, differences thatset important constraints on the genesis of the Russian Peakmagmas. These results also indicate that quartz diorite wasundersaturated with respect to H2O as it reached its solidus,a condition that is consistent with the absence of deutericalteration in this unit. In contrast, granodiorite shows extensivedeuteric alteration and features pegmatites, quartz pods, andradial dikes as might be expected for H2O-saturated conditions. Although calcalkaline plutonic complexes present serious difficultiesin estimating the intensive parameters of crystallization, judiciousapplication of appropriate methods may result in the successfulevaluation of the conditions of crystallization of such complexes.  相似文献   

17.
The adsorption behaviour of uranyl onto seven different samples of quartz was studied in batch experiments. Sea-sand (0.1–0.3 mm), Fil-Pro 12/20 (1–2 mm) and five Min-U-Sil samples with smaller particle sizes (5, 10, 15, 30 and 40 μm) were used. The uptake curves show “pH adsorption edges” in the range of pH 4–5. A good agreement of the new data with literature data was found when plotting surface-normalised distribution coefficients versus pH. Differences in the adsorption behaviour for pre-treated and untreated sea-sand samples were detectable resulting in a shift of the pH edge to higher pH values after treatment. A literature surface complexation model was applied for blind predictions of the experimental results. The simulations described the experimental observations quite well for the Min-U-Sil samples. For the two coarser quartz samples, the calculated over-predictions were explained by the larger-than-expected measured specific surface area and measurable amounts of associated minerals, for Fil-Pro 12/20 and sea-sand, respectively. Dissolution of the samples was studied as a function of pH. After 5 days, the measured Si concentrations were all higher than equilibrium quartz solubilities, but lower than those of amorphous silica. With increasing pH, dissolved silica increased. This strongly suggests that formation of dissolved uranyl–silicato complexes have to be considered based on measured silica concentrations.  相似文献   

18.
In this paper, we examine the distribution of incompatible elements in Earth’s mantle based on data reported for 20 mantle xenoliths collected from 5 localities worldwide. A structural model combined with an element partitioning model forms the basis for our analyses. The former separates a bulk peridotite into mineral crystal lattices, interfaces (grain and interphase boundaries), and intra- and inter-granular inclusions as sites for incompatible elements. The latter relates the distribution of elements among these sites based on lattice strain theory. By treating both intra- and inter-granular inclusions as a melt-like phase, the combined models successfully reproduce the relative concentrations of incompatible elements among minerals, clean rock (reconstituted from mineral compositions and mineral mode), and whole rock. The analyses reveal common signatures in the rocks: (1) incompatible elements in the crystal lattices of olivine, orthopyroxene and clinopyroxene achieved chemical equilibrium. (2) Olivine, orthopyroxene and clinopyroxene grains contain similar amounts of an intra-granular, melt-like component possibly in the form of sub-micron inclusions with weight (≈volume) fractions between 5 × 10−5 and 1 × 10−2. (3) All rocks contain an inter-granular melt-like component with a fraction between 10−4 and 10−2, well above the amount expected to be stored along interfaces. (4) Fractions of the inter- and intra-granular components are positively correlated, indicating that they were originated from the same process. (5) The inter- and intra-granular melt-like phases are chemically equilibrated with other structural components. Based on plausible upwelling rates for mantle xenoliths, it is unlikely that the melt-like component formed during ascent. Instead, its ubiquitous appearance, its invisibility to optical microscopy, and its absorption of the incompatible elements in a manner similar to a melt phase even at sub-solidus condition, all might be explained by the presence of amorphous silica precipitates such as those observed previously in naturally occurring and experimentally annealed mantle composites. From the mineral mode, grain size, and mineral plus whole rock concentrations of incompatible elements, we can ascribe the chemical signatures of xenoliths to achievement of chemical equilibrium at mantle conditions rather than to a consequence of some disequilibrium (metasomatic) effect as has been done previously. Although it should be tested by additional analytical studies, our model will make it possible to determine whether or not a rock is chemically equilibrated in terms of the distribution of incompatible elements or if a metasomatic (disequilibrium) event is required.  相似文献   

19.
Summary Results of a multidisciplinary study on quartz concentrates (mineralogically separated) and etched concentrates (stoichiometric quartz) from three locations at Allchar (Macedonia) are presented. The investigation of quality and composition of these quartz samples is of great interest because the same material has been previously used as monitor for 26Al Acceleration Mass-Spectrometry (AMS) erosion rate estimates. Two genetically different types of quartz are distinguished in the studied samples which petrologically can be described as hydrothermally altered dacites or quartz latites; i.e. volcanic (QV) and hydrothermal (QH) quartz with relative proportions of QH:QV around 3:2. QH is genetically related to the Allchar Sb–As–Tl–S mineralization having very high Sb (85–785 ppm), As (7.6–78 ppm) and (Tl 3.3–4.0 ppm) contents. This type of quartz is also characterized by very high Li (129–138 ppm), Al (2424–2520 ppm) and Ti (153–219 ppm) concentrations. QV appears to be much less enriched in trace elements having Al and K contents ranging from 0 to 280 ppm and from 50 to 85 ppm, respectively. 26Al AMS measurements were done on the samples containing two genetically different types of quartz but this had no effects on the interpretation and erosion rate determinations. However, the extremely high Al concentrations in the analyzed quartz have generally negative effects, mainly by decreasing 26Al/27Al ratios and thus causing an increase of the detection limit. The disagreement between the results of 26Al AMS analyses and quantitative geomorphologic data for one location is probably caused by different geographical position with respect to the direction of cosmic rays.  相似文献   

20.
The major economic types of vein quartz and rock crystals from the Subpolar Urals were studied using electron paramagnetic resonance. Quartz is characterized by widely variable concentrations of aluminum and germanium paramagnetic centers. The average values and ranges of these concentrations increase from older to younger generations of quartz. The lowest content of aluminum and germanium paramagnetic centers is typical of granulated and primary fine-grained quartz; in coarse-grained quartz and rock crystals, the content is much higher. According to the data obtained, granulated and primary fine-grained quartz should be regarded as a potentially high-quality raw material for glass melting, because these quartz varieties are distinguished by the lowest contents of alien structural centers. Once mineral impurities eliminated, high-quality quartz concentrate can be produced from this quartz.  相似文献   

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