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1.
 Using a combination of dielectric spectroscopy and atomistic computer simulation techniques, the dynamical behaviour of the loosely bound (Na+ and K+) channel ions in nepheline has been investigated. The low-frequency dielectric properties of a natural Bancroft nepheline have been studied from room temperature to 1100 K. At each temperature, the dielectric constant, conductivity and dielectric loss were determined over a range of frequencies from 100 Hz to 10 MHz. At high temperatures a distinct Debye-type relaxation in the dielectric loss spectrum was observed; the activation energy for this process was determined to be 1.38 ± 0.02 eV. Atomistic simulation techniques were used to elucidate the mechanism and energetics of cation migration. A mechanism involving the hopping of Na+ ions between oval sites and partially occupied hexagonal (K+) sites, via a bottleneck consisting of a distorted sixfold ring of (Al,Si)O4 tetrahedra, was found to give a calculated energy barrier in very good agreement with the experimentally determined activation energy. These results confirm the nature of the process responsible for the observed dielectric behaviour. Overall, this study demonstrates the intrinsic, microscopic control of cation diffusion processes in rock-forming minerals. Identifying specific energy barriers and preferred diffusion pathways is fundamental to the prediction of diffusion energetics. Received: 8 May 2000 / Accepted: 21 July 2000  相似文献   

2.
In the Tuoshi oilfield,located in the Cenozoic Jianghan Basin of southeastern China ,there have been found hydrocarbon reservoirs hosted in lacustrine sandstones of the Eogene Xingouzui Formation.The main diagenetic features identified in these sandstones include the dissolution of detrital K-feldspar and albite grains,the precipitation of quartz as overgrowths and /or cements ,and the precipitation and /or transformation of clay minerals.These diagenetic features were interpreted to have occurred in early,intermediate and late stages,based on the burial depth.The kinetics of fluid-mineral reactions and the concentrations of aqueous species au each stage of diagenesis were simulated numerically for these lacustrine sandstones,using a quasi-sta-tionary state approximation that incorporates simultaneous chemical reactions in a time-space continuum.During the early diagenetic stage,pore fluid was weakly acidic,which resulted in dissolution of K-feldspar and albite and,therefore,led to the release of K^ ,Na^ ,Al^3 and SiO2(aq) into the diagenetic fluid.The increased K^ ,Na^ ,Al^3 and SiO2(aq) concentrations in the diagenetic fluid caused the precipitation of quartz,kaolinite and illite.At the beginning of the intermediate diagenetic stage the concentration of H^ was built up due to the decomposition of organic matter,which was responsible for further dissolution of K-feldspar and albite and pre-cipitation of quartz,kaolinite,and illite.During the late diagenetic stage,the pore fluid was weakly alkaline,K-feldspar became stable and was precipitated with quartz and clay minerals.When the burial depth was greater than 3000 m,the pore fluids became supersaturated with respect to allbite,but undersaturated with respect to quartz,resulting in the precipitation of albite and the dissolution of quartz.The diagenetic reactions forecasted in the numerical modeling closely matched the diagenet-ic features identified by petrographic examination, and therefore,can help us to gain a better understanding of the diagenetic processes and associated porosity evolution in sandstone reservoirs.  相似文献   

3.
Electrical conductivity measurements on dry polycrystalline K-feldspar were performed at 1.0 to 3.0 GPa and 873 to 1,173 K with a multi-anvil high-pressure apparatus and the Solartron-1260 Impedance/Gain Phase Analyzer in the frequency range of 10?1 to 106 Hz. At each temperature the complex impedance displays a perfect semi-circular arc that represents the grain-interior conduction. Under the experimental conditions, electrical conductivity exponentially increases with increasing temperature and slightly decreases with increasing pressure; however, the effect of pressure on the conductivity is less pronounced than that of temperature. The activation enthalpy decreases slightly from 0.99 to 1.02 eV with increasing pressure, and the activation energy and activation volume for K-feldspar are 0.98 eV and 1.46?±?0.17 cm3/mol, respectively. According to these Arrhenius parameters, ionic conduction is proposed to be the dominant conduction mechanism in K-feldspar at high temperatures and pressures, and potassium ions are the charge carriers transporting by an interstitial mechanism. The diffusion coefficient of potassium at high temperatures was calculated from our conductivity data on K-feldspar using Nernst–Einstein equation, and the results were compared with the previous experimental results.  相似文献   

4.
广汉市平原区浅层地下水化学演化及其控制因素   总被引：4，自引：0，他引：4  
为在广汉市城乡规划过程中提供地下水资源开发利用的基础信息,采用矿物风化系统分析、相关性分析、主成分分析和PhreeQC反向水文地球化学模拟等方法对广汉市平原区的浅层地下水的地下水水化学组分进行分析,确定了浅层地下水的水化学演化及控制因素,完成了地下水资源的质量及时空分布特征分析。分析表明：Gibbs图显示岩石风化主导该区地下水水化学特征,风化过程产生离子和次生矿物又经历水解作用,在矿物风化系统稳定场图中显示水样中铝硅酸盐矿物逐渐趋于溶解,碳酸盐矿物处于饱和状态;PhreeQC反向水文地球化学模拟结果显示在水流模拟路径上主要发生了钙蒙脱石、钾长石溶解和高岭石、石英、钠长石的沉淀,以及Ca-Na2之间的阳离子交换吸附作用;离子相关性和主成分分析进一步的验证了溶滤作用、蒸发浓缩作用和阳离子交换吸附作用是引起浅层地下水水化学过程和矿物组成改变的主要原因。研究区地下水水质总体不会对人体健康造成不良影响。  相似文献   

5.
At very low fluid/rock mass ratios the hydrothermal alteration process corresponds to isochemical recrystallisation of the primary rock. The resulting full equilibrium assemblage with the composition of an average crustal rock contains the phases albite, K-feldspar, K-mica, biotite, quartz and (depending on temperature) epidote, prehnite or one of the Ca-zeolites. Relative Na+, K+, Mg2+ and Ca2+—solution activities in such a rock-dominated alteration system are uniquely fixed and provide useful reference points with regard to the degree of attainment of full fluid/rock equilibrium. With increasing fluid/rock mass ratios the composition of now increasingly fluid-dominated alteration assemblages is determined by the interplay of three major processes: hydrogen metasomatism as a function of CO2 reactivity increasing with the horizontal distance from major fluid upflow zones and leading to the formation of Al-enriched alteration assemblages; potassium metasomatism accompanied by silicification in or close to major fluid upflow zones leading to potassic and phyllic alteration; sodium, magnesium, calcium metasomatism associated with descending and heating solutions leading to propylytic alteration of recharge zones. Two new parameters, reactivity and exchangeability, determining the effectiveness of fluid components with respect to hydrothermal alteration are introduced.  相似文献   

6.
 Computer simulation techniques have been used to investigate the energetics of defect formation in Albite and to calculate O and (OH) migration activation energies. We find that the Na Frenkel defect has the lowest formation energy, whilst interstitials associated with impurities are the most favourable O defects. Water can be accommodated in the albite structure as both OH groups and as H2O molecules with solution energy of 0.73 eV and 0.9 eV respectively. The activation energies for O migration is reduced by up to 50% when the O is migrating as part of an (OH) group. In addition, we find a marked diffusional anisotropy for both O and (OH) in albite. Received: 15 September 1995 / Accepted: 29 April 1996  相似文献   

7.
The mechanism of re-equilibration of albite in a hydrothermal fluid has been investigated experimentally using natural albite crystals in an aqueous KCl solution enriched in 18O at 600°C and 2 kbars pressure. The reaction is pseudomorphic and produces a rim of K-feldspar with a sharp interface on a nanoscale which moves into the parent albite with increasing reaction time. Transmission electron microscopy (TEM) diffraction contrast and X-ray powder diffraction (XRD) show that the K-feldspar has a very high defect concentration and a disordered Al, Si distribution, compared to the parent albite. Raman spectroscopy shows a frequency shift of the Si-O-Si bending vibration from ~476 cm−1 in K-feldspar formed in normal 16O aqueous solution to ~457 cm−1 in the K-feldspar formed in 18O-enriched solution, reflecting a mass-related frequency shift due to a high enrichment of 18O in the K-feldspar silicate framework. Raman mapping of the spatial distribution of the frequency shift, and hence 18O content, compared with major element distribution maps, show a 1:1 correspondence between the reaction rim formed by the replacement of albite by K-feldspar, and the oxygen isotope re-equilibration. The textural and chemical characteristics as well as the kinetics of the replacement of albite by K-feldspar are consistent with an interface-coupled dissolution-reprecipitation mechanism.  相似文献   

8.
The electrical conductivities of alkali feldspar solid solutions ranging in chemical composition from albite (NaAlSi3O8) to K-feldspar (KAlSi3O8) were measured at 1.0 GPa and temperatures of 873–1,173 K in a multi-anvil apparatus. The complex impedance was determined by the AC impedance spectroscopy technique in the frequency range of 0.1–106 Hz. Our experimental results revealed that the electrical conductivities of alkali feldspar solid solutions increase with increasing temperature, and the linear relationship between electrical conductivity and temperature fits the Arrhenius formula. The electrical conductivities of solid solutions increase with the increasing Na content at constant temperature. At 1.0 GPa, the activation enthalpy of solid solution series shows strong dependency on the composition, and there is an abrupt increase from the composition of Or40Ab60 to Or60Ab40, where it reaches a value of 0.96 eV. According to these results in this study, it is proposed that the dominant conduction mechanism in alkali feldspar solid solutions under high temperature and high pressure is ionic conduction. Furthermore, since the activation enthalpy is less than 1.0 eV for the alkali feldspar solid solutions, it is suggested to be a model where Na+ and K+ transport involves an interstitial mechanism for electrical conduction. The change of main charge carriers can be responsible for the abrupt increase in the activation energy for Or60Ab40. All electrical conductivity data were fitted by a general formula in order to show the dependence of activation enthalpy and pre-exponential factor on chemical composition. Combining our experimental results with the effective medium theory, we theoretically calculated the electrical conductivity of alkali feldspar granite, alkali feldspar quartz syenite, and alkali feldspar syenite with different mineral content and variable chemical composition of alkali feldspar at high temperatures at 1.0 GPa, and the calculated results are almost in agreement with previous experimental studies on silicate rocks.  相似文献   

9.
Cathodoluminescence (CL) spectra of unirradiated, He+ ion-implanted and electron-irradiated plagioclase minerals contain the following emission bands: (1) below 300 nm due to Pb2+, (2) at ~320 and ~350 nm to Ce3+, (3) at 380–420 nm to Eu2+, Ti4+ and/or Al–O?–Al/Ti defects, (4) at 560–580 nm to Mn2+ and (5) at 720–760 nm to Fe3+. During the implantation of He+ ion, much of their energy may be dissipated by partial destruction and strain of the feldspar framework, resulting in quenching of CL. Deconvolution of CL spectra acquired from albite and oligoclase reveals an emission component at 1.86 eV (666 nm) assigned to a radiation-induced defect center associated with Na+ atoms. As its intensity increases with radiation dose, this emission component has potential for geodosimetry and geochronometry. Electron irradiation causes Na+ migration in plagioclase, and then a considerable reduction in intensity of emissions assigned to impurity centers, which is responsible for an alteration in the energy state or a decrease in luminescence efficiency following the change of activation energy. Emission intensity at 1.86 eV positively correlates with electron irradiation time for unimplanted and He+ ion-implanted albite and oligoclase, but negatively for the implanted albite above 1.07 × 10?4 C/cm2. It implies that radiation halo produced by α-particles should not be measured using CL spectroscopy to estimate β radiation dose on albite in the high radiation level.  相似文献   

10.
Abstract Microprobe analyses of feldspars in granite mylonites containing flame perthite give compositions that invariably plot as three distinct clusters on a ternary feldspar diagram: orthoclase (Or92–97), albite and oligoclase-andesine. The albite occurs as grains in the matrix, as flame-shaped lamellae in orthoclase, and in patches within plagioclase grains. We present a metamorphic model for albite flame growth in the K-feldspar in these rocks that is related to reactions in plagioclase, rather than alkali feldspar exsolution. Flame growth is attributed to replacement and results from a combination of two retrograde reactions and one exchange reaction under greenschist facies conditions. Reaction 1 is a continuous or discontinuous (across the peristerite solvus) reaction in plagioclase, in which the An component forms epidote or zoisite. Most of the albite component liberated by Reaction 1 stays to form albite in the host plagioclase, but some Na migrates to form the flames within the K-feldspar. Reaction 2 is the exchange of K for Na in K-feldspar. Reaction 3 is the retrograde formation of muscovite (as ‘sericite’) and has all of the chemical components of a hydration reaction of K-feldspar. The Si and Al made available in the plagioclase from Reaction 1 are combined with the K liberated from the K-feldspar, to produce muscovite in Reaction 3. The muscovite forms in the plagioclase, rather than the K-feldspar, as a result of the greater mobility of K relative to Al. The composition of the albite flames is controlled by both the peristerite and the alkali feldspar miscibility gaps and depends on the position of these solvi at the pressure and temperature that existed during the reaction. Using an initial plagioclase composition of An20, the total reaction can be summarized as: 20 oligoclase + 1 K-feldspar + 2 H2O = 2 zoisite + muscovite + 2 quartz + 15 albiteplagioclase+ 1 albiteflame. This model does not require that any additional feldspar framework be accreted at replacement sites: Na and K are the only components that must migrate a significant distance (e.g. from one grain to the next), allowing Al to remain within the altering plagioclase grain. The resulting saussuritization is isovolumetric. The temperature and extent of replacement depends on when, and how much, water infiltrates the rock. The fugacity of the water, and therefore the pressure of the fluid, may have been significantly lower than lithostatic during flame growth.  相似文献   

11.
Naturally deformed feldspars from foliated granites in a shear zone in Newfoundland exhibit transitional brittle-ductile behaviour. Brittle failure is subordinate to dynamic recrystallization, microcracking, strain enhanced diffusion and reaction enhanced ductility during the deformation. Both plagioclase (An28) and K-feldspar are transformed to albite with increasing strain. Interaction of metamorphic and structural processes at the grain scale is emphasised. This is illustrated with examples of quartz-filled veins (segregation bands) in plagioclase and recrystallized polycrystalline aggregates in plagioclase and K-feldspar. The role of microcracking in plagioclase and of pre-existing internal growth structures in the formation of initially coarse grained recrystallized aggregates from large single crystals is suggested.  相似文献   

12.
Qiongdongnan Basin has a tectonic geological background of high temperature and high pressure in a deep reservoir setting,with mantle-derived CO2.A water-rock reaction device was used under high temperature and high pressure conditions,in conjunction with scanning electron microscope(SEM)observations,to carry out an experimental study of the diagenetic reaction between sandstone at depth and CO2-rich fluid,which is of great significance for revealing the dissolution of deep clastic rock reservoirs and the developmental mechanism of secondary pores,promoting deep oil and gas exploration.In this study,the experimental scheme of the water-rock reaction system was designed according to the parameters of the diagenetic background of the deep sandstone reservoir in the Qiongdongnan Basin.Three groups of single mineral samples were prepared in this experiment,including K-feldspar samples,albite samples and calcite samples.Using CO2 as a reaction solution,a series of diagenetic reaction simulation experiments were carried out in a semi-closed high temperature and high pressure simulation system.A field emission scanning electron microscope(SEM)was used to observe the microscopic appearance of the mineral samples after the water-rock reaction,the characteristics of dissolution under high temperature and high pressure,as well as the development of secondary pores.The experimental results showed that the CO2-rich fluid has an obvious dissolution effect on K-feldspar,albite and calcite under high temperature and high pressure.For the three minerals,the main temperature and pressure window for dissolution ranged from 150℃to 300℃and 45 MPa to 60 MPa.Scanning electron microscope observations revealed that the dissolution effect of K-feldspar is most obvious under conditions of 150℃and 45 MPa,in contrast to conditions of200℃and 50 MPa for albite and calcite.Through the comparative analysis of experimental conditions and procedures,a coupling effect occurred between the temperature and pressure change and the dissolution strength and calcite.Under high temperature and high pressure,pressure changed the solubility of CO2,furthermore,the dissolution effect and strength of the sandstone components were also affected.The experiment revealed that high temperature and high pressure conditions with CO2-rich fluid has a significant dissolution effect on aluminosilicate minerals and is conducive to the formation of secondary pores and effective reservoirs.Going forward with the above understanding has important implications for the promotion of deep oil and gas exploration.  相似文献   

13.
Dehydration melting of muscovite in metasedimentary sequences is the initially dominant mechanism of granitic melt generation in orogenic hinterlands. In dry (vapour-absent) crust, muscovite reacts with quartz to produce K-feldspar, sillimanite, and monzogranitic melt. When water vapour is present in excess, sillimanite and melt are the primary products of muscovite breakdown, and any K-feldspar produced is due to melt crystallization. Here we document the reaction mechanisms that control nucleation and growth of K-feldspar, sillimanite, and silicate melt in the metamorphic core of the Himalaya, and outline the microstructural criteria used to distinguish peritectic K-feldspar from K-feldspar grains formed during melt crystallization. We have characterized four stages of microstructural evolution in selected psammitic and pelitic samples from the Langtang and Everest regions: (a) K-feldspar nucleates epitaxially on plagioclase while intergrowths of fibrolitic sillimanite and the remaining hydrous melt components replace muscovite. (b) In quartzofeldspathic domains, K-feldspar replaces plagioclase by K+–Na+ cation exchange, while melt and intergrowths of sillimanite+quartz form in the aluminous domains. (c) At 7–8 vol.% melt generation, the system evolves from a closed to open system and all phases coarsen by up to two orders of magnitude, resulting in large K-feldspar porphyroblasts. (d) Preferential crystallization of residual melt on K-feldspar porphyroblasts and coarsened quartz forms an augen gneiss texture with a monzogranitic-tonalitic matrix that contains intergrowths of sillimanite+tourmaline+muscovite+apatite. Initial poikiloblasts of peritectic K-feldspar trap fine-grained inclusions of quartz and biotite by replacement growth of matrix plagioclase. During subsequent coarsening, peritectic K-feldspar grains overgrow and trap fabric-aligned biotite, resulting in a core to rim coarsening of inclusion size. These microstructural criteria enable a mass balance of peritectic K-feldspar and sillimanite to constrain the amount of free H2O present during muscovite dehydration. The resulting modal proportion of K-feldspar in the Himalayan metamorphic core requires vapour-absent conditions during muscovite dehydration melting and leucogranite formation, indicating that the generation of large volumes of granitic melts in orogenic belts is not necessarily contingent on an external source of fluids.  相似文献   

14.
The behaviour of two silicates (albite and olivine) toward aqueous dissolution has been investigated with emphasis to two aspects: (1) similarities and differences with silicate glasses; and (2) influence of ion bombardment which transforms the initially crystalline structure to a glass-like one. The occurrence on leached materials of hydrated layers enriched with metallic elements is checked by using high-energy ion beam analytical techniques.

It is shown that for both amorphous and crystalline silicates, dissolution involves competing processes responsible for either hydrated-layer generation (hydration by ion exchange and/or water permeation), or its destruction (hydrolysis of siloxane bonds and detachment of silica units). At moderate temperature ( 100°C), the latter processes dominate for crystalline silicates and the resulting dissolution is congruent. However, these silicates tend to behave like glasses upon ion bombardment above a critical dose. At higher temperature (> 200°C), hydration is markedly enhanced and thick altered layers are generated even on undamaged minerals such as albite and orthoclase, where alkali cations are easily exchangeable.  相似文献   


15.
Textural and geochemical studies of inclusions in topaz from greisens in the Hensbarrow topaz granite stock (St. Austell, Cornwall) are used to constrain the composition of fluids responsible for late stage greisening and mineralisation. The topaz contains an abundant and varied suite of inclusions including aqueous liquid + vapour (L + V), quartz, zinnwaldite, albite, K-feldspar, muscovite, ilmenorutile, apatite, columbite, zircon, varlamoffite [(Sn, Fe)(O, OH)2] and qitianlingite [(Fe+2,Mn+2)2(Nb,Ta)2W+6O10]. Primary L + V inclusions in topaz show relatively high T h (mainly 300 to >500 °C) and a narrow range of salinities (23–30 wt % NaCl equivalent) compared with those in greisen quartz (150–450 °C, 0–50 wt % NaCl equivalent). Textures indicate that topaz formed earlier than quartz and the fluid inclusion data are interpreted as indicating a cooling of the hydrothermal fluids during greisenisation, mixing with meteoric waters and a decrease in pressure causing intermittent boiling. The presence of early-formed albite and K-feldspar as inclusions in the topaz is likely to indicate that the greisen-forming fluid became progressively more acid during greisenisation. The most distinctive inclusions in the topaz are wisp- and bleb-shaped quartz, < 50 μm in size, which show textural characteristics indicating former high degrees of plasticity. They often have multiple shrinkage bubbles at their margins rich in Sn, Fe, Mn, S and Cl and, more rarely, contain euhedral albite, K-feldspar, stannite or pyrrhotite crystals up to 40 μm in size. The quartz inclusions show similar morphologies to inclusions in topaz from quartz-topaz rocks elsewhere which have been interpreted as trapped “silicate melt”. Their compositions are, however, very different to those expected for late stage topaz-normative granitic melts. From their textural and chemical characteristics they are interpreted as representing crystallised silica colloid, probably trapped as a hydro gel during greisenisation. There is also evidence for the colloidal origin of inclusions of varlamoffite in the topaz. These occurrences offer the first reported evidence in natural systems for the formation of colloids in high temperature hydrothermal fluids. Their high ore carrying potential is suggested by the presence of varlamoffite and the occurrence of stannite, pyrrhotite and SnCl within the quartz inclusions. Received: 9 April 1996 / Accepted: 12 November 1996  相似文献   

16.
To investigate the mechanisms for the cubic-tetragonal phase transition in leucite (KAlSi2O6), the frequency dependence of the dielectric constant (), and of the electric conductivity (), have been measured as a function of temperature. The dielectric loss function, tan , contains two main features: (i) a classical Debye peak, with activation energy of 0.77 eV, ascribed to hopping of K-atoms between their channel (W) sites, via the vacant side-channel (S) sites; (ii) a heavily-overdamped relaxational mode which softens when the crystal is cooled towards the phase transition temperature. The latter relaxational mode shows a critical behaviour, and is thus directly correlated with the transition mechanism. As it is only the potassium ions that could relax at frequencies well below the optical phonon branches, it appears that their movement is relaxational (i.e. heavily overdamped) rather than phonon-like. At temperatures above the transition point, the relaxation of K+ in an electric field is analysed in terms of collective motions within tetragonal domains. Direct evidence for the existence of such domains follows from the presence of diffuse intensity in single-crystal X-ray diffraction experiments.  相似文献   

17.
As a part of the safety assessment of the geological disposal of high-level radioactive waste, the effects of dry density and exchangeable cations on the diffusion process of Na+ ions in compacted bentonite were studied from the viewpoint of the activation energy for diffusion. The apparent self-diffusion coefficients of Na+ ions in compacted Na-montmorillonite and in a Na- and Ca-montmorillonite mixture were determined by one-dimensional, non-steady diffusion experiments at different temperatures and dry densities. A unique change in activation energy as a function of dry density was found for the Na+ ions in compacted Na-montmorillonite. The activation energy suddenly decreased from 18.1 to 14.1 kJ mol− 1 as the dry density increased from 0.9 to 1.0 Mg m− 3, whereas it increased to 24.7 kJ mol− 1 as the dry density increased to 1.8 Mg m− 3. Examination of the effect of exchangeable cations on the activation energies determined that the activation energies were almost constant, approximately 25 kJ mol− 1, for the montmorillonite specimens at a dry density of 1.8 Mg m− 3. However, three different activation energy values were obtained at a dry density of 1.0 Mg m− 3. These findings cannot be explained by the conventional diffusion model (the pore water diffusion model), which suggests that the predominant diffusion process alternates among pore water diffusion, interlayer diffusion, and external surface diffusion.  相似文献   

18.
The possibility of shock wave-induced interaction between meteoritic iron was estimated based on the results of experiments on the shock wave loading of mixtures of kamacite from the Sikhote Alin iron meteorite with quartz, albite, oligoclase, enstatite, olivine, and serpentine. The experimental samples were then examined with the application of optical microscopy, microprobe analysis, and M?ssbauer spectroscopy. As a result of shock wave load, the metal was proved to become enriched in Si, while the quartz, albite, and oligoclase melted glasses acquired bivalent Fe ions. The products of our experiments with quartz and feldspar mixtures with kamacite were determined to contain paramagnetic metallic iron, and the surroundings of iron atoms in the silicate constituent of the olivine and enstatite mixtures with kamacite become locally more heterogeneous. Our results indicate that shock waves induce redox reactions between Fe and silicates according to the scheme 2Fe+2 + Si+4 = 2Fe+2 + Si0, where Fe0 and Si0 are iron and silicon in metal and Fe+2 and Si+4 are iron and silicon in the sillimanite matrix.  相似文献   

19.
The diffusion properties of Na, Cs, Sr, Ba, Co, Mn, Fe and Sc ions in a basaltic and an andesitic melt have been determined experimentally using the radiotracer residual-activity method, and narrow platinum capillaries, over the temperature range 1,300–1,400° C. Diffusion of all cations follows an Arrhenius relationship; the values of the activation energies range from 24 kcal mol–1 for Na to 67 kcal mol–1 for Co in the andesitic melt, and from 39 kcal mol–1 for Na to 65 kcal mol–1 for Cs in the basaltic melt. Relative diffusivities in the basaltic melt, but not in the andesitic melt, correlate with assumed ionic radii values. Each cation, except Na+, diffuses faster in the basaltic melt than in the andesitic melt over the studied temperature range. Sodium shows similar diffusivity in the two melts.Compensation diagrams incorporating new and some previously-published data indicate that Cs probably diffuses by different mechanisms in different silicate glass and melt systems. Iron has a relatively high activation energy which is consistent with its part occupancy of tetrahedral co-ordination polyhedra.  相似文献   

20.
Abstract

Purified albite powder (44-53 μm) has been sintered to form an albite polycrystal suitable for deformation studies close to the melting temperature. Experiments have been carried out in Griggs solid medium deformation apparatus at 800, 1020 °C and 700 MPa pressure in a dehydrating pyrophyllite confining medium at constant strain rates of 10?4, 10?5, 10?6 and 10?7/s. At 800 °C the samples were brittle-ductile whereas at 1020 °C they were ductile with a rheology well described by a power law with a stress exponent of 3. The transition from brittle-ductile to ductile also coincided with the order-disorder or low-high albite transition, as indicated by the marked increase in mechanical twinning on the albite law at high temperature. At 1020 °C high dislocation densities (10?10 - 10?11/cm2 and mechanical twinning characterised the original high albite grains, whereas fine recrystallised grains ( < 5 цm) had low dislocation densities (107 - 108/cm2) and often contained polysynthetic albite and pericline (M-type) twins. It is suggested that the recrystallized grains were monalbite (monoclinic) under test conditions which have inverted to high albite (triclinic) and in so doing produced M-twins, and that the recrystallization mechanism involved grain-boundary bulging to nucleate new high angle boundaries. The implications of the order-disorder transition for twinning and grain boundary migration are discussed and it is suggested that the data cannot be simply extrapolated to natural deformation in the low albite field.  相似文献   

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