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1.
The synthesis of an analogue of the organic mineral evenkite included preparation of a mixture of ten individual molecular homologues of normal paraffin C n H2n + 2 (n = 19–28), melting of this mixture, and subsequent slow cooling under controlled conditions. The symmetrical distribution of these homologues by the number of carbon atoms in the molecule (dominant homologue n = 23) and the X-ray pattern of synthesized evenkite—a ten-component solid solution of n-paraffins—correspond to the characteristics of natural evenkite.  相似文献   

2.
Natural normal paraffin hydrocarbons C n H2n + 2 (n = 17–41) of a biological origin were investigated in terms of their composition, structure, polymorph modification, and thermal phase transition. Paraffin compositions were studied from beeswax (honeycombs and their cappings), plant wax (apple and pumpkin rinds), as well as from different parts of rat cerebrum (cortex, subcortex, brainstem, cerebellum, cerebral hemispheres, myelin, chromatin, etc.). X-ray diffraction analysis (XRD), chromatography, and high-temperature XRD, as well as original analytical methods elaborated upon by the authors, were used. The beeswax paraffin composition that corresponds to the six-component solid solution with a superperiodic four-layer orthorhombic cell was subjected to experimental modeling for the first time. The prepared mixture simulated a paraffin composition (n = 23, 25, 27, 29, 31, 33) and the asymmetric distribution of homologues by n.  相似文献   

3.
Stable isotope data of precipitation (δ18Op and deuterium excess), drip water (δ18Od), and modern calcite precipitates (δ18Oc and δ13Cc) from Yongxing Cave, central China, are presented, with monthly sampling intervals from June 2013 to September 2016. Moderate correlations between the monthly variation of δ18Op values (from ??11.5 to ??0.7‰) and precipitation amount (r = ??0.59, n?=?34, p?<?0.01) and deuterium excess (r?=?0.39, n?=?31, p?<?0.01) imply a combined effect of changes in precipitation amount and atmospheric circulation. At five drip sites, the δ18Od values have a much smaller variability (from ??9.1 to ??7.5‰), without seasonal signals, probably a consequence of the mixing in the karst reservoir with a deep aquifer. The mean δ18Od value (??8.4‰) for all drip waters is significantly more negative than the mean δ18Op value (??6.9‰) weighted by precipitation amount, but close to the wet season (May to September) mean value (??8.3‰), suggesting that a threshold of precipitation amount must be exceeded to provide recharge. Calculation based on the equilibrium fractionation factor indicates that the δ18Oc values are not in isotopic equilibrium with their corresponding drip waters, with a range of disequilibrium effects from 0.4 to 1.4‰. The δ18Oc and δ13Cc values generally increase progressively away from the locus of precipitation on glass plates. The disequilibrium effects in the cave are likely caused by progressive calcite precipitation and CO2 degassing related to a high gradient of CO2 concentration between drip waters and cave air. Our study provides an important reference to interpret δ18Oc records from the monsoon region of China.  相似文献   

4.
The isotopic systematics of noble gases (He and Ar) were studied in Neoarchean and Paleoproterozoic lower crustal xenoliths from the Belomorian mobile belt. The xenolith suite is dominated by garnet granulites (Grt + Cpx + Pl ± Opx ± Qtz ± Kfs ± Phl ± Hbl) and two-pyroxene or garnet pyroxenites (Cpx + Pl ± Grt ± Opx ± Hbl ± Qtz). The xenoliths and the host Devonian ultramafic lamprophyres forming diatremes and explosion dikes contain fluid with similar He and Ar isotopic compositions. It was found that the fluid was trapped by the rocks and xenolith minerals approximately simultaneously with the formation of the lamprophyres. This conclusion is based on the identical K-Ar ages of the majority of xenoliths and ultramafic lamprophyres. When the xenoliths were transported toward the surface by high-temperature ultrabasic melts, the noble gases occurring in them were partly (Ar) or completely (He) lost. The melts were contaminated by meteoric waters during their emplacement in the upper crust, which resulted in that the Ar isotopic composition of lamprophyres approached the composition of atmospheric Ar. The fluid phase that was liberated during melt crystallization severely affected the xenoliths, diminishing the difference between the isotope compositions of He and Ar in the xenoliths and ultramafic lamprophyres. The He isotope composition includes an admixture of mantle 3He, which is suggested by the high measured 3He concentrations, exceeding the calculated values, and high 3He/36Ar ratios in the xenoliths and their host lamprophyres. The fraction of the mantle component in the fluid trapped by the xenoliths was estimated as ～20%.  相似文献   

5.
The lower part of Shayi Member (Es1x Sub-Member) composed mainly of dark mudstones and shales is the dominant source rocks for the Lixian Slope. Based on organic petrology, organic and inorganic geochemistry analyses of several mudstone and shale samples selected from Es1x Sub-Member, this research provides an overview on type, origin and thermal maturity of organic matters, as well as depositional environment of Es1x Sub-Member. Kerogen microscopy observation shows that the macerals are dominated by sapropelinite with a significant mixture of vitrinite and inertinite, indicating that aquatic algal-bacterial organic matter inputs are dominate with a significant contribution of terrigenous organic matter inputs. This statement is supported by n-alkane patterns distribution characteristics, high (n-C21 + n-C22)/(n-C28 + n-C29) values (average = 1.77), the plot of high Ph/n-C18 values (average = 4.15) versus low Pr/n-C17 values (average = 1.13), and high proportion of C27 sterane and C29 sterane (average = 37.7 and 42.0%, respectively). In addition, the rather low Pr/Ph values (average = 0.38), high gammacerane index values (average = 0.30), high V/Ni and V/(V + Ni) values (average = 11.84 and 0.89, respectively), high Sr/Ba and Sr/Cu values (average = 8.54 and 108, respectively), indicative of a saline water condition and a anoxic depositional environment. The low C29 sterane ααα 20S/(20S + 20R), C29 sterane αββ/(αββ + ααα), C31 homohopane 22S/(22S + 22R), C32 homohopane 22S/(22S + 22R), Ts/(Ts + Tm) values and relatively high moretane/hopane values show that the level of thermal maturity of organic matters in Es1x Sub-Member are low.  相似文献   

6.
Bubbles grow in decompressing magmas by simple expansion and by diffusive supply of volatiles to the bubble/melt interface. The latter phenomenon is of significant geochemical interest because diffusion can fractionate elements and isotopes (or isotopologues) of dissolved components. This raises the possibility that the character of volatile components in bubbles may not reflect that of volatiles dissolved in the host melt over the lifetime of a bubble—even in the absence of equilibrium vapor/melt isotopic fractionation. Recent experiments have confirmed the existence of an isotope mass effect on diffusion of the volatile element Cl in silicate melt [Fortin et al. (Isotopic fractionation of chlorine during chemical diffusion in a dacitic melt and its implications for isotope behavior during bubble growth (abstract), 2016 Fall AGU Meeting, 2016)], so there is a clear need to understand the efficacy of diffusive fractionation during bubble growth. In this study, numerical models of diffusion and mass redistribution during bubble growth were implemented for both “passive” volatiles—those whose concentrations are generally well below saturation levels—and “active” volatiles such as CO2 and H2O, whose elevated concentrations and limited solubilities are the cause of bubble nucleation and growth. Both diffusive and convective bubble-growth scenarios were explored. The magnitude of the isotope mass effect on passive volatiles partitioned into bubbles growing at a constant rate R in a static system depends upon R/D L, K d and D H/D L (K d = bubble/melt partition coefficient; D H/D L = diffusivity ratio of the heavy and light isotopes). During convective bubble growth, the presence of a discrete (physical) melt boundary layer against the growing bubble (of width x BL) simplifies outcomes because it leads to the quick onset of steady-state fractionation during growth, the magnitude of which depends mainly upon R?x BL/D L and D H/D L (bubble/melt fractionation is maximized at R?x BL/D L ≈0.1). Constant R is unrealistic for most real systems, so other scenarios were explored by including the solubility and EOS of an “active” volatile (e.g., CO2) in the numerical simulations. For plausible decompression paths, R increases exponentially with time—leading, potentially, to larger isotopic fractionation of species partitioned into the growing bubble. For volatile species whose isotope mass effects on diffusion have been measured (Cl, Li), predicted isotope fractionation in the exsolved vapor can be as large as ?4‰ for Cl and ?25‰ for Li.  相似文献   

7.
The purpose of the investigation is to reveal the dependences of P p = f(C p) on reservoir conditions and the lithological composition of rocks. The samples were studied using a set of lithological-petrographic investigations. To obtain the dependences on thermobaric conditions, 90 samples of different porosities (C p) of 15, 20, and 25% were collected. As a result, the general pattern of the change in the rock resistivity during the transition from atmospheric conditions of measuring to reservoir conditions was established. Dependences of porosity parameter P p on porosity coefficient C p were obtained for three values of formation water salinity and three reservoir conditions. The measurement errors of the porosity parameter P p were calculated using dependences obtained under atmospheric conditions.  相似文献   

8.
SDO/HMI and SDO/AIA data for the 24th solar-activity cycle are analyzed using a quicker and more accurate method for resolving π ambiguities in the transverse component of the photospheric magnetic field, yielding new results and confirming some earlier results on the magnetic properties of leading and following magnetically connected spots and single spots. The minimum inclination of the field lines to the positive normal to the solar surface α min within umbrae is smaller in leading than in following spots in 78% of the spot pairs considered; the same trend is found for the mean angle 〈α〉 in 83% of the spot pairs. Positive correlations between the α min values and the 〈α〉 values in leading and following spots are also found. On average, in umbrae, the mean values of 〈B〉, the umbra area S, and the angles α min and 〈α〉 decrease with growth in the maximum magnetic field B max in both leading and following spots. The presence of a positive correlation between B max and S is confirmed, and a positive correlation between 〈B〉 and S in leading and following spots has been found. Themagnetic properties of the umbrae of magnetically connected pairs of spots are compared with the contrast of the He II 304 emission above the umbrae, C 304. Spots satisfying certain conditions display a positive correlation between C 304?L and 〈α L 〉 for the leading (L) spots, and between C 304?L /C 304?F and l L /l F , where l L (l F ) are the lengths of the field lines connecting leading (L) or following (F) spots from the corresponding spot umbrae to the apex of the field line.  相似文献   

9.
This study quantifies the influence of various intrinsic soil properties including particle roundness, R, sphericity, S, 50% size by weight, D 50, coefficient of uniformity, C u, and the state property of relative density, D r, on the compression and recompression indices, C c and C r, of sands of various geologic origins at pre-crushing stress levels. Twenty-four sands exhibiting a wide range of particle shapes, gradations, and geologic origins were collected for the study. The particle shapes were determined using a computational geometry algorithm which allows characterization of a statistically large number of particles in specimens. One dimensional oedometer tests were performed on the soils. The new data was augmented with many previously published results. Through statistical analyses, simple functional relationships are developed for C c and C r. In both cases, the models utilized only R and D r since other intrinsic properties proved to have lesser direct influence on the compression indices. However, previous studies showed that the contributions of S and C u are felt through their effects on index packing void ratios and thus on D r. The accuracy of the models was confirmed by comparison of predicted and observed C c and C r values.  相似文献   

10.
Twenty-eight CS molecular clouds toward HII regions with Galactocentric distances from ～ 4 to 20 kpc have been studied based on observations obtained in the J=2→1 lines of CS and C34S on the 20-meter radio telescope of the Onsala Space Observatory (Sweden) in March 2001. All 28 clouds have been mapped with an angular resolution of ～40″. The peak intensity in the C34S line has been measured for 20 objects. An LTE analysis has been performed and the parameters of the molecular cloud cores derived. The core sizes are dA=0.3–4.8 pc, with a median value of ～1.6 pc. The mean hydrogen densities in the cloud cores are nH2=3.5×102–3.7 × 104 cm?3, with a median value of ～7.2×103 cm?3. The value of nH2 ends to decrease with increasing Galactocentric distance of the cloud. The masses of most clouds are 102?6×103M⊙, with the most probable value being MCS～103M⊙. The data follow the dependence MCS ∝d A (2.4–3.2) . As a rule, the cloud masses are lower than the virial masses for MCS<103M⊙.  相似文献   

11.
A high-pressure single-crystal X-ray diffraction study has been carried out on a P21/c natural Mg-rich pigeonite sample with composition ca. Wo6En76Fs18 using a diamond anvil-cell. The unit-cell parameters were determined at 14 different pressures to 7.14 GPa. The sudden disappearance of the b-type reflections (h + k = odd) and a strong discontinuity (about 2.8%) in the unit-cell volume indicated a first-order P21/c–C2/c phase transition between 4.66 and 4.88 GPa. The P(V) data of the P21/c phase were fitted to 4.66 GPa by a third-order Birch–Murnaghan equation of state (BM3 EoS), whereas the limited number of experimental data collected within the C2/c phase between 4.88 and 7.14 GPa were fitted using the same equation of state but with K′ constrained to the value obtained for the P21/c fitting. The equation of state coefficients are V 0 = 424.66(6) Å3, K T0 = 104(2) GPa and K′ = 8(1) for the P21/c phase, and V 0 = 423.6(1) Å3, K T0 = 112.4(8) GPa, and K′ fixed to 8(1) for the C2/c phase. The axial moduli for a, b, and c for the P21/c phase were obtained using also a BM3-EoS, while for the C2/c phase only a linear calculation could be performed, and therefore the same approach was applied for comparison also to the P21/c phase. In general the C2/c phase exhibits axial compressibilities (β c > β a >> β b) lower than those of the P21/c phase (β b > β c ≈ β a; similar to those found in previous studies in clinopyroxenes and orthopyroxenes). The lower compressibility of the C2/c phase compared with that of the P21/c could be ascribed to the greater stiffness along the b direction. A previously published relationship between P c and M2 average cation radius (i.r.) has been updated using all the literature data on P21/c clinopyroxene containing large cations at M2 site and our new data. The following weighted regression was obtained: P c (GPa) = 26(4) ? 28(5) ×  i.r (Å), R 2 = 0.97. This improved equation can be used to predict the critical pressure of natural P21/c clinopyroxene samples just knowing the composition at M2 site.  相似文献   

12.
The aim of this research was to evaluate the potential of six legumes: Medicago sativa L., Glycine max, Arachis hypogea, Lablab purpureus, Pheseolus vulgaris and Cajanus cajan to restore within a short period of time soil contaminated with 3% crude oil. The legumes in five replications were grown in crude oil-contaminated and crude oil-uncontaminated soil in a completely randomized design. Plants were assessed for seedling emergence, plant height and leaf number. GC–MS was used to analyze the residual crude oil from the rhizosphere of the legumes. Plant growth parameters were reduced significantly (P < 0.05) for legumes in contaminated soil compared to their controls. In the 4th week after planting (WAP), shoot height increased across the species up to the 8th WAP. However, in the 12 WAP no significant increase in the shoot of all species was observed. Two WAP legumes planted in contaminated soil had significantly (P < 0.05) higher leaf number than these planted in uncontaminated soil with the exception of M. sativa. In the 4th WAP, only A. hypogea and P. vulgaris had increased leaf number, while in the 6th WAP, only L. purpureus had increased leaf number and survived up to the 12th WAP while most of the legumes species died. Chromatographic profiles indicated 100% degradation of the oil fractions in C. cajan and L. purpureus after 90 days. For other legumes however, greater losses of crude oil fractions C1–C10 and C10–C20 were indicated in rhizosphere soil of P. vulgaris and G. max, respectively. The most effective removal (93.66%) of C21–C30 components was observed in G. max-planted soil even though vegetation was not established. The legumes especially C. cajan, L. purpureus and A. hypogea are promising candidates for phytoremediation of petroleum hydrocarbon-impacted soil.  相似文献   

13.
The purpose of this study is to quantify the magnitudes of surface conduction and pore water conduction from the measured electrical conductivity of kaolinite, with the ultimate goal of estimating the electrical conductivity of kaolinite with a wide range of pore water conductivities (σ w = 0.013–3.356 S/m) and porosities (n = 0.368–1.0). Therefore, the theoretical background of the electrical conductivity in soils was reviewed, and electrical conductivity measurements on kaolinite were performed using both slurry and consolidation tests in this study. The results of this study demonstrate that the variations of measured electrical conductivity (σ mix) with n are debatable according to the values of σ w, because a decrease in n results in both an increase in surface conduction (K s) and a decrease in pore water conduction (K w); this causes the relative magnitude of K s compared to that of K w to vary with σ w and n. Consequently, this study develops the relation between the porosity-normalized K s/K w and 1/σ w. Additionally, the surface conductivity of the tested kaolinite is back-calculated and compared with the previous relationship between K s and zeta potential of kaolinite. The measured and estimated σ mix values are compared with the varying pore water conductivity and porosity values.  相似文献   

14.
The equilibrium thickness of the isothermal layers of interstellar gas and volume gas densities ρ gas in the plane of the disk as a function of galactocentric distance R are computed for seven spiral galaxies (including the Milky Way) using an axisymmetrical model. In this model, the thickness of the stellar disk varies with R and remains approximately equal to the minimum thickness of a stable equilibrium disk. We found the disk thickness to increase toward the periphery in at least five of the seven galaxies. The density of the stellar disk decreases with R faster than ρ gas , so that gas dominates at the disk peripheries in terms of density. A comparison of the azimuthally averaged star formation rate SFR and the gas density shows the absence of a universal Schmidt law SFR ～ρ gas n for galaxies. However, the SFRs in various galaxies are better correlated with the volume than the gas surface density. The parameter n in the Schmidt law formally calculated using the least-squares method lies in the interval 0.8–2.4, being, on average, close to 1.5. The values of n calculated separately for the molecular gas display substantial scatter, but are, on average, close to unity. The value of n appears to increase with decreasing ρ gas , so that the fraction of gas that actively participates in star formation decreases with n.  相似文献   

15.
Multi-epoch observations with high spectral resolution acquired in 1998–2008 are used to study the time behavior of the spectral-line profiles and velocity fields in the atmosphere and circumstellar shell of the post-AGB star V448 Lac. Asymmetry of the profiles of the strongest absorption lines with lower-level excitation potentials χ low < 1 eV and time variations of these profiles have been detected, most prominently the profiles of the resonance lines of BaII, YII, LaII, SiII. The peculiarities of these profiles can be explained using a superposition of stellar absorption line and shell emission lines. Emission in the (0; 1) 5635 Å Swan system band of the C2 molecule has been detected in the spectrum of V448 Lac for the first time. The core of the Hα line displays radial-velocity variations with an amplitude of ΔV r ≈ 8 km/s. Radial-velocity variations displayed by weak metallic lines with lower amplitudes, ΔV r ≈ 1–2 km/s, may be due to atmospheric pulsations. Differential line shifts, ΔV r = 0–8 km/s have been detected on various dates. The position of the molecular spectrum is stationary in time, indicating a constant expansion velocity of the circumstellar shell, V exp = 15.2 km/s, as derived from the C2 and NaI lines.  相似文献   

16.
Pyroxenes of general stoichiometry Mg(Ge x Si1?x )O3 were encountered in attempts to synthesise Ge-substituted talcs at 0.2 GPa, 650–700 °C. Orthopyroxenes (Pbca) of compositions x = 0.21, 0.30, and 0.34 were identified, and also a P21/c clinopyroxene of composition x = 0.63, and C2/c clinopyroxenes of compositions x = 0.91 and 1. End-member clinoenstatite MgSiO3-P21/c synthesised at 16 GPa, 1300 °C and transformed from C2/c was also included in the study. Crystal structure refinements using single-crystal XRD data showed that unit-cell parameters vary linearly with Si–Ge for the Pbca and P21/c pyroxenes, both of which have two symmetrically non-equivalent tetrahedral chains. Refinement of Si–Ge occupancies at tetrahedral sites showed that the two chains of all primitive pyroxenes have very different compositions, with XGe(TB) ? XGe(TA). This difference arises from the greater flexibility of the B-chain to rotate in response to tetrahedral expansion due to increasing Ge content. The TA-M2 shared polyhedral edge imposes significant constraints on the flexibility of the A-chain, which can accommodate much less Ge than the B-chain. Linear trends of cell parameters, site occupancies, and structural parameters for the primitive pyroxenes, when extrapolated to published data for MgGeO3–Pbca, extend across the entire Si–Ge join.  相似文献   

17.
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18.
The results of isotope-geochemical studies of carbonates of different mineral types from manganese and host rocks of the Famennian manganiferous formation of Pai-Khoi are reported. Kutnahorite ores are characterized by δ13C values from–6.6 to 1.3‰ and δ18O from 20.0 to 27.4‰. Rhodonite–rhodochrosite rocks of the Silovayakha ore occurrence have δ13C from–5.2 to–2.9 and δ18O from 25.4 to 24.3‰. Mineralogically similar rocks of the Nadeiyakha ore occurrence show the lighter carbon and oxygen isotopic compositions: δ13C from–16.4 to–13.1 and δ18O from 24.8 to 22.5‰. Similar isotopic compositions were also obtained for rhodochrosite–kutnahorite rocks of this ore occurrence: δ13C from–13.0 to–10.4‰ and δ18O from 24.6 to 21.7‰. Siderorodochrosite ores differ in the lighter oxygen and carbon isotopic compositions: δ18O from 18.7 to 17.6‰ and δ13C from–10.2 to–9.3‰, respectively. In terms of the carbon and oxygen isotopic compositions, host rocks in general correspond to marine sedimentary carbonates. Geological-mineralogical and isotope data indicate that the formation of the manganese carbonates was related to the hydrothermal ore-bearing fluids with the light isotopic composition of oxygen and carbon dissolved in CO2. The isotopic features indicate an authigenic formation of manganese carbonates under different isotopegeochemical conditions.  相似文献   

19.
The Triassic–Jurassic boundary is characterized by strong perturbations of the global carbon cycle, triggered by massive volcanic eruptions related to the onset of the Central Atlantic Magmatic Province. These perturbations are recorded by negative carbon isotope excursions (CIEs) which have been reported worldwide. In this study, Triassic–Jurassic boundary sections from the southern margin of the Central European Basin (CEB) located in northern Switzerland are analyzed for organic carbon and nitrogen isotopes in combination with particulate organic matter (POM) analyses. We reconstruct the evolution of the depositional environment from Late Triassic to Early Jurassic in northern Switzerland and show that observed negative shifts in δ13C of the total organic carbon (δ13CTOC) in the sediment are only subordinately influenced by varying organic matter (OM) composition and primarily reflect global changes in the carbon cycle. Based on palynology and the stratigraphic positions of isotopic shifts, the δ13CTOC record of the studied sections is correlated with the GSSP section at Kuhjoch (Tethyan realm) in Austria and with the St. Audrie’s Bay section (CEB realm) in southwest England. We also show that in contrast to POM analyses the applicability of organic carbon/total nitrogen (OC/TN) atomic ratios and stable isotopes of total nitrogen (δ15NTN) for detecting changes in source of OM is limited in marginal depositional environments with frequent changes in lithology and OM contents.  相似文献   

20.
Isotope fractionation during the evaporation of silicate melt and condensation of vapor has been widely used to explain various isotope signals observed in lunar soils, cosmic spherules, calcium–aluminum-rich inclusions, and bulk compositions of planetary materials. During evaporation and condensation, the equilibrium isotope fractionation factor (α) between high-temperature silicate melt and vapor is a fundamental parameter that can constrain the melt’s isotopic compositions. However, equilibrium α is difficult to calibrate experimentally. Here we used Mg as an example and calculated equilibrium Mg isotope fractionation in MgSiO3 and Mg2SiO4 melt–vapor systems based on first-principles molecular dynamics and the high-temperature approximation of the Bigeleisen–Mayer equation. We found that, at 2500 K, δ25Mg values in the MgSiO3 and Mg2SiO4 melts were 0.141?±?0.004 and 0.143?±?0.003‰ more positive than in their respective vapors. The corresponding δ26Mg values were 0.270?±?0.008 and 0.274?±?0.006‰ more positive than in vapors, respectively. The general \(\alpha - T\) equations describing the equilibrium Mg α in MgSiO3 and Mg2SiO4 melt–vapor systems were: \(\alpha_{{{\text{Mg}}\left( {\text{l}} \right) - {\text{Mg}}\left( {\text{g}} \right)}} = 1 + \frac{{5.264 \times 10^{5} }}{{T^{2} }}\left( {\frac{1}{m} - \frac{1}{{m^{\prime}}}} \right)\) and \(\alpha_{{{\text{Mg}}\left( {\text{l}} \right) - {\text{Mg}}\left( {\text{g}} \right)}} = 1 + \frac{{5.340 \times 10^{5} }}{{T^{2} }}\left( {\frac{1}{m} - \frac{1}{{m^{\prime}}}} \right)\), respectively, where m is the mass of light isotope 24Mg and m′ is the mass of the heavier isotope, 25Mg or 26Mg. These results offer a necessary parameter for mechanistic understanding of Mg isotope fractionation during evaporation and condensation that commonly occurs during the early stages of planetary formation and evolution.  相似文献   

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