共查询到20条相似文献，搜索用时 31 毫秒
1.
M. R. JOHNSON 《Sedimentology》1994,41(5):985-999
Existing proposals for converting thin section data to their sieve equivalents are all flawed in various ways, while questions concerning the significance of the grain size of spherical grains measured on a volume frequency basis in thin section using φ units, and of non-spherical (ellipsoidal) grains using both millimetres and φ units, have not been -satisfactorily resolved in the literature. It can be shown mathematically that the mean thin section diameters of spherical grains, or axis lengths of a series of parallel sections through ellipsoidal grains, will underestimate the dimensions of the corresponding central section (i.e. one passing through the grain centre) by 0.2023 φ when measured on a volume frequency basis. In order to approximate the effect of measuring particle size on random cuts through ellipsoidal grains, the dimensions of a series of sections cut in 49 unique directions, symmetrically arranged and evenly spread with respect to the ellipsoidal axes, were calculated. This calculation was carried out for five different ellipsoids which between them covered the mean sphericity and thin section axial ratio values normally encountered among naturally occurring quartz grain populations. The data indicated that the mean true nominal diameters (D̄) of ellipsoidal quartz grains can be obtained in thin sections from the mean nominal sectional diameters (d̄′) and major axes (ā′) of the central sections (derived from the observed values by multiplying the millimetre means by 1·1318 and subtracting 0·2023 from the φ means) using the following equations: A rough estimate (to within c. 5%) of both the mean nominal diameters and the mean long axes of ellipsoidal quartz grains can be arrived at by applying a simple correction factor to the mean long axis lengths as measured in thin section using either millimetres or φ units. 相似文献
2.
PETER M. SADLER 《Sedimentology》1982,29(1):37-51
In measured sequences of limestone- and greywacke-turbidites the bed-thickness is found to vary proportionally with the fall velocity of the maximum grain size, found at the base of the bed. A simple theoretical model, based on the decay of isotropic turbulence, suggests that bed-thickness should be a function not only of this fall velocity, but also of bottom slope, flow depth and the concentration and grain-size distribution of the sediment in the turbidity current. The field data do show some influence of these additional factors. Nevertheless, for many natural sequences of turbidites the flows must have carried very poorly sorted sediments and the inferred flow volumes and densities must cluster tightly about modal values. Thus, grain size remains the primary variable and the modal regression curve of bed-thickness on maximum grain size is well defined and resembles a fall-velocity curve. Relatively steep basin floors near to source can, theoretically, cause these modal regressions for distal and very proximal parts of a turbidite to diverge, introducing a crudely parabolic appearance in the form of the total regression curve. The form of this parabolic curve predicts the deposition of thin but relatively coarse proximal beds. Such beds do occur. They are different from the thin, but relatively fine, proximal beds that have been interpreted as the result of a fractionation of a turbidity current during levee-forming processes. 相似文献
3.
A key feature of an ideal granite is the occurrence of grain sequences which are reversible Markov chains. This property was tested using a 2 test on a 2 × 6 contingency table consisting of reversible grain pairs for microcline, plagioclase, quartz, and biotite, and on a 2 × 12 contingency table consisting of reversible grain triads. All 28 samples examined from the Pacolet Mills pluton, South Carolina, passed the 2 test for grain pairs, and all but three of these passed the 2 test for grain triads. The coefficients of the reversibility matrix were examined for statistical significance after normalization, using a logarithmic transformation. For all three phases of the Pacolet Mills pluton, the average coefficients were in the range 1.08–0.89. Elevated and depressed values of these coefficients suggested possible differences among the three phases of the granite in their crystallization paths. 相似文献
4.
Microstructural modification processes like dynamic recrystallization and grain growth can have a major effect on the transient and (semi-)steady state flow behaviour of deforming materials. Work on metals and ceramics suggests that deformation-enhanced changes in grain topology and the corresponding increase in fraction of non-hexagonal grains, called cellular defect fraction, can promote grain growth during deformation. The present study tests this hypothesis, by investigating the evolution of the cellular defect fraction during deformation, accompanied by grain growth, of aggregates with distributed grain sizes. For this purpose, we made use of the ELLE 2D microstructural modeling package. We simulated and quantified microstructural evolution under conditions where both surface energy driven grain boundary migration (GBM) and homogeneous deformation or grain size sensitive (GSS) straining were allowed to occur. The simulations show that contemporaneous GBM and simple geometrical straining of grain aggregates with distributed grain size and coordination number lead to extra grain neighbor switching, an increase in defect fraction, and enhanced grain growth. An increase in defect fraction was also found in a selected set of natural calcite mylonites that, with increasing temperature, show an increase in grain size and contribution of GSS creep. Analysis of defect fraction thus appears to be a good microstructural tool to establish whether or not a material has experienced normal static (defect fraction 0.7) or dynamic grain growth (defect fraction 0.8). 相似文献
5.
6.
7.
为深入认识渗透变形对粗粒土渗透性及压缩性的影响,利用自主研发的加载式大型渗透变形仪对不同级配的粗粒土试样进行渗透变形全过程试验及侧限压缩对比试验,获得了不同试件产生渗透变形的临界水力梯度、渗透系数演化规律及损伤变化特性。研究成果表明：渗透变形过程中渗流路径中下游部位产生了渗透挤密效应,引起局部渗透性降低;在后期增加的水头作用下,先前产生渗透挤密部位的渗透性逐渐增大。试样产生渗透变形后,压缩模量减小;颗粒级配不同,渗透破坏引起的压缩损伤程度亦不相同。试件产生渗透变形后,在重力和渗透力作用下,试件结构将产生重构现象。 相似文献
8.
P. Fielitz G. Borchardt M. Schmücker H. Schneider 《Physics and Chemistry of Minerals》2007,34(6):431-436
Aluminium tracer diffusivities were measured in polycrystalline mullite. The artificial aluminium isotope 26Al was used as tracer isotope. An advanced preparation technique for the 26Al2O3 tracer source allowed to apply secondary ion mass spectrometry (SIMS) in order to analyse 26Al depth distributions in the polycrystalline material. Pre-exponential factors and activation enthalpies were determined
for compositions of 78 wt.% Al2O3, 22 wt.% SiO2 (high-alumina material) and of 72 wt.% Al2O3, 28 wt.% SiO2 (low-alumina material), respectively. A strong dependence of the 26Al grain boundary diffusivity on the composition is observed. The results are discussed in comparison to previous data on
grain boundary diffusivities of oxygen in mullite samples from the same batch.
Dedicated to Prof. Hermann Schmalzried on the occasion of his 75th birthday. 相似文献
9.
W. E. Bardsley 《Mathematical Geology》1978,10(6):643-655
An extreme value model is developed for the situation where a cloud of sediment particles moves away from the boundary of a defined source area while undergoing constant depletion due to deposition of the larger particles. Taking the particles deposited at distance xfrom the source boundary to represent a distribution of largest extremes derived from a parent distribution of smallest extremes, it is possible to express the mean size of the deposited sediment in terms of the parameters of the original distribution at the source area. Thickness functions can be obtained as the product of expected diameter and particle frequency. If the spatial distribution f(x)of particle frequency along a linear transect can be inferred from a physical process, then this provides sufficient information for the construction of particle size and bed thickness prediction equations. Alternatively, the model places some restrictions on distribution selection if an empirical choice of f(x)is necessary. Some generalizations are obtained for trends in the mean and variance of the deposited particles on the basis of the hazard function of f(x). 相似文献
10.
Debris flow density determined by grain composition 总被引：1，自引：1，他引：0
Density is one of the most important parameters of debris flows. Because observing an active debris flow is very difficult, finding a method to estimate debris flow density is urgently needed for disaster mitigation engineering. This paper proposes an effective empirical equation in terms of grain size distribution (GSD) parameters based on observations in Jiangjia Gully, Yunnan Province, China. We found that the GSD follows P(D) = KD -μ exp(? D/Dc), with μ and Dc representing the fine and coarse grains, respectively. In particular, μ is associated with some characteristic porosity of soil in the natural state and increases with increased porosity. Dc characterizes the grain size range of the flow and increases with the grain concentration. Studies show that flow density is related to both parameters in power law. Here, we propose an empirical equation for estimating flow density: ρ = 1.26μ -0.132 + 0.049Dc0.443, which provides not only an estimation of the density for a flow, but also describes the variation in density with the GSD of material composition; this provides important information related to the design of debris flow engineering structures. 相似文献
11.
Traditional grain storage used to be an effective mechanism to combat famine in the rural areas of W Sudan. Of late, this
mechanism has failed to function and a severe famine took place in 1984–1985. This paper endeavours to highlight this crisis,
show the traditional methods of grain storage and point out to ways of retrieving grain storage systems to start again on
a less vulnerable basis. An assessment of grain storage requirements under the new conditions where both market and subsistence
economy are effective in rural areas is made for the different ecological zones of W Sudan. 相似文献
12.
HEINZ BURGER 《Sedimentology》1976,23(3):395-405
A computer program is described for the display of grain size distributions by log-probability plots. These plots are useful to recognize and interpret log-normal subpopulations. Because log-probability plots of the cumulative curves often show two or more straight line segments we suggest a method of ‘log-normal’interpolation for use in computing statistical parameters. The method combines advantages of graphical methods with those of mathematical moments. Finally, the frequency curve is derived by approximating the log-probability plot of the cumulative curve with spline functions. 相似文献
13.
Shape effects in aeolian grain transport 总被引：1，自引：0，他引：1
Wind tunnel experiments are reported, the results of which show that grain transport rate is responsive to characteristic grain shape. It is relatively less sensitive to changes of sorting unless these involve the fraction smaller than 50μm. A cubic relationship between transport rate and velocity gradient appears to be valid only for well worked natural grains having high sphericity. 相似文献
14.
Fast diffusion along mobile grain boundaries in calcite 总被引：1，自引：0，他引：1
Andrew McCaig Stephen J. Covey-Crump Walid Ben Ismaïl Geoffrey E. Lloyd 《Contributions to Mineralogy and Petrology》2007,153(2):159-175
Experimental measurements of grain boundary diffusion are usually conducted on static boundaries, despite the fact that grain
boundaries deep in the Earth are frequently mobile. In order to explore the possible effect of boundary mobility on grain
boundary diffusion rates we have measured the uptake of 44Ca from a layer of 44Ca-enriched calcite powder during the static recrystallization of a single crystal of calcite at 900°C. A region about 500 μm
wide adjacent to the powder layer is heterogeneously enriched in 44Ca, and complex zoning patterns, including sharp steps in composition and continuous increases and decreases in 44Ca content, are developed. In metamorphic rocks, these would normally be interpreted in terms of changes in pressure or temperature,
Rayleigh fractionation, or episodic fluid infiltration. These explanations cannot apply to our experiments, and instead the
zoning patterns are interpreted as being due to variations in grain boundary migration rate. We have applied an analytical
model which allows the product of grain boundary diffusion coefficient and grain boundary width (D
GB
δ) to be calculated from the grain boundary migration rate and the compositional gradient away from the powder layer. The value
of D
GB
δ in the mobile grain boundaries is at least five orders of magnitude greater than the published value for static boundaries
under the same conditions. In order to allow the scale of chemical equilibrium (and hence textural evolution) to be predicted
under both experimental and geological conditions, we present quantitative diffusion-regime maps for static and mobile boundaries
in calcite, using both published values and our new values for grain boundary diffusion in mobile boundaries. Enhanced diffusion
in mobile boundaries has wide implications for the high temperature rheology of Earth materials, for geochronology, and for
interpretations of the length- and time-scales of chemical mass-transport. Moreover, zones of anomalously high electrical
conductivity in the crust and mantle could be regions undergoing recrystallization such as active shear zones, rather than
regions of anomalous mineralogy, water- or melt-content as is generally suggested. 相似文献
15.
Akira Miyake 《Contributions to Mineralogy and Petrology》1998,130(2):121-133
The phenomenon of normal grain growth in pure single phase systems is modeled with the Monte Carlo technique and a series
of simulations are performed in 2- and 3-dimensions. The results are compared with natural and experimental monomineralic
rock samples. In these simulations various lattice models with different anisotropic features in grain boundary energy are
examined in order to check the universality of the simulation results. The obtained microstructure varies with the artificial
parameter T
′ in each lattice model, where T
′ means scaled temperature and controls thermal fluctuation on grain boundary motion. As T
′ (thermal fluctuation) increases, the boundary energy anisotropy characterizing each lattice model becomes less important
for the evolution of the microstructure. As a result the difference in the grain size distribution among the lattice models,
which is significantly large for low T
′, is reduced with increasing T
′. The distribution independent of both the lattice model and the dimension is obtained at sufficiently high T
′ and is very close to the normal distribution when carrying out the weighting procedure with a weight of the square of each
grain radius. A comparison of the planar grain size distribution of the natural and experimental rock samples with the 3-D
simulation results reveals that the simulations reproduce very well the distributions observed in the real rock samples. Although
various factors such as the presence of secondary minerals and a fluid phase, which are not included in the simulation modeling,
are generally considered to influence the real grain growth behavior, the good agreement of the distribution indicates that
the overall grain growth behavior in real rocks may still be described by the simplified model used in the present simulations.
Thus the grain size distribution obtained from the present simulations is possessed of the universal form characterizing real
normal grain growth of which the driving force is essentially grain boundary energy reduction through grain boundary migration.
Received: 7 January 1997 / Accepted: 25 August 1997 相似文献
16.
GERALD M. FRIEDMAN 《Sedimentology》1996,43(1):189-189
17.
A grain of ilmenite enclosed in deformed quartz‐mica‐staurolite schist from Ducktown, Tennessee, shows a variety of optical features produced by natural deformation, recovery and recrystallisation processes. These features are consistent with dislocation processes similar to those observed in metals and other minerals, with the principal deformation modes apparently being slip on (0001) and twinning on (1011). Recrystallisation may have proceeded by sub‐grain rotation. Strain hardening associated with late‐stage deformation twinning has resulted in considerable variation in hardness, with measured Vickers Hardnesses ranging from 362 to 788. This range is substantially greater than those reported to date in standard compilations of ore microscopic data, even though detailed microprobe analysis has shown it to contain no component due to chemical inhomogeneity, and relatively little can be due to structural anisotropy. If other minerals show equally substantial effects of natural work‐hardening and annealing, the diagnostic value of microhardness determinations will be considerably reduced. 相似文献
18.
19.
为使样品加工粒度200目的过筛率平均达到98％以上，满足X荧光分析对样品粒度的基本要求，在实验过程中发现了饭颗粒存在并影响着过筛率，通过分析饭颗粒的成因，找出了有效抑制饭颗粒的办法——加入酒精在一定的条件下能抑制饭颗粒的产生并能保证样品加工粒度达到200目过筛率平均在98％以上。 相似文献
20.
JOHN S. BRIDGE 《Sedimentology》1976,23(3):407-414
Engelund's (1974a, b) theoretical bed topography model for curved channels is modified and shown to fit natural channels when using values for the dynamic bed load friction coefficient of about 0·4–0·5. The dependence of this coefficient on grain size for flow in the lower flow regime is discussed. Using the bed topography model simultaneously, Allen's (1970a, b) theory for grain size variation over point bar profiles is shown to be broadly applicable. The interaction between sediment, flow and bed topography in bends is therefore adequately described. 相似文献