Backscattered Electron Scanning Images Research Articles

Evolution of soil porosity in loess-palaeosol sequences of the Ebro Valley, NE Iberia

Structure development and porosity evolution are closely related key processes in the formation of soils on loess. In order to better understand changes in soil porosity with time, four loess-palaeosol sequences (LPS) of the Ebro Valley were investigated using micromorphological and multiscalar images of soil thin sections combined with luminescence dating. Scanned and high-resolution mosaic images of thin sections using circular polarized light and backscattered electron scanning images were used to characterize the structural and textural porosity. The results were compared with physical and chemical analyses, and detailed micromorphological descriptions. The main results of our research revealed that 1) the change in loess particle packing, from enaulic to porphyric coarse/fine (c/f) related distribution, is the first process giving cohesion to the loess material and 2) that soil development during stabilization periods is associated with higher bioporosity. Gypsum accumulation is also a relevant process contributing to the increase of packing porosity within channels and chambers in the studied soils. Our results provide new insights into soil formation processes in Mediterranean loess and open new fields of research such as the possibility of using the quantification of bioporosity as an additional palaeoenvironmental marker.

2-D image analysis: A complementary tool for characterizing quarry and weathered building limestone

Abstract The understanding of weathered processes and, more generally, of transfer properties of building stones, requires a detailed knowledge of porosity characteristics. This study aims to analyze two-dimensional (2-D) images of stones by using mathematical tools that enable the description of the pore and solid phase distribution. We selected two limestones that have been widely used for different types of buildings: a quarried and weathered Tuffeau stone (the latter being used in most châteaux of the Loire) and a quarried Sebastopol stone selected for numerous buildings in Paris. Backscattered electron scanning images obtained on thin sections of the stones were studied by using autocorrelation function analysis and chord distributions. Results showed that these mathematical tools are able to discuss, quantitatively and statistically, differences of pore and solid distributions between quarried limestones, and to discuss the degree of weathering of stones collected from buildings. Thus, very small differences of pore and solid phase distribution between the samples studied were revealed by chord distribution analysis and autocorrelation function analysis. Resulting characteristics obtained with such an analysis are promising information for a better understanding of weathering mechanisms.

Mass Proportion of Microaggregates and Bulk Density in a Brazilian Clayey Oxisol

The physical properties of Brazilian Oxisols are closely related to the development of their microstructure, which typically consists of stable microaggregates >1 mm. There is no model available to predict changes in microstructure in Oxisols. The objective of this work was to relate the proportion of microaggregates to the bulk density (Db) in the soil studied. Five sites of a typic Haplustox under native vegetation (two sites) and pasture (three sites) were sampled. Soil bulk density, sand, silt, and clay content and aggregate‐size distribution were measured from the surface to 1.6 m deep in increments of 0.1 m. Thin sections were prepared from undisturbed samples collected in duplicate from 0‐ to 0.1‐, 0.3‐ to 0.4‐, 0.8‐ to 0.9‐, and 1.5‐ to 1.6‐m depths, and backscattered electron scanning images (BESI) were generated. Clay content ranged from 672 to 798 g kg−1 and bulk density between 0.87 and 1.18 g cm−3 among the 80 samples studied. Bulk density was poorly correlated with clay content (R2 = 0.358) and at any depth was not significantly smaller under native vegetation than under pasture. Visual assessment of BESI revealed that soil material corresponded to either microaggregates (<0.1 mm) in loose arrangement or to much larger aggregates. Quantification of BESI from the deepest sampling depth of all soils showed that 96.2 and 95.7% of microaggregates were <0.8 mm with 73.2 and 95.7% between 0.1 and 0.5 mm under native vegetation and pasture, respectively. The mass proportion of microaggregates can be estimated using the <0.84‐mm soil material that is obtained by dry sieving (Φ < 0.84). Finally, our results showed that Φ < 0.84 varied with Db Linear regression coefficients were calculated for the relationship between Φ < 0.84 and the reciprocal of bulk density (1/Db) [Φ < 0.84 = 1.97 (1/Db) − 1.52, R2 = 0.82], assuming no interaction between microaggregates and macroaggregates, the porosity of these two structural types was estimated as 0.71 and 0.51, respectively.

Backscattered Electron Scanning Images of Soil Porosity for Analyzing Soil Compaction around Roots

Thin sections of soil surrounding maize roots were studied in scanning electron microscopy using the backscattered electron mode. The soil is a fine-loamy, mixed, mesic, Typic Eutrochrept. Backscattered electron scanning images (BESI) of the porosity surrounding a selected maize (Zea mays L.) root were studied by image analysis at the scale of micro- and mesopores. Image analysis of BESI revealed that the porosity was 22 to 24% less within the soil surrounding the root than in the bulk soil. The bulk density increased up to 1.80 Mg m−3 close by the root-soil interface, although it was 1.54 Mg m−3 in the bulk soil. The porosity reduction consisted of a removal of the mesopores and a decrease in micropores, which resulted from the packing of skeleton grains with the porous clayey phase. The micropores were affected, although they are usually thought to be altered only with difficulty under natural conditions. A model which was developed earlier for soil compression around roots of plants growing on remolded soils was fitted to the experimental data by nonlinear regression analysis.

Variation of textural porosity of a clay‐loam soil during compaction

SummaryThe effect of compaction on the porosity of aggregates was studied in the laboratory using mercury porosimetry and backscattered electron scanning images (BESI) of polished thin sections. Porosity was divided into structural and textural porosity, and then textural porosity into lacunar and clayey porosity.For aggregates equilibrated at a water potential of −1 kPa and pressures of 50 and 200 kPa, compaction of structural porosity resulted in an increase of textural porosity. For aggregates equilibrated at a water potential of −1 kPa and an applied pressure of 600 kPa, the structural porosity strongly decreased but did not result in a variation of textural porosity. For aggregates equilibrated at water potentials of −63 and −103 kPa and the three values of pressure studied, textural porosity was unaffected by compaction whatever the evolution of structural porosity. The BESI indicated that the increase in textural porosity which was recorded by mercury porosimetry for aggregates equilibrated at a water potential of −1 kPa and applied pressures of 50 and 200 kPa was due to the formation of relict structural pores during compaction. The relict structural pores resulted from partial distortion of the structural pores within the original aggregates. These relict structural pores still had the morphology of structural pores on BESI, but they were accessible to mercury through the necks of textural pores identified as lacunar pores.Results also indicate that massive structure frequently seen in the field for these soils and interpreted as resulting either from structural collapse during rewetting or from compaction actually resulted mainly from wheel compaction.

Porosity measurements and form analysis of mineral grains in thin sections from oil- gas reservoir rocks using quantimet 720 and besi

Submicroscopic and image analyzer techniques have been applied for the measurement of the porosities of oil—gas—water reservoir rocks from thin sections. Backscattered electron scanning images (BESI) were made predominantly at low magnifications of × 30 and × 60. Porosity data from linear traverses were obtained and differences in porosities between two sandstone reservoirs and a carbonate reservoir were determined. A large number of the measured pores contained clayey material. The surface occupied by this material was subtracted from the apparent porosity measurements with the image editor of the Quantimet 720 to give the real porosity. A sample from one of two sandstone reservoirs contained oil and could not be hardened using the common procedure for impregnation of the sample by a polyester resin. Gamma radiation was applied and hardening occurred in one weekend after 5 Mrad (50 kGy) had been absorbed. Hardening of this sample gave no problems, if the oil had been removed by monostyrene before a start was made with the impregnation process; such a sample was used for the present porosity study. Form analysis was done of individual mineral grains using BESI and Quantimet. The grains were represented in a number of shape classes of a diagram used for shape classification. Form-separation of individual minerals was also possible.

The characterization of the shape of mineral grains in thin sections of soils by quantimet and besi

Silt-sized mineral grains were analysed for their shapes or forms in thin sections of soils using backscattered electron scanning images (BESI) and an image analyzer (Quantimet 720). This work was done with an image-editor and required manual editing of the shape of mineral grains on the display screen of the Quantimet. Shape analysis of coarse and fine silt grains could thus be performed at magnifications of x 240 and x 480. The shape of the mineral grains could be characterized by five parameters: area ( A), perimeter ( Pe), largest and smallest Feret's diameters ( Fmax and Fmin), lobation ratio ( Lr) and indentation ratio ( Ir). A relation was found, however, between the APe 2 ratio and the Lr and Ir ratios. Consequently, shape analysis of mineral grains can be done by measuring three of the five parameters, i.e. A, Pe and Fmax/ Fmin. These parameters, however, are still not be able to reproduce the shape of individual mineral grains and images of them therefore remain necessary. Several of the 54 mineral grains were found to have different shapes but the same A Pe 2 ratio. An explanation for this phenomenon is given by using a theoretical model in which two-dimensional forms change their shapes from a circle ( A Pe 2 = 79 ) to a line ( A Pe 2 = 1 ). Form separation of mineral grains with the same ( A Pe 2 ) ratio can be done by using the F max F min ratio. The measurement of the shape of the mineral grains, the theoretical model and observation of the micrographs of the individual mineral grains, made it possible to draw a diagram with shape classes which can be used for shape classification purposes. Knowledge gathered in soil micromorphology on shape analysis of pores and soil aggregates has also been used to obtain boundaries of shape classes in the diagram. The present study is intended both as a proposal as to how two-dimensional shape classification can possibly be done, and as a work-model. The boundaries of the shape classes can easily be adjusted if data from forthcoming studies require it.

Sem-edxra investigation of tubular features and iron nodules in lateritic soils from malaysia

Tubular features were observed with SEM in unhardened pieces of iron nodules found in some lateritic soils of Malaysia. EDXRA measurements demonstrated that the tubes were coated with predominantly Fe and Al. They probably represent mycelia encrusted as a result of lateritization or ferrallitization processes. The iron nodules frequently exhibited fluidal structures in backscattered electron scanning images. White areas in the images contained most iron, whilst grey areas of the nodules were composed of predominantly Al, Si, K and Fe, similar to the composition of the soil matrix surrounding the iron nodules. The nodules are interpreted as having formed in clayey material most of which is still present in these features.

The development of soil porosity in experimental sandy soils with clay admixtures as examined by quantimet 720 from besi and by other techniques

For this study thin sections have been prepared of natural and artificial sands mixed with 20% and 40% clay. These and other samples had already been examined using soil physical and soil mineralogical methods for loose samples. Initial thin section studies, with the help of the light microscope alone, have been enlarged upon in this investigation using backscattered electron scanning images and an image analyzer (Quantimet 720). This allowed quantitative information to be obtained on the development of different types of soil porosity patterns using various clay admixtures. Information was also obtained on the influence the form of mineral grains had on the shape of pores in the mixtures of sand and clay. It was also possible to obtain quantitative information on the amount of clays and larger mineral grains in the thin sections. Much of the present data cannot be obtained automatically but must be manually controlled by using the image editor of the Quantimet. This type of work, however, may provide basic data which can subsequently be used to help explain various processes which occur in soils.

Quantimet 720 analysis of porosities in backscattered electron scanning images made with different phototechniques

Three photo-techniques were used to make backscattered electron scanning images (BESI) of the same area in a thin section of soil using a scanning electron microscope (SEM). Subsequent porosity analysis with an image analyzer (Quantimet 720) showed that different results could be obtained in a number of diameter classes while the total porosity was virtually the same, a phenomenon which may be related to beam instability of SEM. Under such conditions, one needs a dependable photo-technique in which soil constituents on a BESI micrograph can be compared with the same constituents on the display screen of SEM and Quantimet. Such a photo-technique is suggested and can be used to obtain a limited number of measurable BESI micrographs.

Exsolution of cummingtonite, actinolite and sodic amphibole in hornblende in high-pressure metamorphism

Exsolution phenomena in amphiboles indicate the existence of miscibility gaps1,2. The phase relationships of hornblende, the commonest amphibole in igneous and metamorphic rocks, to the other monoclinic amphiboles such as cummingtonite, actinolite and sodic amphibole has particular petrological importance. In these systems a solvus has been directly confirmed by exsolution in the hornblende–cummingtonite system3–5. Chemical discontinuities suggesting miscibility gaps have been shown for other systems6–18. In the hornblende–actinolite system, which was recently confirmed to have a solvus by experimental synthesis19, fine lamellar texture that is too fine for microprobe resolution is expected to result from exsolution9,10. However, there has been no report of amphibole exsolution from high-pressure metamorphic rocks. We have found exsolution lamellae of three kinds of clinoamphibole, cummingtonite, actinolite and sodic amphibole, in a zoned hornblende from high-pressure metamorphic rock, and present here direct evidence to confirm the miscibility gaps between hornblende and these three clinoamphiboles in high-pressure metamorphic conditions. The genesis of exsolution lamellae discussed below is based on back-scattered electron-scanning images and microprobe analyses.

The characterization of microporosity in a ploughpan by submicroscopic and quantimet techniques

Ploughpans are usually recognizable in thin sections because of their massiveness (density) when compared with adjacent parts of the soil. When pores can be measured in the ploughpan such massiveness can be documented and compared with the porosity of the underlying soil profile. Such measurements, however, concern only meso- and macropores when done by Quantimet on micrographs obtained with the light microscope. This is due to the thickness of the thin section and the necessity to work with transmitted light. The measurement of the smaller mesopores may also be problematic with the light microscopy—Quantimet technique. The introduction of backscattered electron scanning images allows to obtain micrographs of a very thin layer and this made it possible to measure micro- and mesopores by Quantimet. If only small magnifications are used, macropores can also be measured. In the present investigation, micro- and mesopores were quantified by a combination of backscattered electron and Quantimet techniques and the macropores by light microscopy and Quantimet. It was demonstrated that macro- and mesopores were less frequent in a ploughpan than outside, whereas capillary or micropores increased somewhat.