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III. Applications of Electron Microscopy

III. Applications of Electron Microscopy

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66



E. B. A. BISWM



analysis, and electron spectroscopy for chemical analysis) are discussed

(Bisdom, 1981a,b) .



1. General



The submicroscopic study of unhardened soil samples can be done by TEM,



STEM,and SEM. The TEM and STEM can give magnificationsover X 1,OOO,OOO



depending on the type of soil particles that are studied. The TEM and STEM are

usually used for very small soil particles that are present in ultrathin sections or in

pretreated and disturbed samples. Ultrathin sections are discussed in Section

III,B, but pretreated and disturbed samples form no significant part of this article.

The SEM can reach magnifications of more than X100,OOO. The maximum

magnification is again dependent on the type of soil particle that is investigated.

The SEM is an ideal instrument for three-dimensional studies of soil constituents

and therefore it is frequently used for morphological examination. Much attention

has also been paid by specialists in soil mechanics and soil microscopy to the

spatial relationships between individual constituents in soil peds.



2 . Clay Minerals

Individual clay minerals in soil peds or aggregates are usually difficult to

recognize with the SEM because they commonly form stacks that can be partly or

wholly coated with other fine soil constituents. X-raypowder diffractograms of

bulk and disturbed samples and TEM studies of pretreated and disturbed individual clay minerals are usually performed simultaneously with SEM studies of

materials in soil peds.

Keller (1976a,b,c, 1977a,b, 1978a) and Keller and Haenni (1978) studied

kaolinite in various deposits around the world and were able to classify these

deposits into transported and residual types on the basis of texture differences

found in scanning electron micrographs. Gillott (1974) and Tessier and Berrier

(1978) recognized that the in situ investigation of clay minerals in soil peds

required ‘special preparation techniques such as freeze-drying or critical-point

drying if air-drying does not give the required results. Smart and Tovey (1982)

discuss these and other techniques for electron-microscopic work.

Spherulitic halloysite in volcanic deposits was examined with the SEM and

TEM by Sudo and Yotsumoto (1977) and Violante and Violante (1977). Differences in shape and mineralogical properties were found to exist between the

spherulitic halloysite bodies. Sudo and Yotsumoto (1977) called the bodies

“chestnut-shell-like” on a morphological basis and “allophane-halloysite-



SUBMICROSCOPIC EXAMINATION OF SOILS



67



spherules” on a genetic basis (i.e., halloysite formed from allophane which had

originated from volcanic glasses). Violante and Violante (1977)explained that

the spherulitic halloysite possibly may have formed inside vitreous bubbles as a

result of processes exerted by surrounding minerals.

Palygorskite in acid-etched carbonate nodules was studied with the SEM by

Yaalon and Wieder (1976)and in calcareous crusts by Nahon et al. (1975).The

morphology of allophane, immogolite, and halloysite in volcanic ash soils was

studied with the SEM by Eswaran (1972). Various scanning electron micrographs of clays were presented by Smart and Tovey (1981).Scanning electron

microscopy-energy dispersive X-ray analysis has obtained information on chemical elements present in clay of soil peds. The maximum magnification at which

this is possible is XlO,OoO, and the analysis can be performed on a l-Fmdiameter spot.

3 . Weathered Minerals



Much SEM work has been done on various types of weathering minerals.

Although clay minerals also weather, most such studies concern the larger primary minerals such as feldspar, olivine, mica, and quartz. Minerals like feldspar,

mica, and quartz can also form a part of the clay fraction of a soil, but it is the

larger particles which are studied because they can be compared with the results

obtained from light-microscopic investigation of the same or similar samples.

a. Feldspars. Scanning electron micrographs of feldspars weathering to halloysite and kaolinite have been published by Eswaran and de Coninck (1971).

The feldspars weathered to halloysite in an Entisol and to kaolinite in an Ultisol.

Feldspar altered to kaolinite and gibbsite in a granite profile from Malaysia. No

intermediary crystalline or amorphous phase was found during such weathering,

whereas the amorphous phase was present during the transformation of feldspar

into halloysite (Eswaran and Wong, 1978). Weathered feldspar in decomposing

basalt was photographed with the SEM by Benayas and Alonso (1978).In Israel,

weathering of plagioclase gave halloysite pseudomorphs in the vesicularly

weathered basalt and smectite pseudomorphs in the saprolite profiles (Singer,



1973).

Feldspars in Scottish soils showed holes and pits due to continuous dissolution

and etching (Wilson, 1975). Such holes and pits were thought to have originated

where crystal dislocations met the surface of the feldspars. No residual layer was

observed at the boundary between the unweathered feldspar and a void or crack.

The existence of such a residual layer was found to be unlikely. Experimental

etching of a microcline perthite (Wilson and McHardy, 1980) confirmed that

etch marks developed along crystal dislocations emerging on the surface (i.e.,

dislocations associated with perthitic lamellae). Analyses with ESCA (see Section I1,C) by Berner and Holdren (1977)of surface layers of feldspars c o n f i e d



68



E. B. A. BISDOM



that weathering occurs along dislocations, cleavages, and fractures (i.e., a residual layer, which requires an equal rate of attack on all parts of a feldspar grain,

is not necessary and probably does not exist). Pitted feldspar in an altered rock

fragment was thought to be the result of dissolution (Taupinard, 1976), whereas

Keller (1978b) explained this pitting as the result of uneven dissolution and

nonuniformity in composition. Millot et al. (1977) indicated that such pits in

feldspar could originate when secondary calcite replaced feldspar, a process

which was called epigenesis.

b. Quartzes. Many SEM studies concern the morphology of weathered

quartz grains. If the degree of weathering of individual quartz grains can be

assessed, it is possible to use such information to deduce the developmental

history of individual horizons in a soil profile. Legigan and Le Ribault (1974)

studied the evolution of quartz in a humic and fermginous podzol in France that

was developed in aeolian sands. Surface features of the quartz grains were

related to the sedimentary and pedological history of the profile. Well-polished

surfaces indicated transport in streams, whereas polished surfaces found with

shock imprints indicated a fluviatile or wind-transported origin. Striae with a

certain density on the surfaces of quartz grains were interpreted as being formed

by the rubbing of quartz grains against each other during glacial activity. If the

quartz grain had dissolution figures on its surface or an iron crust with or without

organic matter, it was thought to be caused by pedogenesis. This type of approach permitted the indication of various environments in the studied profile

and also helped to unravel the history of the sands.

Eswaran and Stoops (1979) worked with a zero phase in a weathering sequence of quartz, established in a Xerochrept formed on Keuper marls in Spain.

The quartz crystals were idiomorphic to hypidiomorphic. The surface textures of

quartzes were studied in a 19-m-deep profile developed on granite in a tropical

environment. Weathering of the quartz started a few centimeters above the fresh

rock with fragmentation of the quartz grains and the presence of hairline cracks

in the weathered quartz. Etching of the quartz grains occurred at a depth of 18.5

m and the quartz showed large dissolution pits that were interconnected by

grooves and hairline cracks. Some idiomorphic secondary quartz was precipitated between the depths of 9.5 and 16 m on the surface of heavily etched

primary quartz grains. Triangular dissolution pits were developed in primary

quartz grains at a depth of about 9.5 m and heavily etched quartzes with linear

striations were found at a depth of 1 m. These linear striations differed from the

etch grooves found on the surfaces of quartzes at greater depths in the profile.

The surfaces of quartz grains from Neogene sands in the Ivory Coast were

examined by Leneuf (1972). The quartzes came from depths of 3,30, and 90 m.

Two classes of weathering figures were distinguished, one related to the crystal

lattice of quartz and the other apparently not related to it. The first class comprised cavities with the same alignments; cavities with tetrahedral, rectangular,



SUBMICROSCOPIC EXAMINATION OF SOILS



69



irregular polyhedral, and wedge-shaped forms; fissures with a concentric outline;

cubic figures in a regular network; and lines with a relief and at 30, 60, and 120

degrees. The second class contained irregular cavities, fissures related to desquarnation, vermiform fissures, fine particles on the surface of quartz grains,

and newly formed secondary quartz from silicon which had passed through the

profile. These surface features on the quartz grains indicated that silicon had

been mobilized in the upper part of the strata and that only part of the silicon

participated in the formation of kaolinite. Secondary quartz could form from the

transported silicon at deeper levels in the profile.

Scanning electron micrographs of quartz particles in surface soils of the Hawaiian Islands were studied by Jackson et ul. (1971). These soils developed over

quartz-free mafic (basic) rocks. Wind deposition of the quartz was inferred by

comparison of the sharp angular, chip- or shard-like morphology of the grains

with that of quartzes in aerosolic dust and pelagic sediment. The percentage of

quartz varied with the elevation of the soil, the age of the soil, and the amount

and source of annual rainfall. Scanning electron microscope and X-ray diffraction (XRD) analyses of airborne particles indicated that the coarser ones, with

radii of 10-100 pm, consisted predominantly of quartz, whereas the finer particles, with radii of 1-10 pm, were mainly clay minerals. The clay minerals were

found in the air as constituents of aggregates, as coatings on quartz grains, and as

individual platelets, and were derived from the soil by sandblasting.

Riezebos (1974) studied weakly cemented Miocene sands of deposits from

South Limburg, the Netherlands, with the SEM. Secondary quartz was found not

only at grain contacts but also around detrital quartz grains. Overgrowth of

secondary quartz on the larger grain surfaces formed steps and striations. Such

steps and striations were therefore not the result of glacial environments. Douglas and Platt (1977) investigated the surface morphology of quartz and the age of

soils in glacial material from Wisconsin. Quartz in late Pleistocene (Wisconsin)

deposits was only slightly weathered with a mainly broad, flat or conchoidal

breakage surface, and sharp or slightly rounded upturned plates. Quartzes in

sands of Illinoian age showed both sharp and rounded upturned plates. Precipitation of secondary silica had occurred on the quartz grains and a modification of

the surface morphology was the result. Some solution pits were also present.

Corroded surfaces with solution Vs and highly rounded upturned plates were

found to be associated with quartz grains of Kansan age. The rounded forms

were caused by dissolution and precipitation of silica. Flaking was also found,

representing intense chemical weathering.

Moss and Green (1975) pointed out that the concept of deformation sheeting

(i.e., forming plates, steps, etc. on the surface of quartz grains) probably is more

realistic than explanations based on existing cleavages in quartzes. Attention was

also paid to microfractures and the laminae of quartzes between them called

“sheets.” Such a sheet of quartz, usually 2-20 Fm thick, was considered to be



E. B. A. BISDOM



70



the smallest weathering entity. Microfractures can subdivide the sheet into small-



er particles that are clay sized. In nature, however, quartz is frequently common

in the 2- to 20-pm silt fraction and does not occur in a dominant form in the clay

fraction. It was also pointed out by Moss and Green (1975) that quartz grains can

already be well-rounded when they leave the source rock and that it is therefore

unrealistic to always assume angular particles that gradually become more rounded with increasing maturity. Conversely, angular quartz can often be found in

soils and sediments. Magaldi (1978) indicated that two contradictory interpretations exist, one which cites the more rounded and another that cites the more

angular quartz grains during weathering.

A cathodoluminescent (CL) study of quartz sand grains was made by Tovey

and Krinsley (1980), who pointed out that the common secondary electron (SE)

micrographs (emissive mode micrographs) do reveal surface information on the

quartz grains but no subsurface information as seen in cathodoluminescent micrographs. The surfaces of quartz grains, cross sections of quartzes, etched

grains, and heated quartzes were studied with the SE and CL modes. Cathodoluminescence is significantly affected by slight changes in the chemical composition of the quartz grain, and cracks that are not visible in the SE mode can

often be recognized in the CL mode. Study of the spatial distribution of narrow

and broader dark bands, of dark patches, and of other characteristics in the CL

micrographs, together with information obtained from SE micrographs, allowed

some insight into the various processes which affected the quartz grains.

c. Micas. Scanning electron microscope studies of weathered micas are often

done in combination with nonsubmicroscopic techniques. Jackson and Sridhar

(1974) studied Li exfoliated and freeze-dried phlogopite flakes. Scanning electron micrographs indicated that the osmotic force and swelling created by Li+

resulted in the gliding out of interstratified saponite layers which became twisted

and curled during this process. Saponite was formed from phlogopite with vermiculite as an intermediate. Gliding out of layers only occurred when salt was

removed from the solution and electric double-layer swelling took place in distilled water during the experiments.

Scanning electron microscopy allowed the study and portrayal of tracks and

holes in micas (Lee et al., 1974). The tracks were produced by spontaneous

fission of 238U under natural conditions (235U

must be activated to give thermal

neutron bombardment and induced fission particle tracks). Upon splitting of the

uranium nucleus, large fragments can move through the micas with considerable

energy and leave behind trails of damage called “tracks” which are about 20 pm

long and have diameters of about 0.015 pm. These fission tracks play a role

during the weathering of micas and also influence cation exchange capacity.

Tarzi and Protz (1978) studied the weathering of micas obtained from rocks.

Upon the start of weathering, the micas split at their edges and this process

proceeds inward along planes. The exfoliated stage is reached when the layers

+



SUBMICROSCOPIC EXAMINATION OF SOILS



71



become separated. During the exfoliation process bending may affect the individual layers which may then take various forms. Holes in the micas were

thought to have been occupied by quartz and other minerals, rather than to have

resulted from spontaneous fission processes as advocated by Lee et al. (1974).

Secondary material could accumulate in the spaces provided by the weathering

micas. Crusts could form in them and roots could penetrate the mica.

Secondary micas are frequently observed in weathering micas. Verheye and

Stoops (1975) made a scanning electron micrograph of kaolinite between biotite

lamellae in a soil from the Ivory Coast. Illite was distinguished by Taupinard

(1976) on weathering biotite flakes of an altering granite in France together with

dissolution, new formation, and disaggregation features. Sousa and Eswaran

(1975) found that large biotite flakes in a saprolite from Angola were pseudomorphically altered to goethite. Scanning electron microscope observations indicated that microdroplets of goethite covered the surfaces of weathered biotite.

d. Other Minerals. Dissolution of olivine to deeply etched and pitted weathered olivine probably occurred at particular sites where structural dislocations

emerged in the olivine, similar to weathering feldspars (Wilson, 1975). This

weathering mechanism was confirmed during experimental studies by Grandstaff

(1978). The initial dissolution of freshly crushed olivine was where lattice imperfections occurred (e.g., dislocations and cleavage planes). Pits and rounded

edges were found in altered forsterite. Dissolution was more rapid along surface

discontinuities than along the general surface in the initial phases of weathering,

whereas surface dissolution could dominate the overall rate of reaction in subsequent phases.

Berner et al. (1980) studied the weathering features of augite, hypersthene,

diopside, and hornblende. In the initial phase, only part of the surface of the

altering pyroxenes and amphiboles was affected, as was the case with olivine and

feldspar. Lens-shaped etch pits formed parallel to the long and short axes of the

minerals, according to SEM observations, and this gave different alteration

patterns of deeply striated surfaces with end-to-end alignment along the long

axes and rough-walled cracks with side-by-side alignment along the short axes.

Secondary clay could be found in cracks of the weathered minerals. Tooth- or

needle-shaped walls were present in the cracks because primary mineral fragments were maintained between expanding lens-shaped pits during weathering.

Scanning electron micrographs of weathered amphiboles from Israel also showed

tooth- and needle-shaped walls of cracks and pores (Williams and Yaalon,

1977).

Detrital garnets from fluviatile, littoral, and aeolian desert sands were studied

with the SEM by Magaldi (1977). Furrows, V-shaped pits, triangular pits, quadrangular pits, clusters of polygon-shaped pits, and coalescent etch figures were

found. Flicoteaux et al. (1977) studied the alteration of phosphate minerals in

phosphate-containing Cretaceous-Tertiary sediments of the Senegalese-



72



E. B. A. BISDOM



Mauritanian basin. Pseudomorphous transformation of wavellite to crandallite

was found. Crandallite crystallites could take different orientations with respect

fo wavellite. Scanning electron microscopy also demonstrated an increase in

porosity during the transformation of wavellite to crandallite. M o m (1978)

studied isotropic phosphatic nodules, probably weathered guano fragments, in

the A1 horizon of a soil developed on basaltic colluvium on Santa Fe Island of

the Galapagos archipelago. Small craters and globules were present in the

nodules.



4 . Newly Formed Minerals



A considerable number of newly formed minerals in unhardened samples of

soils have been studied by SEM. Submicroscopy has mainly been used to obtain

information on the surface morphology of the minerals. Nonsubmicroscopic

techniques were used primarily for identification purposes.

a. Carbonate, Gypsum, Anhydrite, and Celestite. Needle-shaped calcite

from Turkey, called lublinite, was studied with the SEM by Stoops (1976).The

individual lublinite crystals were stacked in an echelon with their c-axes in a

parallel position. This explained certain optical characteristics as determined in

thin sections with the light microscope. Various scanning electron micrographs

of lenticular gypsum, weathered lenticular gypsum with a comb structure, gypsum microlites, and a rosette-like aggregate of prismatic gypsum crystals were

published by Stoops et al. (1978). The authors also studied anhydrite fibers,

which were parallel to each other, on gypsum in soils from Peru. Celestite was

found as long square prisms elongated according to (100)and had a well-developed (011)form. Stoops et al. (1978)found celestite in gypsiferous soils from

Algeria, Iran, and Iraq. Upon weathering of celestite, grooves could develop

normally to the prism faces.

b. Halite, Thenardite, Bloedite, Hexahydrite, and Barite. The morphologies

of halite, thenardite, and bloedite were studied with the SEM by Driessen (1970)

and Driessen and Schoorl (1973).These salts came from the Konya basin in

Turkey and were present in salt crusts. Mirabilite was recognized in the field but

could not be transported to the lab because of its high water content.

The porosity of the salt crusts could also be investigated, and it was found that

the needle-shaped thenardite gave more porosity to the crust than the platy

bloedite. Halite could seal the surface of the soil. Vergouwen (1981)studied salts

from the same basin in Turkey with the SEM-EDXRA. Crystallographic properties and morphologies of individual salt crystals were examined. The relations

between different salt crystals in salt assemblages were also studied. It was found

that identification on the basis of morphology alone is not always possible;

microchemical in situ analysis with the EDXRA is then necessary. Thenardite

occurred in two crystal forms, as needles and in another crystal form when



SUBMICROSCOPIC EXAMINATION OF SOILS



73



associated with other salt minerals. Trona, bloedite, and hexahydrite made the

salt crust very fluffy. Halite formed a smooth crust and sealed the soil. Several

scanning electron micrographs of various morphologies of halite in soils were

published by Eswaran et al. (1980). Attention was also given to crust formation

by halite.

Tursina et al. (1980) published scanning electron micrographs of thenardite in

hydromorphous Solonchaks from the Soviet Union. Scanning electron microscopy also permitted the effect of salt crystallization on soil fabric and structure to

be studied. Stoops et al. (1978), using the SEM, found hexahydrite on ped

surfaces of a salic Gypsiorthid from Iran. Hexahydrite was mixed with gypsum

crystals. Barite (microlites consisting mainly of prism) was found by Stoops and

Zavaleta (1978) in a typic Haplustalt of Peru.

c. Pyrite, Jarosite, and Gypsum. Scanning electron micrographs of pyrite,

jarosite, and gypsum in a paleosol of eastern Nigeria were published by Moormann and Eswaran (1978). Pyrite framboids were found associated with organic

matter, and fine gypsum needles could protrude from these. van Breemen and

Harmsen (1975) photographed jarosite by SEM before and after dialysis with

distilled water over a period of 4 months. Miedema et al. (1974) studied pyrite,

jarosite, and gypsum in four soils of inland polders of the Netherlands. Paramananthan et al. (1978) investigated the effects of drainage on pyrite-containing

marine clays in the coastal area of Malaysia and presented SEM photographs of

pyrite, jarosite, gypsum, ferriorganans, fungal mycelia, and diatoms.

d. Iron- and Manganese-Containing Minerals. Iron-containing minerals in

laterites have been the subject of a number of SEM studies. Schmidt-Lorenz

(1974a,b, 1975) studied many laterites of tropical regions and remnants of laterites in paleosols of Europe. Several scanning electron micrographs of hematite

and various types of goethite were presented. The process of lateritization was

subdivided into primary and secondary ferrallization. Kuhnel et al. (1975) studied goethite in laterite profiles and found that the highest crystallinity of the

mineral was found near the surface of the laterite and the lowest at the base of the

profile between the soil and bedrock (i.e., at the start of weathering). Poorly

crystalline goethite could also contain nickel, chromium, and aluminium.

Hematite and goethite crystallites were studied with the SEM in plinthite by

Moormann and Eswaran (1978) and Eswaran et al. (1978).

Iron-containing minerals have also been studied in nonlateritic soils. Lepidocrocite was found in the upper part of the B horizon of Molkenpodzols in the

Vosges of France (Guillet et al., 1976). Lepidocrocite was a weathering product

of hematite and occurred as stacks of subparallel platelets, with local intermineral porosity, on scanning electron micrographs. Babanin et al. (1976) indicated that very fine goethite particles with diameters of less than 5-6 nm were

dominant in Ortstein. The forms and compositions of iron compounds in various

soil concretions in a number of soils from the Soviet Union were investigated.



74



E. B. A. BISDOM



Scanning electron microscope studies of manganese-containing minerals such

as lithiophorite, nsutite, birnessite, and feitknechtite were made by Eswaran et

al. (1978). The minerals were present in nodules found in tropical soils.

e. Other Minerals. Various forms of gibbsite were studied in tropical soils by

Eswaran et al. (1977). Gibbsite can be present in very small amounts in tropical

soils but may also form gravel-sized aggregates or sheets that are recognizable in

the field. Dobrovolsky (1977) studied gibbsite crystals with a diameter of 1-20

pm in peaty soils of the Kilimanjaro area of Africa at an altitude of 2950 m above

sea level; he favored a biological origin of the mineral.

Biogenic opal has been studied with the SEM in soils of the United States

(Wilding and Drees, 1971, 1973, 1974; Wilding and Geissinger, 1973). Opal

isolated from trees differed considerably in amount and size and was dependent

on the tree species. Only hackberry produced enough opal to be incorporated in

the soil. Scanning electron micrographs demonstrated that there was a characteristic difference between tree-leaf opal and grass opal. Opaline isolates of wet

soils often contained sponge specules and diatoms. Wilding et al. (1977) presented a review on silica present in soils and the conversion of silica hydrogel to

silica polymorphs (opal, chalcedony, quartz, cristobalite, and tridymite).

5 . Organic Matter

Humic and fulvic acids (HA and FA, respectively), inclusive of metal and clay

complexes, were studied with the SEM by Chen and Schnitzer (1976). Fulvic

acid morphologies were investigated at pH 2-10 and those of HA at pH 6-10.

Metal-FA and clay-FA complexes were also studied at different pH. The SEM

was used by Bruckert et al. (1974) to investigate organomineral complexes in

aggregates from Andosols of the Canary Islands and France. The morphology of

these aggregates was different from that of aggregates consisting of a clayhumus complex. Benayas et al. (1974) published a scanning electron micrograph

of plant remains and small soil components in the upper part of an Andosol in the

Canary Islands. Organomineral complexes in alkaline extracts of soil were investigated by Dormaar (1974). Fungal aggregates in sand-dune soil from Canada

consisted of threads of branching mycelium from fungi to which sand grains

adhered (Clough and Sutton, 1978). It was also found that amorphous material

could form a sheet on the hyphae and act as an adherent between fungal hyphae

and sand grains. The amorphous material consisted of polysaccharides and was

possibly produced by fungi or bacteria. Aggregates formed when the fungal

mycelium was in active symbiosis with the host plant.



6. Soil Structure and Fabric

Numerous studies have been performed with the SEM to obtain information on

various aspects of soil structure and fabric. Specialists in soil mechanics have



SUBMICROSCOPIC EXAMINATION OF SOILS



75



done considerable work to obtain information on the behavior of especially

clayey soils under different experimental conditions, whereas soil micromorphologists frequently have had a closer look at soil constituents in soil peds

and aggregates.

a. Arrangements, Orientations, and Behavior of Soil Components under

Various Conditions. Scanning electron micrographs and X-ray diffraction measurements were made of oriented clay samples obtained at different pF values by

Tessier and Pedro (1976). Micrographs were made parallel or perpendicular to

the orientation plane of the clay platelets of calcium kaolinite, calcium montmorillonite, and calcium illite. It was seen that considerable changes in the clay

structures could occur with only minimal changes in the measured ranges of pF

values; changes were greatest in the lower pF ranges.

Structural changes in soil pore systems induced by Na/Ca exchange were

studied with the SEM by Chen et al. (1976). At a low sodium adsorption ratio

(SAR), fine material adhered to the sand grains or formed large aggregates. At a

higher SAR, the fine material separated from the sand grains and filled pores.

Another result was that calcium montmorillonite formed large irregular porous

aggregates when a suspension was quickly frozen and dried, whereas sodium

montmorillonite gave very thin sheets that were usually folded. An explanation

for this phenomenon was presented.

Sheeran and Yong (1974) indicated that rearrangement of soil particles in the

soil environment is relatively simple as long as the soil is porous, but can only

occur by way of individual minerals if only little porosity is left. Experiments

indicated that virtually all changes in the orientation of clay particles occurred at

lower pF levels, a result which was also obtained by Tessier and Pedro (1976).

Much work on the quantification of individual clay particle alignments, including those in scanning electron micrographs, has been done by Tovey (1974,

1980) and Tovey and Wong (1974, 1980). Attention was given to photogrammetric and quantification techniques used in TEM, light microscopy, and XRD.

A film measuring technique and digital computer techniques for the quantitative

analysis of the orientation of clay particles in scanning electron micrographs of

peds and aggregates were discussed. Such techniques can help in the quantification of soil fabric types in such micrographs. Attention was also given to particle

alignments in scanning electron micrographs caused by mechanical stresses during experiments and various preparation techniques such as oven-drying, airdrying, substitution-drying, freeze-drying, and critical-point drying.

The SEM has also been used to study the broken surfaces of soil fragments

from a thin iron pan, the argillic horizon of an alfisol, and the cambic, argillic,

and oxic horizons of tropical soils formed on basalt (Eswaran, 1971). Argillans

have also been examined with the SEM (Osman and Eswaran, 1974; Callot,

1978; Koppi, 1981). Using the SEM, an impression of the degree of orientation

of clay and silt in pores can be gained; whether microlayers are present or absent

in the argillan can also be ascertained.



76



E. B. A. BISDOM



b. Aggregates and Crusts. Scanning electron microscope studies have been

done by Moreno et al. (1978) of aggregates from black earth in Southern Spain.

Clay minerals exhibited platy intermineral pores when observed at higher magnifications with the SEM. The aggregates also contained a few cylindrical pores

with diameters of 0.5-2 pm. The aggregates in the soil had a similar microstructure. Buol and Eswaran (1978) investigated aggregates in oxisols and found that

inter- and intraaggregateporosity could be considerable. Moura and Buol(l975)

studied a Eutrustox in Brazil. The soil originally had a porosity of between 15

and 34%; continuous cultivation over a period of 15 years had decreased the

porosity to 10-22%. The type of porosity was studied with the morphology of

the minerals on the surfaces of pores, fractures, and aggregates. Only a few fine

pores were present in clay balls, which had a higher density than the surrounding

soil materials.

Aggregates and weathered complexes in Andosols of France were studied in

detail, with the inclusion of SEM and TEM, by Hktier (1975). Organomineral

complexes were extracted from the aggregates in a step-by-step method, and

each of the residues was studied. The aggregates had diameters of 5-50 pm and

contained minerals, organic matter, and embedding cement which were apparent

at higher magnifications. The minerals were coated with organomineral complexes. Extractions removed virtually all of the coatings, but an insoluble humic

debris consisting of humin remained on part of the mineral surfaces. It was also

demonstrated that humic acids, which were the most condensed and stable, were

situated in clay-humus spherules, the central part of which were often occupied

by glomerated halloysite.

Toogood (1978) performed studies on aggregate stability in Ap horizons of

various soils in Alberta, Canada. Only very weak correlations were found between the stability of the aggregates and their organic-matter content, clay percentage, carbonate content, or specific surface. On a microscale, considerable

differences between individual aggregates were indicated by SEM,and the suggestion was made that general rules should be developed to explain aggregation

and cementation for each individual soil type in separate regions and under

different management systems. This could form a basis to obtain techniques for

improving aggregate stability for individual soils.

Intergranularcontacts were examined in sands, loesses, and clays by Barden et

al. (1973) to study collapse phenomena when wetted under load. The SEM

allowed the investigation of the arrangement of individual and of combinations

of clay platelets on and between larger soil components at various magnifications. Ducloux and Ranger (1978) examined aggregates in fragipan horizons of

French soils with the SEM. Strands and bridges of clay minerals and iron oxides

were found between the aggregates. Such a structure can be rigid, and if it is

broken will break by brittle failure. Wang et al. (1974) studied a large number of

fragipans in Nova Scotia, Canada. Clay bridges were indicated by SEM between



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III. Applications of Electron Microscopy

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