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III. Applications of Electron Microscopy
E. B. A. BISWM
analysis, and electron spectroscopy for chemical analysis) are discussed
(Bisdom, 1981a,b) .
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
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,
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
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
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
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
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,
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-
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
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
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
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.
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
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
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.
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
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