Clay Min. Bull. (1964), 5, 338.
THE
IN
OF
S1TU
SOME
FORMATION
KAOLINITE
AND
DEVELOPMENT
MACROCRYSTALS
L. R. M O O R E
Department of Geology, University of She)field
Read 8th November 1963
ABSTRACT: Tonsteins, which are kaolinite-rich rocks with an amorphous groundmass containing much organic material, occur widely
in European Coal Measure deposits. The nature of these rocks and
their peculiar fabric is considered in detail and evidence is adduced
which suggests that they were derived from soils and that the original
organic components (micro-organisms and fungi) played a significant
part in the formation of the kaolinite macrocrystals.
INTRODUCTION
Rocks referred to as Tonsteins are well known within the European Coal
Measure Basins; they occur within the coal seam or in the accompanying
fireclay or roof shales. The term merely signifies an argillaceous rock,
but a structural significance has become implicit in that it indicates a
well-consolidated homogeneous rock, often with conchoidal fracture
and distinct from thinly bedded or fissile shale bands which may have a
similar relationship to the coal seam. These rocks vary in colour from
dark brown or black, to grey or white, and are usually thin (1-3 in.) but
very exceptionally may reach a thickness of 3 or 4 ft. An account of
their nature and occurrence (Moore, 1963) has also reviewed the
literature concerning their study. The importance of Tonsteins follows
from the remarkable regularity of their occurrence at definite horizons
in the succession of Coal Measures rocks and hence their value in
correlation within a coalfield, or as a means of correlation between
disconnected coal basins. Their recognition, apart from physical
characteristics, also depends upon a content of kaolin-type minerals,
first noted by Termier (1923) who referred to the presence o f the mineral
'leverrierite.' This distinctive mineralogical characteristic led to an
impetus in their study and SchiJller & Grassrnan (1949) identified both
"leverrierite' and kaolin, whilst Grim (1953) later suggested the term
'leverrierite' should be discontinued and referred to kaolin alone. The
338
Kaolinite macrocrystals
339
habit of the kaolin present led to a mineralogical classification (Schtiller,
1951) in which the rounded forms or boules characterized the 'Gruppen'
type of Tonstein, whereas prismatic and vermiform kaolin macrocrystals served to identify the 'Kristal' Tonstein. Upon this basis, and
in conjunction with consideration of other mineralogical constituents,
the Tonstein bands are separately identified and used in stratigraphic
subdivision. Many workers attracted by the mineralogy, physical
characteristics and widespread lateral occurrence of these rocks have
considered the problems of genesis. Thus, the ash of forest fires, finely
degraded felspathic materials deposited as sediments (Termier, 1923;
Pruvost, 1934), and kaolin derived f r o m weathered fireclays
(Teichmtiller, Meyer & Werner, 1952) have been considered. Other
authors (Stach, 1950; Petraschek, 1942) sought the source in volcanic
ash, whilst Scheere (1955) believed that, under certain physico-chemical
conditions in shallow water environments, kaolin would be formed from
fine-grained sediments consisting of mica- or illite-type clay minerals,
Schtiller (1951) and Hoehne (1954) thought the rocks originated from
alumina-rich aqueous solutions or gels deposited within the peats.
Although Tonsteins occur rarely, their distribution is widespread
within the Carboniferous and Permo-Carboniferous coal-bearing rocks
of the northern and southern hemispheres (Moore, 1963). Their
frequent occurrence within coals, or in close association with a coal
seam, suggests an origin connected with the process of peat formation,
development, and possibly with erosion. This relationship is substantiated in records of their presence within Tertiary coals.
The chemical composition of these rocks (Pruvost, 1934; Scheere,
1955; Hoehne, 1951) shows a significant range; those most typically
developed and with the physical characteristics enumerated above are
often very pure and consist mainly of silica and alumina. In general
terms the SiO2 may range from 35 to 50% with A1203 30 to 45%. The
high SiO2 and A1203 is characteristic, and although MgO, CaO, Na20,
K20 are invariably low each may range from less than 1% to 2-3%.
The iron content is variable, but often low, whilst fractional percentages
of MnO, and P205 may be present and TiO2 is frequently around 2-3 %.
The amount of H20 is generally in the range 10-15% and the organic
matter content is very variable. The analyses of some of the purer
Tonsteins are directly comparable with kaolinites such as that from
Murfreesboro, Arkansas, U.S.A., which contains 97-2% kaolin.
Despite the apparent homogeneity, the lack of bedding and the
conchoidal fracture, minor structures, such as included lenses or wedges
of darker or lighter coloured material, are often visible in hand specimens.
Flow structures, sometimes emphasized by broadly folded curves of
included organic material, indicate an originally plastic medium.
340
L. R . Moore
In thin section a pale yellow or brown groundmass is present. This
may form, e.g. in Tonstein from Erda, a very high proportion of the
rock with extensive and relatively continuous distribution and little
other mineralogical content than kaolin macrocrystals, although there
is invariably some organic material. In other Tonsteins, e.g. Graziella,
the groundmass is more restricted or patchily developed. Included
within it or separating individual areas are larger masses of organic
matter and a range of minerals which may variously include quartz,
mica, clay minerals, pyrite, anatase and kaolin macrocrystals. The
groundmass, particularly that of the former type, consists of small
rounded ill-defined floccules (seen best with partially crossed nicols)and
carries small dark cells (1 t0 or colonies of such cells which resemble
bacterial colonies; in places larger cells or chains of cells and fungal
hyphae are present. The groundmass is predominantly amorphous
and isotropic and is colloidal in nature. Rotation under crossed nicols
presents a patchy clouded effect with a suggestion of linear arrangement
which might be due to streaming double refraction (Baver, 1956). The
original consistency was that of a gel, paste or aggregate of plastic
material composed largely of alumina and silica together with an
organic content of degraded humic material and included micro-biological remains. This resembles the composition of a mixed organicinorganic soil colloid and by processes which include dehydration and
separation of the mixed colloid the kaolin macrocrystals develop within
the groundmass.
GENESIS
OF
SOME
TONSTEINS
The study of a large number of European and British specimens has
shown that a feature common to all is intense aerobic microbiological
degradation of the organic matter. The microbiological details have
been described (Moore, 1963) for the German Erda Tonstein. This
evidence is important in assessing the conditions under which the rocks
were formed and provides information on the chemical composition of
the rocks and of the processes leading to the kaolinite content. The
recognition of micro-organisms simulating bacteria, fungi and actinomycetes, and analogous to those of a soil population, when taken in
conjunction with other physical and chemical properties leads to the
conclusion that these rocks were formed as soils. By analogy with
modem tropical soils of comparable chemical and kaolinitic mineralogical constitution the method of formation might have been as podzol
or 'lateritic type soils' (Mohr & Van Baren, 1954; Van der Merwe,
1949) or by other processes already described (Moore, 1963). Mohr &
Van Baren (1954) describing podzolization processes in tropical forest
Kaolinite macrocrvstals
341
profiles, referred to the formation of a mottled clay in the A horizon.
This mixed colloidal kaolinite-iron-oxide hydrate complex was loose,
spongy and pervious to air and water in the non-dispersed state. With
leaching of the iron, kaolin and kaolinitic materials remained and the
consistency became plastic, tough and impervious.
Many authors including Robinson (1949), Mohr & Van Baren (1954),
Joffe (1949) and Baver (1956) have described soil colloids containing the
trihydrate gibbsite, kaolinite and related minerals as forming important
constituents of podzols and lateritic soil types. A consideration of
literature on the environment of kaolin formation is instructuve in
indicating the conditions necessary for the formation of kaolinite-rich
soils. Ross (1943) stresses the stability of kaolin as an end product
associated with other stable minerals, and indicates that it is formed in
conditions where leaching is effective and where strong oxidation of
organic matter by an active soil microflora working in a humid climate
produces humic acids. Mohr & Van Baren (1954) consider humic
acids play a major r61e since they not only control pH but are also
effective in removing bases. Keller (1956) has discussed the influence
of various environments on the formation of clay minerals. With
regard to kaolinite he considers the environment as a 'chemical microclimate' controlled by factors such as the biota, concentration of active
ions, oxidation potential, temperatures of the reacting system, relationship to ground water and the nature of the water environment, e.g.
lacustrine or marine. Kaolin formation requires excess H+ and the
removal of Ca2+, Mg/+, Fe3+, Na+ and K+ (in fact all cations other
than aluminium and silicon) from a permeable substrate with strong
leaching by water that must not be stagnant. The high AI:Si ratio
leads to the removal of silica which is stabilized in solution by Na+
and K+, and iron if present must be removed from the system by
oxidation to oxide or sulphide. The conclusions of Nord & Vittucci
(1947) and Nord & Schubert (1961) are pertinent in considering the part
played by microbiological organisms in these processes. They studied
the chemical nature of the microbiological attack on cellulose by fungi
and showed the enzymatic action to be one of hydrolysis which by
stages resulted in acetic, oxalic and related acids from simple decomposition. Mohr & Van Baren (1954) state that bacteria, actinomycetes
and fungi are all concerned in the mineralization of organic matter.
Bacteria are principally concerned with initial breakdown of organic
matter whilst the fungi convert the material into new compounds which
remain in the soil. The micro-organisms require only limited air or
oxygen but are nevertheless also dependent upon the presence of some
calcium, magnesium and potassium as essential metabolic elements.
The same authors referred to the effect of pH on microbiological
342
L. R. Moore
activity, and state that below a critical value of pH 4.5 bacterial activity
ceases, the optimum range lying between pH 5.5 and pH 7.5. Fungi,
however, are dominant members of a soil microflora at pH 3"5-5.5 and
can flourish under extremely acid conditions. These authors conclude
that the critical pH value of 4-5 which limits bacterial activity is normally
indicative of a low level of mineral plant foods, such as calcium, magnesium and potassium, and below such a limit organic matter accumulates
and peat formation occurs. This critical pH value is also significant
in other ways for in sub-aquatic soils Mohr & Van Baren observed the
constitution of the soil colloid to be kaolin plus silicic acid. This fact
is in accord with the experimental evidence of Mason (1960), who
noted a marked change in the relative solubilities of alumina and silica
at a pH of about 4, at which value alumina is readily soluble and silica
only slightly soluble. Within the pH range 5-9 the solubility of silica
increases and alumina becomes practically insoluble and remains to
form lateritic soils and bauxites. At a pH of 4.5-5.0 iron forms complex
soluble compounds with organic matter (Mohr & Van Baren, 1954) and
given adequate sub-surface drainage iron may be removed from the
system and is deposited as concretions at lower levels or removed to
greater distances.
By analogy with these known conditions in modern tropical soils it is
possible to make suggestions concerning the environment under which
the Tonsteins were formed. That the medium was acid is indicated by
the predominance o f fungal attack and that it was conducive to metabolic
activity is demonstrated by the continued attack upon groundmass
organic material; thus the medium provided the minimal requirements
o f atmospheric or dissolved oxygen, and essential quantities o f calcium,
magnesium and potassium for metabolic activity were also present as
shown by analyses of these rocks. The pH was therefore above 4-5 and
ranged nearer pH 5.5-7 with high humidity and presumably high
temperatures. Rainfall, possibly of a monsoon type, with a fluctuating
water table was adequate for leaching--but since the SiO2 : AlzO3 ratio
and kaolinite content decreases with increasing rainfall (Mohr & Van
Baren, 1954) this would not be excessive. For Hawaian soils developed
from the same parent rock material at 500 and 1000 mm rainfall,
respectively, the optimum SiO2 and A1203 percentages in the composite
colloid were 43.92% SiO2, 31.33~o A1203 and 40.25~o SiO2, 33.04%
A1203.
These observations make it possible to integrate the various biological
and physico-chemical requirements which were conducive to the formation of the mixed inorganic-organic colloidal complex, constituting the
original gel-like material of the Tonstein. They illustrate also the bio=
chemical and biophysical nature of the environment, in which the
Kaolinite macrocrystals
343
macrocrystals of kaolin developed and provide a basis for the consideration of crystal development described subsequently.
D E V E L O P M E N T OF K A O L I N I T E M A C R O C R Y S T A L S
The smallest recognizable crystalline areas with a vitreous appearance are
ovoid in plan, lens shaped, and range between 2 and 10 t~. Larger examples simulate the 'boule' or Gruppen habit and either include coccoid
cells, filaments and fragments of organic material, or this material is in
part present on the surface. The organic material may be irregularly
distributed, partly radially arranged, or distributed in a concentric
manner. The inorganic matter is similarly arranged and optically is
only partially crystalline, consisting of isolated or separated crystallites.
These crystalloid or semi-crystalline mineral aggregates and their
associated organic matter have already been illustrated (Moore, 1963)
for the Erda Tonstein. Other Tonsteins exhibit these characteristics to
a more marked degree, and the organism Anthracomyces cannellensis
(Renault, 1900) is included within the 'boule' from the Hermance
Tonstein (Fig. 2a).
Tabular crystals of kaolinite in thin plate-like forms are common;
they do not exhibit a preferred orientation and are frequently seen in
juxtaposition over a considerable size range. Their in situ development
results from the reorganization of the constituents of the mixed inorganic
and organic colloidal groundmass and takes place in what are considered
to be developmental stages in the formation of the macrocrystal.
Initially over the rectangular area of a developing crystal the groundmass appears lighter in colour and the poorly defined borders merge into
the groundmass without suggestion of crystal growth pressure at this
stage. The organic content of this area consisting of bacteria-like cells,
fungal hyphae and small carbonaceous fragments, is either extruded to
the surface of the developing crystal or is included within it but shows a
minimum of orientation. At a later stage the inorganic matter, though
still of only a partially ordered nature, forms lamellae approximately at
right angles to the long axis of the crystal. The organic material is
disposed in prominent planar fashion on the surface of the crystal and
between the lamellae in the position of cleavage planes of the macrocrystal (Figs. la and lb). With increasing organization of the kaolinite
and consequently with developing crystal pressure at the margins, the
macrocrystal assumes a definite crystal outline. The outline may then
be marked by a rim of organic material, either extruded from the developing crystal margin, or resulting from compression of the groundmass parallel to the crystal margin. The planar distribution of organic
matter may be uniform and regular throughout the macrocrystal, but is
more commonly impersistent. Further reorganization of the cry-
344
L. R. Moore
stalline material takes place from local centres within the macrocrystal
boundary, with the development of lens-shaped crystalline areas o f
uniform optical properties. The consequent displacement of surrounding organic matter by crystal pressure results in discrete patterns of
organic and inorganic material. The reorganization is frequently less
regular and a spherical crystalline mass with radially arranged optical
properties resembling the 'boule' habit may occur within the tabular
macrocrystal. The included organic matter of the macrocrystal is then
compressed and displaced in sweeping curves around the developing
crystalline mass (Moore, 1963). Very few completely formed tabular
crystals without organic content are seen, but such as are observable
have less developed cleavage and more uniform optical properties.
The vermicular macrocrystals vary considerably in size and occur as
simple curved crystals, or as forms with amazing complexity. Their
development follows the trends described above for tabular crystals, but
the form is vermicular even at the earliest stages and the groundmass is
reorganized in this form. Thus the small vermicular crystals do not
show marked planar distribution of organic matter, but the larger
vermicular crystals exhibit marked planar distribution of the bacterialike cells, filaments, and fragments of organic matter in the cleavage
directions (Figs. lb and le). This direction is at right angles to the long
axis of the crystal at a given point. Where large wood fragments are
incorporated into the crystal f r o m the groundmass, reorientation is
often not affected and the fragments (Fig. lb) lie at angles across the
cleavage. Similar features concerning local centres of reorganization
within the macrocrystal and the formation of a definite crystal outline
are noticeable: alignment of the groundmass parallel to the margins o f
the vermicular crystal (Fig. lc) is a common feature.
Fie. 1. (a) Constance Tonstein x 500. Tabular kaolinite macrocrystal
showing planar orientation of organic matter in direction of cleavage planes,
between inorganic larnellae.
(b) Hermance Tonstein x 500. Tabular kaolinite macrocrystals joined at
right angles. Note general alignment of organic matter, and position of
non-aligned larger wood fragment.
(c) Hermance Tonstein x250. Vermiform kaolinite macrocrystals.
Note (i) alignment of groundmass parallel to borders of macrocrystal at
lower margin of figure; (ii) planar distribution of orgalaic material within
main macrocrystal, and incipient development of rounded 'boule' on its
lower border; (iii) dark zone of organic material in groundrnass with little
(or longitudinal) orientation of contents, also wedge-shaped area indicating
flow and compression ofgrourzdmass between crystals; (iv) small amount of
orientation in the upper crystal and smaller amount of included organic
material.
Kaolinite maerocrystals
345
346
L. R. Moore
There are many other irregular forms which masses of macrocrystalline kaolinite may assume and which appear to be various combinations
o f the three previously described habits. These occur more commonly
in restricted or irregularly shaped areas of groundmass and in those
rocks with a greater amount of organic matter present as large entities.
A common form depicted in Fig. 2b has an irregular rounded but
crenulate margin suggestive of plastic flow conditions. The margins o f
such crystalline areas are well defined and the adjacent groundmass
shows compressional features. Optically there is a lack of uniformity
and incipient crystallization with patchy extinction indicates a radial
direction of the crystallites developing from several individual centres.
The included organic matter is similarly oriented in a very general radial
direction and intersects the margins at right angles--a direction at
considerable variance to that of similar organic matter in the surrounding
groundmass. Fig. 2c illustrates the complexity of other kaolin macrocrystalline masses, and shows the very considerable changes in the
direction of alignment o f the organic matter over a small area. It is
clear that this alignment is in part due to the development of the separate
kaolin macrocrystals and that there is no preferred orientation resulting
from external causes during macrocrystal formation. Some degree o f
directional control appears to have been exercised upon the form of the
resulting macrocrystal by the nature or manner of distribution of the
organic content :included within the groundmass. Thus in Fig. 2c the
wood fragments along the upper margin controlled the direction of the
developing crystal at this point and the organization of the degraded
material and hyphae associated with it suggests a form typical of the
degradation of tracheid material. This is strikingly so in the lower part
o f the figure where the fungal remains represent a 'Phellomyces' pseudomorph after tracheid structure (Moore, 1963). The curved remains
have led to the development of a vermicular crystal, with a breadth that
FIo. 2. (a) Hermance Tonstein x 650. 'Boule'-like form of kaolinite containing colonies of Anthracomyces cannellensis arranged in partly radial
manner; these may have contributed to the aggregation and form of the
kaolinite mass.
(b) Constance Tonstein x 250. An irregular form of kaolinite. Represents part of a continuous succession of such forms, suggestive of plastic
flow. Note radial and lateral orientation of included organic matter, sharp
contact with, and compression of, groundmass.
(c) Hermance Tonstein x 800. Note (i) control of upper margin of crystal
by wood fragment, and its displacement by later rounded 'boule'; (ii)
'Phellomyces' pseudomorph after tracheid structure and control on size of
central vermicular crystal; (iii) large mass of wood fragments and hyphae
(left centre), and poorly oriented and indecisive form of crystalline material.
Kaolinite maeroerystals
347
348
L. R. Moore
of the distance between the wood elements. The larger mass of wood
fragments and associated fungal hyphae to the left centre of the figure
are only partially oriented and this mass may have controlled the
indecisive form of the crystalline material thereabout.
The size and form of macrocrystals are frequently controlled by large
resistant masses of carbonaceous material or wood cells in the adjacent
groundmass. Where these objects are less resistant they are often
plastically displaced by the developing crystalline area particularly by
the rounded 'boule' habit (Fig. 2c) and by vermicular crystals. Macrocrystals may occur in juxtaposition or even interfere one with another,
and groups of tabular crystals may have the plastic groundmass packed
against them or appearing to have flowed over them often with the
formation of later vermicular crystals. It is possible to suggest the
relative order of formation of macrocrystals under these conditions.
The evidence described above indicates that the kaolinite macrocrystals were formed #1 situ by processes involving macrocrystallization
from an aluminium silicate gel; the latter was either formed or contained
within the groundmass and intimately associated with the humic products of microbiological decay in the form of a mixed colloid. The
development of macrocrystals from a colloidal groundmass is an unusual
feature and the processes described differ significantly from those
normally associated with the formation of crystals from solutions or
melts. Baver (1956) has stated that in most soil colloids there is a
mutual interaction of the organic and inorganic colloidal fractions on
the physico-chemical properties of the mixture. By polar adsorption
the organic colloid particles are closely packed on the surface of the
orienting substance and the mineral particles joined together in this
process. The platy structure and physico-chemical properties of the
kaolin particle leads especially to such alignment. These observations
appear to be of particular significance to the present work.
POSSIBLE
CAUSES OF MACROCRYSTAL
FORMATION
There is a very close analogy on microbiological, physical, chemical and
mineralogical grounds, between Tonsteins and some of the characteristics
of certain soil types. In seeking a possible explanation of the presence
of macrocrystals of kaolinite and of their wide range of form, and also in
considering the processes involved in their formation it is pertinent to
bear in mind the evidence of soil science.
The occurence of mixed colloids containing an inorganic constituent
of kaolinite, and an organic constituent of humic degradation products
which carries microbiological organisms is well known. The interrelationship of these two constituents is however of considerable interest;
Kaolinite macrocrystals
349
it is known that there is a mutual interrelationship between the two
fractions and that this controls the physico-chemical properties of the
mixture. Since micro-organisms are an essential part of the organicmatter content, and are intimately concerned in macrocrystal formation
their relationship to the original kaolinite particle is of interest. Thus,
are the micro-organisms directly concerned, in a vitalistic physicochemical sense, with the actual production of the kaolinite particle as a
result of their metabolic processes, or by means of enzymatic degradation
products? There does not appear to be an answer to this question.
The micro-organisms do, however, produce the necessary environment,
in terms of requisite pH control and humic degradation products, for
the genesis of the kaolin particle and its association in the mixed colloid,
and in this sense the environment is biochemical.
Concerning the broader aspects of the interrelationship of the organic
and inorganic fractions of the colloid, there is evidence of the physical
relationships. Thus Sideri (1936) observed organic molecules to be
adsorbed and oriented on the surface of clay particles by reason (Myers,
1937) of the attraction of the slightly ionized polar humic compounds
by the electronegative inorganic colloid particles; the resulting polar
adsorption leads to close packing of organic colloids on the surface
whilst the inorganic particles are joined together beneath. Dehydration of
the adsorbed organic matter effects a close union between the inorganic
and organic material and the tenacious cementation which follows is responsible for the formation of aggregates. In this manner the organic
fraction brings about an orientation of the inorganic matter which could
lead to the processes of inorganic linkage and incipient crystallization.
The formation of stable aggregates in colloids provides a means of
organization of particles; in soils this is due (Baver, 1956) to the presence
of cementing agents amongst which organic matter, colloidal clay and
dehydrated colloidal oxides of iron and alumina are the most important.
Organic colloids (Bayer, I956) cause a high degree of aggregation of the
clay particles, produce large aggregates and are most effective in aggregating the finer clay fractions.
Many authors have referred to the effects of microbiological activity
in aggregate formation. Martin & Waksman (1940) have reported that
the growth of micro-organisms leads to aggregation of soil particles, the
degree of this being dependent upon the nature of the organism, and
amount of its growth, and the nature of the substrate. They also claim
that fungi cause both mechanical binding by mycelial growth and
aggregation resulting from the synthesis of organic compounds; these
conclusions have been substantiated by Martin (1945) and McCaUa
(1942). Peele & Beale (1940) innoculated clay loam with various
organisms and discovered that aggregation varied with different species
350
L. R. Moore
o f organism; fungal species representative of Penicillium and Fusarium
were much more effective than bacteria. Geoghegan (1950) believes the
metabolic products of micro-organisms to be much more effective in
aggregate formation than the cells of the organisms, and McCalla (1942)
has noted that some of the metabolic products render the aggregates
impermeable to water.
It is likely that some effects of aggregation by organic matter are to
be seen in the form of macrocrystals in the Tonsteins. Thus, the close
association of colonies o f fungal hyphae and bacterial-like cells with the
form of some of the irregular crystalline areas and rounded aggregates
may reflect the binding effects of the organism (Fig. 2). The pattern of
microbiologically degraded cellular structure controls the form of the
crystals in cases such as Fig. 2c. It is difficult to demonstrate any effects
of metabolic processes. A considerable degree of reorganization due to
incipient crystallite formation has taken place but this does not appear
to be inevitable (e.g. Fig. lb and Fig. 2a). These controls within the
macrocrystalline mass are additional to those exercised by large objects
in the groundmass described above.
The occurence of tabular or vermicular macrocrystals may follow
from the initial form o f the kaolinite colloidal particles. These are
hexagonal, compact and plate-like with colloidal properties determined
only by external surfaces; the unsatisfied valencies on the edges o f the
particle are responsible for ionic reactions, and such particles can be
readily aligned. The processes by which alignment is achieved may
follow from the polar adsorption of organic matter referred to above but
it is also likely that viscosity or plasticity plays a part. Plate-like
particles of small dimension constitute the most plastic colloidal
materials. Upon the application of pressure the particles are orientated
and slide over one another; with the release of pressure the particles are
held in the new position by water tension and there is no return to the
original position. The orientation pressure might be external and of a
geological nature, but the in situ development from a gel-like medium,
lack of pre-orientation and juxtaposition of the crystals, renders outside
pressure unlikely and certainly not severe.
Internal pressure of a
localized character and of variable degrees o f intensity from place to
place would follow from the strain and stress relationship which occurs
during alternative wetting and drying. According to Peterson (1944)
this could be expected to produce plate-like aggregates in a soil colloid.
Alternate wetting and drying causes aggregation, and cementation
follows drying when the colloidal content is high; reversibility of the
process may be slow. Such a method provides a means by which
regular orientation of particles is effected, and following which crystaUization may commence. There is still the problem of what defines
Kaolinite macrocrystals
351
the form and the boundaries of a tabular crystal which has developed
from the groundmass and is unhindered by any obstacles within it.
Vermicular macrocrystals probably owe their origin to pressure conditions coincident with wetting and a higher state of plasticity which
leads to plastic flow and which has created a basic orientation of particles
upon which crystallization develops. It is singularly difificult to explain
on other grounds the complex arrangements of crystalline masses such
as that Fig. 2c. Examples such as Fig. 2b are reminiscent of plastic flow,
their form, the orientation o f inclusions and the incipient crystallization
directions resemble features associated with glacier ice. The presence
o f roots, or burrows would also produce the local pressure conditions
resulting in reorganization of particles in a manner likely to produce
vermicular or irregular crystalline areas.
Plasticity is to a large extent governed by moisture content, but many
other factors have considerable effects upon it. Thus (Baver, 1956) it is
directly related to particle size, and dependent upon the amount o f
colloidal matter, and the nature of the inorganic content; it also varies
with the organic content, the plastic limits tending to be higher with the
presence of organic matter. The exchangeable cations present in a
colloid, and the temporary presence of electrolytes have an effect on
plasticity. These factors may not be significant in governing the various
effects seen in any one Tonstein but they may be of importance in
assessing the differences seen in the kaolin macrocrystals from different
Tonsteins.
It may be concluded, therefore, that the macrocrystallization of
kaolinite has taken place, or was initiated within a still plastic medium,
represented by a gel, paste or aggregate. Loss of water either intermittently by wetting and drying, or by a slow process of drying and
attendant dehydration appears to have been an important factor in
providing the final diagenetic features. Other factors enumerated
above would play some part but their actual contribution is difficult or
impossible to define.
REFERENCES
BAVERL.D. (1956) Soil Physics, 3rd edn. Chapman & Hall, London.
GEOGHEGANM.J. (1950) Trans. 4th int. Congr. Soil Sci. 1, 198.
GRIMR.E. (1953) Clay Mineralogy. McGraw-Hill, New York.
HOEHNEK. (1951) Decheniana 105-106, 33.
HOEHNEK. (1954) Chem, d. s
17, 6.
JOFrE J.S. (1949) Pedology, 2nd edn. Pedology Publications, New Brunswick.
KELLERW.D. (1956) Bull. Amer. Ass. Petrol. GeoL 40, 2689.
MARTINJ.P. (1945) Soll Sci. 59, 163.
MARTINJ.P. & WAKSMANS.A. (1940) Soil Sci. 50, 29.
MASONB. (1960) Principles o f Geochemistry, 2nd edn. Wiley, London.
MCCALLAT.M. (1942) Proc. SoilSci. Soc. Amer. 7, 209.
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