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On the Chemistry of Clay.

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Far more serious for FE microscopy is the fact that
the work function is a quantity which can only be
defined macroscopically and which, like the surface
potential and the Helmholtz equation, loses its meaning if - as is done with the probe-hole technique the adsorption of a single atom on a minute face
element consisting of only a few atoms is considered.
It is also doubtful whether the theory of the metallic
state can be applied to surfaces largely covered with a
metalloid adsorbate. As is known, there is much to be
said for the supposition that the surface atoms are
demetallized through adsorption [41,53,49,903.
For FI microscopy, in which the individual surface
atom plays an even more predominant role than in FE
microscopy, the question is raised as to whether the
band model of the metallic state may still be applied.
What is certain is that the Sommerfeld approximation
of the quasi-free electron is an unsuitable starting
point, as in this simplest model the parameters of the
atoms are amalgamated to those of the band,
whereas FI microscopy, conversely, resolves the metal
continuum into discrete atoms. Adescription in terms of
the band model, therefore, is acceptable only if it is based
on approximations in which the atom cores embedded
in the “electron gas” are explicitly considered.
[90] K. D. Rendulic and Z . Knor, Surface Sci. (Amsterdam) 7,205
(1967).
It should be borne in mind that all theories of the
metallic state have primarily been designed for the
interior of the three-dimensional metal. Dowden [9*1and
Bond 1921 are of the opinion that extrapolation to the
metal surface is most promising if it is based on approximations taking into account the spatial orientation of the metal bonds. From this point of view, the
models formulated by Goodenough 1931 and Trost 1941,
in which the spatial orientation of the bonds in the
metal is regarded as a consequence of the crystal field,
are of particular interest.
As long as no molecular orbital theory of the metal
surface or even a theory of the surface demetallized
through chemisorption is available, the physicochemist must, of necessity, continue to use the rather
crude theoretical tools with which FI microscopy has
reached its present, impressive, degree of development.
Received: March 21, 1968
[A 648 IE]
German version: Angew. Chem. 80, 673 (1968)
[91] D. A. Dowden in: Coloquio sobre Quimica Fisica de Procesos en Superficies Solidas. Libreria Cientifica Medinaceli, Madrid
1965, p. 177.
[92] G. C. Bond, Discuss. Faraday SOC.41, 200 (1966).
[93] J. B. Goodenough: Magnetism and the Chemical Bond.
Interscience, New York 1963.
1941 W. R. Trosr, Canad. J. Chem. 37, 460 (1959).
On the Chemistry of Clay[**]
BY U.HOFMANNr*1
Kaolinite, illite, chlorite, montmorillonite, and vermiculite are among the most important
of the clay minerals. Cations are embedded between the silicate layers and on the basal
faces of thecrystals. Whereas only the cations onthe outer faces are exchangeable in the first
three of the above minerals, those between the layers can also be replaced by others in
montmorillonite and vermiculite. These characteristics and the ability of montmorillonite
and vermiculite to undergo intracrystalline sweiling and to form inclusion compounds are
responsible for the industrial importance of kaolin and clay. The structure of halloysite
is particularly interesting, since the silicate layers in this case are rolled up to form tubes.
The possible role of the clay minerals as catalysts in the formation of petroleum and in
the beginning of life is finally discussed.
1. Form a n d Crystal Structure of Some Clay
Minerals
Only 40 years ago, practically nothing was known
about the clay minerals. The reasons for this are that
their crystals are so small that they cannot be observed
under the optical microscope and that a clay or kaolin
[*I
Prof. Dr. U. Hofmann
Anorganisch-Chemisches Institut der Universitat
69 Heidelberg, Tiergartenstr. (Germany)
[**I Based on a plenary lecture to the General Meeting of the
Gesellschaft Deutscher Chemiker in Berlin, September 1967.
Angew. Chem. internat. Edit. / VoZ. 7 (1968) 1 No. 9
is generally a mixture of several -minerals. The-discovery of the clay minerals was only made possible by
X-ray interferences, and the electron microscope soon
afterward made it possible to observe their crystals.
The clay minerals are very numerous, but we shall
deal here only with a few of the important ones.
1.1. Electron Micrographs
The electron micrograph of a well formed kaolinite,
which is the principal clay mineral in kaolins, is shown
68 1
in Figure 1. The crystal platelets, which are often
hexagonal, have a diameter of about 5000 A and a
thickness of about 500 A.
Fig. 4. Electron micrograph of a chlorite.
Fig. I , Electron micrograph of a kaolinite, shadowed with chromium
at a n angle of 20’.
Figure 2 shows the electron micrograph of a kaolinitic
clay, which is characterized, above all, by the presence
of very small and very thin crystal platelets that are
mostly no longer hexagonal. Their average diameter is
1200 A and their average thickness 250 A.
Figure 4 shows the electron micrograph of a chlorite,
which occurs in high concentrations e.g. in shales.
Chlorite also occurs as the clay mineral in arable soils.
The crystals are generally thin lamellae whose diameters are not very small, and which have an irregular
outline.
1.2. Crystal Structures
The crystals of the clay minerals mentioned above are
platelets and, accordingly, have a layer structure.
Figure 5 shows the crystal structure of kaolinite. The
structure of the silicate layer was predicted by Puuling 111
as early as 1930; a (lower) tetrahedral layer of Si and 0
is condensed with an (upper) octahedral layer of A1
and OH, thus giving the formula A12(OH)&i20~1.
Hendricks c21 and Brindley (31 established the sequence
of the stacked silicate layers, and suggested that the
octahedral layer is bound to the tetrahedral layer
above by hydrogen bonds to the 0 atoms.
Fig. 2. Electron micrograph of a kaolinitic clay.
-A
Figure 3 shows the electron micrograph of a micaceous
clay mineral often known as illite. This mineral rarely
occurs in high concentrations, but is often present in
clays and sometimes in kaolins, and is particularly
frequently the clay mineral present in arable soil. The
Crystals are irregularly shaped, and are Often very
small and very thin.
41 .A(
00
60H
Fig. 5. Crystal structure of kaolinite. A: hydrogen bonds, B: octahedral layer; C: tetrahedral layer. b = 8.93 A, distance between layers =
7.15 A.
In fine-particle clay, the silicate layers of the kaolinite exhibit
a parallel displacement in relation to one another. The clay
is then known as the “fireclay type” [41. This lattice disturbance is also observed, though to a smaller extent, in kaolins.
It is intensified in particular by dry grinding.
[l] L. Pauling, Proc. nat. Acad. Sci. U.S.A. 16, 578 (1930).
[2] S . B. Hendricks, Z. Kristallogr., Mineralog. Petrogr., Abt. A
95, 247 (1936).
Fig. 3. Electron micrograph of a n illite.
682
[ 3 ] G. W.Brindley and K . Robinson, Mineralog.Mag.J.minera1og.
SOC.27, 242 (1946); R. E. Newnham and G. W. Brindley, Acta
crystallogr. 9, 759 (1956); 10, 88 (1957); G . W. BrindIey and M .
Nakahira, Mineralog. Mag. J. mineralog. SOC.31, 781 (1958).
141 G . W. Brindley and K . Robinson, Trans. Faraday SOC.42 B,
198 (1946); Trans. Brit. ceram. SOC.46, 49 (1947); U . Hofmann,
Silikattechnik 8, 224 (1957).
Angew. Chem. internat. Edit. / Vol. 7 (1968) j No. 9
The micaceous clay mineral illite was extensively
studied in 1937 by Hofmann and Maegdefrau[51 and
by Grirnr61. Its structure can be derived from that of
coarsely crystalline mica [7 31; the silicate sheet consists
of two tetrahedral layers with the vertices of the tetrahedral pointing inward, these two layers being joined
by an octahedral layer (Figure 6). Some of the Si
atoms in the tetrahedral layers are replaced by A1
atoms, thus giving the silicate sheet a negative charge,
which is neutralized by potassium ions situated between
the sheets. A1 in the octahedral layer may also be
replaced by Mg, FeII, Fe111, etc.
K’
K’
K‘
K’
K‘
K‘
K’
K’
K’
K’
close to the trioctahedral type, though dioctahedral
types also occur.
The crystal structure of montmorillonite LlO1 is shown
in Figure 7; the structure of the silicate layers is the
same as in mica sheets. An interesting characteristic
of montmorillonite is its ability to absorb water
(according to the water vapor pressure) between the
silicate sheets, with simultaneous unidirectional intracrystalline swelling, perpendicular to the sheets. This
intramolecular swelling also occurs with other polar
liquids.
OSI
Fig. 6. Crystal structure and cation exchange of mica. Top: a free
basal surface with exchangeable cations ( 0 ) ;the K ions of the t w o inner
layers are not exchangeable.
Mite is much more finely crystalline than the coarse mica (cf.
Fig. 3). It generally has a lower content of potassium ions, as
well as distortion of the lattice by parallel displacement of the
silicate sheets, so that it can often be indexed as orthorhombic [51. If the octahedral layer contains mainly trivalent
ions such as AP+, the mineral is described as “dioctahedral”,
whjle if this layer contains mainly bivalent ions such as Mg2+,
the mineral is “trioctahedral”.
Chlorite consists of an alternation of micaceous silicate
sheets with magnesium hydroxide or aluminum
hydroxide sheets [1,91. As in illite, the chemical
composition may be varied by the replacement of Si
by Al and of Mg by Al, FeII, FeIrI, etc. The micaceous
silicate sheets and the hydroxide sheets are probably
oppositely charged and held together in this way.
Hydrogen bonds from the hydroxide sheets to the 0
atoms of the tetrahedral layers may also contribute to
the bonding between the layers. Most chlorites are
151 E. Maegdefrau and U. Hofmann, Z . Kristallogr., Mineralog.
Petrogr., Abt. A 98,31(1937); EMaegdefrau, Sprechsaal Keram.,
Glas, Email 74, 369 (1941).
[6] R. E . Grim, R. H . Bray, and W.F. Bradley, Amer. Mineralogist
22, 813 (1937).
171 L . Pauiing, Proc. nat. Acad. Sci. USA 16, 123 (1930).
[8] W. W. Jackson and J. West, Z. Kristallogr., Mineralog.
Petrogr., Sect. A 76, 211 (1930); 87, 160 (1933).
191 K . Robinson and G. Brindley, Proc. Leeds philos. lit. SOC.,sci.
Sect. 5, 102 (1948); G. W. Brindley, B. M . Oughton, and K . Robinson, Acta crystallogr. 3, 408 (1950).
Angew. Chem. internat.
Edit.1 Vol. 7 (1968) J No. 9
O A I 00
6 0 ~
Fig. 7. Crystal structure and intracrystalline swelling of montmorillonite. @ = exchangeable cations; b approx. 9 A; distance between
layers approx. 10-20 8, (15.5 A).
Montmorillonite is occasionally found in clays and kaolins.
Clays having high montmorillonite contents are called
bentonites. The cations in the tetrahedral and octahedral
layers may be replaced in the same way as in micas. Montmorillonite with a high iron content is known as nontronite,
whereas the trioctahedral form with a high magnesium
content is called saponite or hectorite”11. As a result of
intracrystalline swelling, the silicate sheets lose their orientation with respect t o one another, and one then obtains only
the interferences of the distance between sheets (001) and the
cross-lattice interferences ( h k ) of the individual silicate sheets.
Vermiculite corresponds to a trioctahedral montmorillonite, though its crystals are much larger, occasionally
having diameters of up to many cm, and the crystal
lattice is more regular. Vermiculite containing no iron
is called batavite [121.
Mites having high iron contents are green, and are
known as glauconite or seladonite; they are the pigment
of veronese green. The Mossbauer spectrum of a
glauconite containing 15 % of iron calculated as Fez03
and 1 % as FeO[l31 showed only the y absorption of
FeIII in the octahedral arrangement, and, in agreement with the chemical analysis, no FeI1. Fell1 in the
octahedral layer possibly colors the mineral green.
The same is true of green nontronite, chlorite, and
many others.
[lo] U. Hofmann, K. Endell, and D . Wilm, Z. Kristallogr., Mineralog. Petrogr., Sect. A 86, 340 (1933).
1111 H. Strese and U. Hofmann, 2. anorg. allg. Chem. 247, 65
(1941).
(121 Armin Weiss and U. Hofnann, Z . Naturforsch. 66, 405
(1951).
[131 U. Hofmann, E. Fluck, and P. Kuhn, Angew. Chem. 79, 581
(1967); Angew. Chem. internat. Edit. 6, 561 (1967).
68 3
Table 1. Formulas of some clay minerals
Mineral
Kaolinite
Illite
(micaceous clay)
Montmorillonite
Vermiculite
I
Formula
= KT,., { (Ab, Mg3, Fe;",
A12(0H)4ISi20~1
Fe:')(OH)zISi,.p A10.70101 }
0-7-
= { (Alz, Mg3. Fey', Fe:')(OH)z[Sixs A10.40101 } 0.4= { (Mg3, Alz, Fey, Fe~')(OH)~[Si3.,,Alo.650,01> 0.65octahedral layer
+ 0.4 M+
+ 0.65 M i
tetrahedral layer
The structure of halloysite is discussed in Section 1.4.
The formulas of the clay minerals are summarized in
Table 1.
1.3. Exchangeable Cations
All clay minerals contain exchangeable cations.
Weiss [I41 found that in very finely cleaved micas and
in illite, only the potassium ions on the free basal planes
are exchangeable. Within the limits of accuracy, the
quantity of exchangeable cations agrees quantitatively
which there are no cations between the silicate layers,
to the micas. An extract from this table is shown in
Table 2.
Intracrystalline swelling is observed in clay minerals
having a medium cation density between the silicate
layers. It is caused by the hydration of these cations,
which draw the water in between the silicate sheets.
If there are no cations between the silicate sheets, the
mineral cannot swell. Moreover, if the cation density
between the sheets is too high, as e.g. in muscovite,
the Coulomb forces between the cations and the silicate
layers are too great; these forces hold the sheets
Table 2. Intracrystalline swelling of layer silicates (after A . Weiss 1161.
Mineral
Equiv. cations
per formula
unit [a]
Pyrophyllite, Talc
Montmorillonite
Vermiculite
lllite
Muscovite
Layer distance (A) with
Alkaline earth ions
dry
in HzO
1
K+-Ions
dry
in H z 0
Na'-Ions
dry
in HzO
constant 9.2
0
0.24-0.5
0.65
11.5
11.5
0.7
-
0.8-1.0
-
20
10
co
14.5
10
10
10
10
:8
11
-
li:
[a1 (M:'.
with ;the quantity of potassium ions between two
silicate sheets in the interior of the crystal. These
potassium ions cannot be exchanged. Cation exchange
at the edges of the silicate sheets is normally improbable, and may be disregarded (cf. Fig. 6).
The exchangeable cations in nature are mostly calcium
and magnesium ions (action of hard water).
The chemical analysis of kaolinite agrees to within the
limits of accuracy with the formula, and n o substitution of ions that might lead to a negative charge on the
crystals has been detected. Weiss[15] was able to
dissolve kaolinite crystals from the basal surface
(octahedral layer) with ammonium fluoride solution.
The cation-exchange capacity, based on the original
quantity of kaolinite, remained unchanged. It therefore appears that exchangeable cations are practically
confined to the basal surface, where the tetrahedral
layer is exposed. It is possible that only this one
outermost silicate layer carries a negative charge as a
result of substitution of its ions.
Weiss[161 has drawn up a table of minerals that in
principle contain the same silicate layers. The series
extends from pyrophyllite and talc, with the formulas
A12(OH)zSi4010 and Mg3(OH)zSi4010 respectively, in
\
03
15
-
-
My) (OHh [(Si. AI).,O~~]
together, and no swelling takes place. The silicate
sheets are held together particularly strongly by
potassium ions (see e.g. vermiculite). Potassium ions
are less strongly hydrated than sodium ions or alkaline
earth metal ions, and their size is such that they fit
very well into the 12-coordination offered them by
the 0 atoms of the two tetrahedral layers 1171.
Alkaline earth metals ions allow only limited swelling
in water; up to four layers of water are incorporated
in rnontmorillonite, and up to two layers in vermiculite. Sodium and potassium ions give infinite swelling
1141 Armin Weiss, 2. anorg. allg. Chem. 297, 257 (1958).
Fig. 8. Electron micrograph of a montmorillonite having exchangeable
alkaline earth ions.
[I51 Armin Weiss,2. anorg. allg. Chem. 299, 92 (1959).
[I61 Armin Weiss, G. Koch, and U.Hofmann, Ber. dtsch. keram.
Ges. 32, 12 (1955); U. Hofmann, Kolloid-2. 169, 58 (1960).
1171 Armin Weiss, Lecture to the Chemical Society Heidelberg,
Jan. 23, 1968.
684
Angew. Chem. internat. Edit. 1 VoI. 7 (1968) No. 9
in montmorillonite, i.e. the crystal separates into
individual silicate layers.
Figure 8 shows the electron micrograph of a montmorillonite having exchangeable alkaline earth ions;
as usual, the sample was prepared from a suspension.
In the suspension, the silicate layers move toward one
another and dry in such a way that their original
outline is indistinct.
The electron micrograph of a freeze-dried montmorillonite gel having exchangeable sodium ions gives a
picture of the type shown in Figure 9. Owing to the
intracrystalline swelling, the silicate sheets had separated singly o r a few at a time. In the gel they form a
framework structure with water trapped in its cavities.
The silicate layers are flexible, and can be folded like
silk ribbons.
consists only of kaolinite layers (A12(0H)4[Si205]) 1181.
The crystal structure 1191 of the highly hydrated
halloysite is shown in Figure 10.
Q
O
Q
Q
Q
Q
O
Q
Fig. 10. Crystal structure of the highly hydrated form of halloysite.
Distance between layers = 10.1 A.
The electron micrograph of halloysite having a low
water content (Fig. 11) shows fine needles. A needle
shape can also be discerned in the highly hydrated
halloysite by means of replicas. Closer examination
shows that the needles are in fact tubes1201, smaller
tubes often being inserted in larger ones. The tubes
are most clearly seen in surface replicas [*I ,221; carbon
Fig. 9. Skeletal structure of a freeze-dried gel of montmorillonite having
exchangeable sodium ions.
If a suspension of a montmorillonite with exchangeable
sodium ions is evaporated to dryness for examination
in the electron microscope, a film of flat silicate layers
is obtained.
Table 3 shows that the content of exchangeable cations
in kaolinite, chlorite, and illite increases with decreasing thickness of the crystal platelets. In montmorillonite and vermiculite, the cations between the silicate
layers can also be exchanged.
Table 3. Content of exchangeable cations
(M') in the clay minerals.
Kaolinite
Chlorite
Mite
Montmorillonite
Vermiculite
1-20
3-40
10-50
60- 130
100-170
1.4. Halloysite
Halloysite occurs in two forms, one having a high and
the other a low water content. In the form with the
high water content, Al~(OH)&3205].2 H20, [d(OOl) =
10.1 A], the kaolinite and water layers alternate. The
water is eliminated at about 50°C to give the form
having the low water content [d(OOl) = 7.3 A], which
Angew. Chem. internat. Edit. Val. 7 (1968) J No. 9
Fig. 1 I . Electron micrograph of halloysite.
is vaporized onto a piece of halloysite, and the
halloysite is subsequently dissolved out with hydrofluoric acid. Halloysite tubes remain embedded in the
carbon layer, and it is occasionally possible to see
them very clearly in end view (Fig. 12).
It can be seen from Table 4 that the outside diameter
as found under the electron microscope gives a smaller
calculated surface area if the tubes are assumed to be
[I81 U. Hofmann, K . Endell, and D . Wilm, Angew. Chem. 47,539
(1934).
[19] U. Hofmann, Kolloid-Z. 69, 356 (1934); S. B. Hendricks,
Amer. Mineralogist 23, 295 (1938); G. Ruess, Mh. Chem. 76,
168 (1946).
[20] T. F. Bates, F. A . Hildehrand, and A. Swineford, Amer. Mineralogist 35, 463 (1950).
[21] T. F. Bates and J . J. Comer, Proc. 3rd Nat. Conf. Clays and
Clay Minerals, Nat. Acad. Sci., Washington 1955, p. 1; Th. Nemetschek and U.Hofmann, Z. Naturforsch. 166, 620 (1961).
[22] U. Hofmann, S. Morcos, and F. W. Schembra, Ber. dtsch.
keram. Ges. 39, 474 (1962).
685
Kaolinite that has been split into extremely thin layers via
inclusion compounds can be rolled into halloysite-like tubes
by careful treatment [251.
The b axis of the kaolinite lattice generally becomes the fiber
axis for the halloysite tube.
If the water is driven out of the highly hydrated form of
halloysite, the spirals roll up more tightly. They occasionally
split in the longitudinal direction to give narrow halloysite
tubes inside wider tubes.
Halloysite has a higher cation-exchange capacity in the highhydration than in the low-hydration state, since some of the
cations in the latter are shut in. The content of exchangeable
cationsis generally between 5 and 30 mequiv/100 g of mineral.
1.5. Dimensions of the Crystals of the Clay Minerals
Fig. 12. Electron micrograph of a surface replica of Stolberg halloysite.
1, 2, and 3: tubes inserted one inside another; 4: rolled-up tube;
5: example of a tube in cross section.
solid than that found by nitrogen adsorption measurementS (BET, AREA^^^^^), better agreement being
Obtained
if the inside diameter is taken into account 1231.
’
To determine the average diameter Of the crystal platelets
of kaolinite and illite, about 1500 crystal platelets were
measured in the electron micrograph of each kaolin or clay.
The specific surface area 0 of the clay mineral crystals has
been determined by adsorption of nitrogen at its boiling
point as described by Brumuer, Emmett, and Teller (BET
method). The best preparations for this purpose are those
obtained from gels by freeze-drying, the nitrogen evidently
penetrating into the gaps between the crystals of the clay
Table 4. Specific surface areas of halloysites.
Sample
Argile Djebel I fine
Argile Djebel I coarse
Djebel Debar I fine
Djebel Debar I coarse
Novo Bdro Jugoslav.
Span. “kaolin”
Marocco
“kaolin” from Ceylon
( r < 3 vm)
Bergnersreuth
( r < 10vm)
Lawrence, Illinois
49.7
51.7
53.7
56.2
41.1
47.0
14.7
22.3
23.1
29.7
20’6
30.5
25.8
36.6
39.2
47.9
44.7
51.4
54.0
36.5
40.6
I”:
1
31.1
These tubes are probably formed as follows. An individual
kaolinite silicate layer is formed first, but this layer carries a
negative charge as a result of substitution of its ions, and it
also carries exchangeable cations. The hydrated cations
prevent the formation of hydrogen bonds and hence also
combination with another sjlicate layer. In the single silicate
layer, the a and b axes along the tetrahedral layer (8.95 A)
are theoretically greater than the a and b axes of the octahedral layer (8.65 A). In the kaolinite crystal, the hydrogen
bonds brace the octahedral layers against the tetrahedral
layers of the adjacent silicate layer, and so keep the crystal
planar. In halloysite, on the other hand, the individual
kaolinite-like silicate layers are able t o roll up into a spiral
tube, taking with them a layer of water between the silicate
layers as a result of the hydration of t h e cations (Figure
13)[22,241.
,~65U3
(a)
(b)
(c)
Fig. 13. Cross section through the spiral of the halloysite tube. (a) Highly
hydrated form; (b) low water content form; (c) low water content form,
silicate layer broken into three tubes.
[23] R . Reingraber, Dissertation, Universitat Heidelberg, 1968.
1241 Unpublished work by A . Weiss.
686
243
252
226
213
309
256
551
381
1
342
j ‘1:
I
66
mineral. However, the determination is meaningful only if
there is n o intracrystalline swelling of the mineral, since the
inner surface is not reached in the adsorption of nitrogen.
The same area is obtained on determination by the adsorption of phenol from decalin solution.
The thickness h of the crystal platelets can be determined
from the surface area and the diameter 1261:
h
=
(2a//d2)/((p.O-B/dz)-4)
The density p was taken t o be 2.6 g/cm3. It is assumed in this
equation that the thickness h is proportional t o the average
diameter 2 for any clay mineral crystal. If the calculation is
carried out by another equation, in which all particles are
assumed to have the same average thickness h, the values
obtained for the average thickness are smaller. However, the
assumption made for the equation given above is more
probable.
The results agree very satisfactorily with the average thickness found by measurement of the shadow in the electron
micrograph after oblique shadowing. Though the method
used for the calculation of the thickness is not very accurate,
[25] J, RKSSOW,
Dissertation, Universitat Heidelberg, 1965.
[26] U . Hofmann, H . P . Boehm, and W. Gromes, Z . anorg. allg.
Chem. 308, 143 (1961); U . Hofmonn ef at., Ber. dtsch. keram.
Ges. 44, 131 (1967).
Angew. Chem. internat. Edit. / Vof. 7 (1968) / No. 9
this comparison shows that the value obtained is approximately correct, and is too high rather than too low. It can
be seen from Table 5 that kaolins are coarser than clays.
activity of the oxalate ions. The activity in this case is
equated to the activity of the Ca ions which remain on
the kaolinite.
Table 5. Average diameter 2 and average
thickness 6 of the crystals of kaolins and
clays.
Table 6 shows the activities of cations on a clay mineral
that is covered half with the cation in question and
half with Na ions. These activities are one or more
orders of magnitude lower than the activities of the
same cations over the sparingly soluble salt in pure
water.
Material
7000
2500
2000
1000
Very coarse kaolin
Very fine kaolin
Very coarse clay
Very fine clay
1500
340
500
I 50
1.6. Activities of the Exchangeable Cations
When kaolin is liquefied with soda, calcium ions in
the kaolinite are replaced by sodium ions, and sparingly soluble calcium carbonate precipitates out. A
bentonite is activated in the same manner.
The cation exchange involves a n equilibrium, which
is clearly discernible even when a sparingly soluble
salt is formed. The equilibrium in the aqueous solution
can be described e.g. as follows:
Ca-kaolinlte t- 2 Na-
+ CzO42-
-$
This method naturally cannot be used to find the
activity, in pure water, e.g. of the Ca ions on a kaolinite whose exchangeable cations have been completely replaced by Ca ions. However, the curves in
Figure 14 show that even when 90 % of the exchangeable cations are calcium, the activity of the Ca ions is
still very low. Exchangeable K, Mg, Ca, Ba, and La
ions bound on clay minerals form compounds that
are more sparingly soluble than, or roughly as sparingly soluble as K[B(C6H&], MgF2, CaC204.Hz0,
BaS04, and La2(C204)39Hz0 respectively.
Naz-kaolinite + CaC204
The existence of an equilibrium can be demonstrated
by shaking a sodium kaolinite with an excess of calcium
oxalate in water. Calcium kaolinite and NazC204 are
formed. Alternatively, when sodium kaolinite is
shaken with BaS04, a barium kaolinite and NazS04
are obtained E271.
The concentrations or activities of the Na and oxalate
ions in the above equilibrium can be determined in the
separated aqueous solution. If a known quantity of
NazC204 has been added to a pure calcium kaolinite,
it is also possible to determine the position of the
equilibrium. The activity of the Ca ions in the solution
can be calculated from the solubility product of the
sparingly soluble calcium oxalate and the measured
Fig. 14. Activity u c a of exchangeable calcium on clay minerals as a
function of the covering ratio Na:Ca. ---- activity at a covering ratio of
1 :l;
glauconite (R); -A-A-: illite; - x - x - :kao1inite;-0-0-: glauconite (U.S.A.).
-o-n-:
Table 6. Effective activities of the exchangeable cations on clay minerals.
in the
sparingly
soluble
compound
in pure
HzO
Clay mineral
on the clay
mineral
half
covered
with the
cation and
N a g K > Mg > Ca > Ba
I
K-kaolinite
K-illite
Mg-kaolinite
Mg-illite
Ca-kaolinite
Ca-illite
Ba-kaolinite
Ba-illite
La-kaolinite
La-illite
The higher the activity of the cation, the more readily is it
exchanged. The lower the activity, o n the other hand, the
more readily is the cation taken up by the clay mineral. This
is clearly shown when e.g. a n ammonium kaolinite is treated
with equivalent solutions of two cations and the ratio of the
exchanged cations determined. The order of the activities
of the cations corresponds to the Hofmeister series.
1.82.10-4
1.82.10-4
1.25.10-3
1.25.10-3
4.5 .lO-5
4.5 .10-5
1.00.10-5
1.00.10-5
2.3 .10-6
2.3 .10-6
s.o .10-5
3.2 .io-5
1.65.10-6
3.00.10-6
1.25.10-6
1.65.10-6
I . .10-7
~
2.7 -10-7
2.84.10-10
4.~0.10-10
1271 U. Hofmann and W. Burck, Angew. Chern. 73, 342 (1961);
H . Friedrich and U.Hofmann, 2.anorg.allg.Chern. 342, 10 (1966).
Angew. Chem. internat. Edit. / Vol. 7 (1968) J No. 9
>
La
It must be a s u m e d that, unlike the N a ions, K , Mg, Ca, Ba,
and La ions simply form a diffuse cloud around the clay
mineral crystal.
If a natural kaolin or clay covered with Ca and Mg ions is
to be covered with N a ions and then washed until free from
salt, it is not advisable to use a sodium salt that gives sparingly
soluble C a and Mg salts, since the sparingly soluble salt
would react again with the sodium ions during washing. On
the contrary, a large excess of a sodium salt that gives soluble
C a and Mg salts should be used, these salts then being
washed out with the excess of sodium salt.
687
1.6.1. A c i d S t r e n g t h o f E x c h a n g e a b l e
Hydrogen Ions
Clay minerals are attacked by acids. The best method
of covering a clay mineral with exchangeable hydrogen
ions is to use a strongly acidic cation exchangerrzsl.
Clay minerals containing the readily exchangeable Na
ion react particularly well. The H-clay mineral can be
titrated with COz-free sodium hydroxide, potassium
hydroxide, or barium hydroxide solution in polyethylene containers under nitrogen with the quinhydrone electrode.
It can be seen from Figure 1 5 that the H-clay minerals
react as acids, though they are less acidic than equivalent acetic acid 1291. (However, the value given by the
titration is the average pH value of the clay mineral
suspension. The concentration of H3O ions is undoubtedly much greater immediately adjacent to the
surface of the clay mineral.)
3r
illite and montmorillonite, as well as quartz, iron oxide,
calcium carbonate, humus, etc. Clays often contain clay
minerals in high concentrations.
The formation of a kaolinite crystal can be explained
as follows. A silicate sheet is formed first. The second
silicate sheet then grows with its octahedral layer
toward the tetrahedral layer of the first sheet, and with
formation of hydrogen bonds. The crystal thus grows
layer by layer until the concentration of foreign ions
in the mother liquor becomes so high that the last
silicate layer receives a negative charge as a result of
ion substitution. This silicate layer binds exchangeable
cations, the hydration of which prevents the formation
of hydrogen bonds and hence the growth of a new
silicate layer. Consequently, the crystal can grow no
further.
A hypothesis proposed to explain the formation of
halloysite tubes was described in Section 1.4.
Illite is formed by weathering of potassium feldspar
and of micas if the potassium ions cannot be removed
from the mother liquor. The weathering of feldspars
yields roughly equal quantities of sodium and potassium ions. Ground water, however, contains e.g. only
4 mg of potassium ions/l as compared with 30 mg of
sodium ions& and sea water contains 11 g of sodium
ions/l as opposed to only 0.3 g of potassium ions/l.
The potassium content of land plants is not sufficient
to account for the missing potassium ions, particularly
since much of it is recycled between the soil and the
plants. The potassium ions have in fact been consumed by the formation of illite and other micas
containing potassium.
As a result of weathering in the soil, illite provides the
additional potassium required by land plants. Potassium feldspar is less common in soil and is coarser
(0.1 to 0.01 mm diameter) and so weathers more slowly.
8L
9
0
1
2
3
L
5
6
rn equiv NaOH/100gKaolmite0.01 N CH&OOH i0.1 N
Fig. 15. Neutralization curves. -:
N a 0 H ; - x - x - : 1.0mequivof H-kaolinite/100ml of HzO+ 0.1 N NaOH;
-0-0-:0.1 equiv of H-kaolinitef100 ml of HzO 0.01 N NaOH.
+
2. Formation of the Clay Minerals
It seems probable from geological observations and
from the synthesis of clay minerals in the laboratory [11,301 that these minerals are formed from
aqueous solution.
Kaolinite, A12(0H)4[Si205], is formed by the weathering of feldspar, e.g. K(AlSi308), in the presence of
water containing acid, e.g. COz. The alkali metal and
alkaline earth metal ions liberated are carried away;
the excess Si02 is also carried away, or forms cristobalite or quartz in the course of time.
Kaolins generally occur in primary deposits. The rock often
contains only 30 % of kaolinite, the remainder being quartz,
feldspar, mica, etc. Clays are generally found in secondary
deposits, to which they have been carried by water. Apart
from kaolinite (usually of the fireclay type), they contain
___[28] K. Friihauf and U.Hofmann, 2. anorg. allg. Chem. 307, 187
Montmorillonite results from the weathering of
volcanic ashes in the presence of sodium ions, which
promote the intracrystalline swelling. Bentonite deposits also contain vitreous weathering residues. The
Lower Bavarian bentonite deposits may owe their
existence to the impact of the great meteorite in the
Tertiary, which formed the giant crater at Nordlingen
in the Alb. The crater has a diameter of 20 km. The
flying “volcanic” ash was carried south-eastward by
the wind, collected in the Lower Bavarian basin, and
changed into bentonite.
Investigation of the sediments in the sea around the mouths
of large rivers led many workers to believe that montmorillonite sedimented first and then gradually changed into
illite[31], taking up potassium ions from the sea water and
increasing the negative charge on the sheets e.g. by substitution of magnesium for aluminum in the octahedral
layer.
3. Properties and Use of Kaolin and Clay
(1961).
1291 H . Friedrich and U.Hofmann, Z . anorg. allg. Chem. 342, 20
(1966).
1301 W.Noll, Z . Kristallogr., Mineralog.Petrogr., Sect. A 48, 210
(1936); S. Henin and 0.Robichet, C. R. hebd. Seances Acad. Sci.
236, 517 (1953); S. Cailldre, S.Henin, and J . Esquevin, ibid. 237,
1724 (1953); H . Harder, Naturwissenschaften54, 613 (1967).
688
Kaolins are generally washed before use to reduce the
content of quartz, feldspar, and mica. Clays are
usually worked without being washed. Coarse kaolins
1311 Cf. W. v. Engelhardf, Geol. Rdsch. 5 1 , 4 7 5
(1961).
Angew. Chem. internat. Edit. J Vol. 7 (1968) 1 No. 9
are not very plastic, and for this reason the coarser
grades are not used in ceramics. They are used as
fillers for paper, as brightening fillers for plastics, as
vehicles for insecticides, and for the preparation of
ultramarine.
3.1. Plasticity
Kaolins and frequently clays are not worked alone in
ceramics, but are mixed e.g. with quartz and feldspar
for the manufacture of porcelain. Kaolins and clays
containing a suitable quantity of water are plastic, i.e.
they can be shaped, often into thin-walled vessels. They
retain the shape of the vessel or model after being
formed, and harden on drying.
There is unfortunately no satisfactory method of
measuring the plasticity of clays. One method that is
widely used is the Pfefferkorn method[32J, in which
the water content of a wet clay that is flattened to a
given thickness by a falling plate is measured. Another
method is Cohn's method in which a rod must sink
into a wet clay at a given speed, the water content of
the clay then being determined. Thus, both of these
methods involve the measurement of the water content
of the wet clay at a given resistance to deformation.
Clays that are too wet or too dry have lower plasticities.
The best method would be to measure the strength of
a wet clay at a deformability that corresponds to the
most plastic state 1337. Unfortunately, however, the
strength cannot yet be determined with sufficient
accuracy in the plastic state, since the clay flows. The
potter's thumb is therefore still often the measuring
instrument used in practice.
Pure quartz sand is fairly plastic in the moist state; the
water adheres t o the surface of the quartz sand and
holds the quartz grains together by its surface tension.
When dried, however, the quartz sand becomes mobile,
since the only contact between the grains is at points.
Clay or kaolin coats the quartz and feldspar grains
with its crystal platelets, so providing large contact
areas both in the wet and in the dry states, which give
the wet mix its strength. The quartz in Figure 16,
however, contains only a small quantity of clay, such
as is present e.g. in a casting sand produced with
bentonite. In a plastic ceramic mass, the interstices
between the quartz grains are also filled with clay or
kaolin platelets.
According to Weiss[341, a coarse kaolin can be made very
plastic by incorporation of compounds, such as urea, that
break hydrogen bonds between the layers. By careful treatment,
the kaolinite crystal can then be split into very,thin lamellae
and the compound added can finally be removed. The Chinese
probably prepared their kaolin in this way during the Sung
dynasty (960 to 1279) for the production of eggshell porcelain. They obtained the inclusion compound by allowing
decomposing urine to act on kaolin.
Ceramic objects are formed either freehand or on
potter's wheels, which were known as early as 3000 B.C.
Alternatively a slip may be cast in porous plaster-ofParis molds; the action of the exchangeable cations
can be seen in this caser35J. A plastic clay can be
liquefied to a slip without changing its water content
by the addition of a small quantity of soda and/or
water glass.
In the natural or washed state, kaolin or clay generally
contains exchangeable alkaline earth metal ions from
the hardening constituents of water or from the
flocculation with calcium hydroxide after washing.
Soda enters into a n equilibrium with the alkaline earth
metal ions of the kaolinite or clay mineral, with
exchange of sodium ions and precipitation of alkaline
earth metal carbonate. Water glass reacts in a similar
manner. The alkaline reaction of soda or water glass
is necessary for the liquefaction only if the clay or
kaolin also contains exchangeable hydrogen ions as a
result of contact with acid soil water, e.g. humic acid.
Alkaline earth metal ions carry a double positive
charge. They cannot completely screen the negative
charges on the surface of the clay mineral crystals [35,361.
The negative potential extends outward between
I\ i\\'-\ \ i\\i\\i\1
@@@a@
Fig. 17. Saturation of the negative surface charge of the clay minerals
by monovalent cations (e.g. Na+) and by divalent cations (e.g. CaZ').
(a) Monovalent cations, better charge neutralization, screening
of the surfaces; (b); divalent cations, poorer charge neutralization.
inadequate screening of the surfaces; (c) diffuse cation cloud over the
surface; (d) divalent cations, better charge neutralization by combination of t w o layers.
B
Fig. 16. Binding of quartz sand (A) by clay (B).
1321 F. Zapp, Ber. dtsch. keram. Ges. 34, 12 (1957).
1331 E. Scharrer and U.Hofmann, Ber. dtsch. keram. Ges. 35,278
(1958); U.Hofmann, Keram. Z . 14, 14 (1962).
Angew. Chent. internat. Edit. f Voi. 7 (1968) j
The better the clay or kaolin conforms, the more
plastic is the mixture. The flexibility of the platelets is
therefore important. Clays are more plastic than
coarse kaolins, but less plastic than fine kaolins.
No. 9
[34] Armin Weiss, Angew. Chem. 75, 755 (1963); Angew. Chem.
internat. Edit. 2, 597 (1963).
[35] U. Hofmann, Silikattechnik 8, 224 (1957); W . Czerch, K.
Fruhauf, and U . Hofmann, Ber. dtsch. keram.Ges. 37,255 (1960).
1361 Armin Weiss, Kolloid-Z. 158, 22 (1958); U.Hofmann, ibid.
169, 58 (1960); U . Hofmann, E. Scharrer, W. Czerch, K . Friihauf,
and W. Burck, Ber. dtsch. keram. Ges. 39, 125 (1962).
689
the alkaline earth metal ions (Fig. 17b). Clay mineral
crystals therefore adhere to one another via two layers
of water lying between them (Fig. 17d). The adhesion
occurs, not only between faces, but also between faces
and edges. This structure, which resembles a house of
cards, is shown in a greatly exaggerated form in
Figure 18a. Because of the water layers between them,
the platelets can be displaced in relation to one
another and subsequently become fixed again in their
new position. This is the explanation of the plastic
deformability and the dimensional stability.
parallel to one another, the soil can contract to a very
dense state on drying, and the fertility of the soil
decreases or disappears. It then takes a considerably
long time for the alkaline ions to be replaced by the
hardening constituents of water. When the soil is fed
with moderate amounts of potassium chloride, the
clay minerals take up potassium ions as part of their
complement of exchangeable ions and prevent them
from being washed out of the soil, so that they are
available for the plants.
3.3. Dry Bending Strength
Na
@m3
(a)
-
Plastic clay mixes consist essentially of the solid particles and water; they contain practically no air. On
drying, the water is removed, the clay shrinks, and air
pores are simultaneously formed; the smaller the
number of pores or the greater the solid content of
the material, the higher is the dry bending strength 1391.
(b)
Fig. 18. (a) Plastic structure in the presence of Ca ions; (b) slip in the
presence of Na ions (schematic section).
Sodium ions are monovalent, and can screen the
negative potential of the clay mineral surface much
more effectively. They are also strongly dissociated in
water, and form a cation cloud around the clay mineral
crystal (Fig. 17a and c). The clay mineral crystals
repel one another and assume a parallel orientation,
the clay thus becoming liquid. This is shown, again
exaggerated, in Figure 18b.
When a slip is poured into the plaster mold, the
porous plaster absorbs the water. It can be shown that
the clay mineral platelets arrange themselves as nearly
as possible parallel to the wall[371. When the body
becomes sufficiently thick, the remaining, unwanted
slip is poured out of the mold.
Clay minerals containing exchangeable hydrogen ions
also form a plastic mix, since they can form hydrogen
bonds, and, since they can dissolve aluminum ions out
of the octahedral layers of the crystals, these ions, as
Al(OH)2+, then act in the same manner as alkaline
earth metal ions 1381.
In clay, the small platelets lie in the interstices between
the large platelets in the house-of-cards structure of
the plastic mix. The differentiation between small and
large platelets is not so pronounced in kaolins, and the
interstices between the large platelets are consequently
not so well filled. This is shown exaggeratedly in
Figure 19. On drying, therefore, the clays give higher
densities, higher solid contents, and higher dry bending strengths than the kaolins.
I
Emj5
(a)
(b)
Fig. 19. House-of-cards structure in (a) kaolin and (b) clay (in section).
The dry materials obtained by slip casting have particularly high densities because of the parallel arrangement of the platelets; consequently, higher dry bending
strengths are obtained by casting than by shaping of
the same plastic material.
3.2. Action of Exchangeable Cations in Arable Soil
Owing to the alkaline earth metal ions normally
present in arable soil, the clay minerals have the
house-of-cards structure, giving the soil the desired
loose and crumbly texture, so that it is permeable to
water and to air. If the soil becomes flooded with sea
water o r if it is fed with too much potassium chloride,
the alkaline earth ions are replaced by sodium or
potassium ions, which liquefy the clay mineral skeleton
in the presence of water having a low electrolyte
content. Since the clay mineral platelets are now
[37] A . DietzeI and H . Mostetzky, Ber. dtsch. keram. Ges. 33,115
(1956).
[38] K . Friihauf and U. Hofmann, Z. anorg. allg. Chem. 307, 187
(1961).
690
Table 7. Plasticity and dry bending strength as a function of the nature
of the exchangeable cations (purified Zettlitz kaolin. 6.5 mequiv of
exchangeable cations/100 g).
Exchangeable
cations
Pfefferkorn
plasticity
(g HzOilOO
dry kaolin)
Na+
Kf
CaZ+
Ba2+
La,+
43.5
47
46.5
47.3
50.6
g
1
Dry bending
strength
(kg,cm21
30
18
16.3
10.2
8.4
1
Solids
(Vol: %)
61
58
59
57
54
[39] U.Hofmann, W. Czerch, and E. Scharrer, Ber. dtsch. keram.
Ges. 35, 219 (1958); U. Hofmann, E. Scharrer, W. Czerch, K.
Fruhauf, and W. Burck, ibid. 39, 725 (1962); U. Hofmann, F. W.
Schembra, M . Schatz, D . Scheurlen, H . Friedrich, and J . Dammler, ibid. 44,131 (1967).
Angew. G e m . internat. Edit.
Vol. 7 (1968) 1 No. 9
Halloysite has a given dry bending strength at a lower solids
content than kaolinitic and illitic kaolins and clays. This is in
agreement with the fact that a skeleton constructed from
tubes (halloysite) can be made lighter for a given strength
than can a skeleton constructed from plates (kaolinite and
illite).
The effect of the exchangeable cations on the dry
bending strength and the plasticity is shown in Table 7.
The higher the water content of a kaolin for a given
degree of flattening in the Pfefferkorn tester, the lower
is the dry bending strength.
3.4. Use of Vermiculite
When vermiculite takes up potassium ions, it loses its
capacity for intramolecular swelling; hence potassium
ions are more readily exchanged in vermiculites that
were previously swellable. Since this behavior is even
more pronounced with rubidium and cesium ions,
vermiculite is therefore used in the treatment of
nuclear fuels to remove 137Cs ions (half life 30 years)
from solution.
Vermiculite forms fairly large crystals. When swellable
vermiculite with one to two layers of water between
the silicate layers is rapidly heated, the water between
the layers cannot escape quickly enough. It boils and
forces the silicate layers apart, so that the vermiculite
swells. If the crystals have a small diameter e.g. 1 mm
or less, they give worm-like structures, from which the
name vermiculite is derived. Crystals greater than 1 cm
in diameter give a very loose material having a bulk
density of e.g. only 100 g/l. The material retains its
loose structure up to about 1000°C. It is a good
packing material, and can also be used as a thermal
insulator.
3.5. Use of Montmorillonite and Bentonite
Montmorillonite is generally used in industry in the
sodium form [401. Na-montmorillonite exhibits infinite
swelling in water, forms very thin lamellae, and so
disperses extremely readily in a wet clay. Sodium
montmorillonites are sometimes found in nature.
However, they can also be obtained by reaction of the
more abundant natural alkaline earth metal montmorillonites in the moist state with soda, which leads
to the equilibrium mentioned.
In spite of its high plasticity and dry bending strength,
little use is made of bentonite in ceramics, since it
usually contains some iron in its lattice and so has a
brownish to reddish color after firing.
However, 5 % of bentonite is sufficient to make a
casting sand sufficiently strong in the wet or dry state,
while the permeability to gases is still excellent (cf.
Fig. 16).
Sodium montmorillonite gives a suspension a high
viscosity. At about 10 wt- % of montmorillonite, the
suspension thickens to a gel at rest (thixotropy). This
[40] U . Hofmann, Angew. Chem. 68, 53 (1956).
Angew. Chem. internat. Edit. 1 Vol. 7 (1968) / No. 9
affords a good, strong drilling mud and at the same
time, the montmorillonite lamellae seal off the wall of
the borehole and prevent the drilling mud from flowing
into the rock. Owing to the supporting and sealing
effect on the wall, a bentonite suspension is introduced
into the slits cut out in slit wall construction before the
concrete is poured in.
The use of montmorillonite for the removal of radioactive 137Cs ions from solutions during the treatment
of nuclear fuels in nuclear power stations is also being
consideredr411 In order to encase the cesium ions in
an insoluble ceramic product, it would then only be
necessary to fire the montmorillonite.
Still better exchange is obtained with long-chain
organic ammonium ions. (These ions even cause intramolecular swelling in illite or muscovite [14,421, though
this takes some weeks to occur.) One such montmorillonite is supplied in organic solvents under the
name “Bentone” as an additive for paint suspensions
and for lubricating greases. Protamines, e.g. salmine,
which has a high arginine content, are very readily
exchanged. However, proteins are also exchanged in
acidic solution via the basic amino acidsE431. Montmorillonite is therefore used to free wine and export
ale from proteins and protein degradation products,
which would otherwise flocculate on standing. Montmorillonite absorbs up to its own weight of protein.
It is probable that only one end of the protein molecule with basic amino acids is incorporated between
the silicate layers of the montmorillonite.
4. Clay Minerals as Catalysts
Clay minerals, particularly montmorillonite are good
catalysts [43c3441. For example, in the presence of air,
montmorillonite oxidizes diphenylamine to the blue
benzidinium ion or aniline to aniline black. Peptides
are cleaved by montmorillonite. This led us to consider the formation of petroleum and of oil shale.
Oil shale contains clay minerals and “kerogen”, an
organic material, and affords a petroleum-like product
on slow distillation. The earth probably contains four
times as much potential oil in oil shale as it does free oil.
Evidence in favor of the biological origin of kerogen
and petroleum is provided inter aliu by the detection
of porphyrins by A . Treibs. Proteins, fats, and possibly
also carbohydrates from plankton, algae, and similar
microorganisms change in the sludge at the bottom
of the sea into gyttja as a result of H2S-induced oxygen
deficiency and into sapropel in the absence of oxygen.
[41] W. Hoffmann and U . Hofmann, Nukleonik (Berlin) 3, 195
(1961).
[42] Armin Weiss, A. Mehler, and U. Hofmann, Z . Naturforsch.
I l b , 435 (1956).
[43]a) L. E. Ensminger and J . E. Gieseking, Soil Sci. 48, 467
(1939); b) 0.Tafibudeen, Nature (London) 166, 236 (1950);
c) Armin Weiss, Angew. Chem. 75, 113 (1963); Angew. Chem.
internat. Edit. 2, 134 (1963).
[441 Armin Weiss, Beitr. Silikose-Forsch., special volume, Grundfragen Silikose-Forsch. 3, 45 (1958).
69 I
It is possible that petroleum is formed from sapropel,
and kerogen from sapropel and gyttjaL451. Clay
minerals simultaneously sediment into the sludge. The
petroleum has emerged from the clay minerals in the
course of time, and is now found in porous rocks such
as sandstone.
Weiss and RoIojW61 have shown that hemin incorporated as a cation into the layer lattice of montmorillonite is stable up to about 300°C, instead of
only up to 200°C as in the free state. However, clay
minerals act as catalysts even above 200 “C on organic
substances. Organic ammonium ions, which may be
derived from decaying protein, change in the layer
lattice of montmorillonite above 200 “C into a mixture
of hydrocarbons that corresponds to petroleum and
natural gasL471. The same is true of protein incorporated into montmorillonite. Montmorillonite carrying organic ammonium ions also absorbs fats. The
hydrocarbons are formed above 200 “C in every case.
Thus petroleum can be produced in the laboratory
from organic ammonium ions, protein, and fats by the
catalytic action of montmorillonite. Does this also
happen in nature?
We have determined the clay minerals in a series of oil
shales [481. The older shales (Cambrian to Jurassic)
contained illite and the younger shales (Jurassic to
Middle Eocene) montmorillonite.
In the young Middle Eocene oil shale from Messel, the
kerogen is at least partly situated between the silicate
sheets of the montmorillonite.
Illites also have a large surface and can act as catalysts.
However, the geological sequence suggests that the kerogen
was initially formed in the montmorillonite and migrated
[45] K . Krejci-Grafi ErdoI. 2nd. Edit., Springer, Berlin 1955.
[46] Armin Weiss and G. Ro[off,Z . Naturforsch. 196,533 (1964).
[47] Armin Weiss and G . Roloff,Int. Clay Conf., Proc. Stockholm
2, 313 (1963).
[48] H. Friedrich and U. Hofmann, Z . Naturforsch. Zlb, 912
(1966).
692
slowly, while the montmorillonite changed into illite as a
result of the absorption of potassium and an increase in the
charge on the layers. The free petroleum may also have been
formed from the kerogen of the oil shale.
When montmorillonite is heated with organic ammonium ions, an ammonium montmorillonite is
formed; above 350 “C and in the absence of air, this
changes into an H-montmorillonite(471. The H ions
migrate into the silicate layers, probably into the
octahedral layer, and the montmorillonite, loses its intracrystalline swelling capacity, and like pyrophyllite
and talc, can no longer swell.
In Trinidad asphalt we 1481 found an unswellable layer
silicate having a distance of 9.9 8, between the illite
layers, but which contains only 0.34 K per formula
unit. This may be a montmorillonite with 0.34 potassium ions that has lost the ability to swell because
of the temperature of about 300°C at which the
kerogen was formed and driven out to form the asphalt.
It is also possible that montmorillonite or the clay
minerals in general were involved as catalysts in the
creation of life on earth. Three or four billion years
ago, the earth probably had a “primordial atmosphere” of hydrogen, methane, ammonia, and water.
The ultraviolet radiation from the sun (which was not
absorbed by oxygen as it is now) and electric discharges cooperated in this atmosphere to form the
components of the proteins and nucleic acids, e.g.
formaldehyde, prussic acid, glycine, alanine, aspartic
acid, adenine, and sugars. Algae and microorganisms
have been detected in rocks as much as three billion
years old 1491. The clay minerals, which existed even in
the Precambrian, may have catalyzed the formation of
“viable” compounds of high molecular weight [501.
Received: March 4, 1968
[A 653 IEI
German version: Angew. Chem. 80, 736 (1968)
Translated by Express Translation Service, London
1491 Cf. F. Oberlies and A. A . Prashnowsky, Naturwissenschaften
55, 25 (1968).
[SO] U. Hofmann, Ber. dtsch. keram. Ges. 38, 201 (1961).
Angew. Chern. intenrat. Edit.
Vol. 7 (1968) / No. 9
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