Livro Holfman (parte)

Prévia do material em texto

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 mean- 
ing 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*1 and 
Bond 1921 are of the opinion that extrapolation to the 
metal surface is most promising if it is based on ap- 
proximations taking into account the spatial orienta- 
tion 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 physico- 
chemist 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 Proce- 
sos 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 and 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 
is generally a mixture of several -minerals. The-dis- 
covery 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. 
[*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. 
1.1. Electron Micrographs 
The electron micrograph of a well formed kaolinite, 
which is the principal clay mineral in kaolins, is shown 
Angew. Chem. internat. Edit. / VoZ. 7 (1968) 1 No. 9 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. 
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 diam- 
eters are not very small, and which have an irregular 
outline. 
Fig. I , Electron micrograph of a kaolinite, shadowed with chromium 
at an 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. 
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: octa- 
hedral 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 disturb- 
ance is also observed, though to a smaller extent, in kaolins. 
It is intensified in particular by dry grinding. 
Fig. 3. Electron micrograph of an illite. 
682 
[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). 
[ 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 derivedfrom that of 
coarsely crystalline mica [7 31; the silicate sheet consists 
of two tetrahedral layers with the vertices of the tetra- 
hedral 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’ 
Fig. 6. Crystal structure and cation exchange of mica. Top: a free 
basal surface with exchangeable cations (0); the K ions of the two 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 ortho- 
rhombic [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 . Robin- 
son, Acta crystallogr. 3, 408 (1950). 
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 intra- 
crystalline swelling, perpendicular to the sheets. This 
intramolecular swelling also occurs with other polar 
liquids. 
O S I O A I 0 0 6 0 ~ 
Fig. 7. Crystal structure and intracrystalline swelling of montmoril- 
lonite. @ = 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. Mont- 
morillonite 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 orienta- 
tion with respect to one another, and one then obtains only 
the interferences of the distance between sheets (001) and the 
cross-lattice interferences (hk) of the individual silicate sheets. 
Vermiculite corresponds to a trioctahedral montmoril- 
lonite, 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 agree- 
ment 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., Mine- 
ralog. 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). 
Angew. Chem. internat. Edit. 1 Vol. 7 (1968) J No. 9 68 3 
Table 1. Formulas of some clay minerals 
Kaolinite 
Illite 
(micaceous clay) 
Montmorillonite 
Vermiculite 
Mineral I Formula 
A12(0H)4ISi20~1 
= KT,., { (Ab, Mg3, Fe;", Fe:')(OH)zISi,.p A10.70101 } 0-7- 
= { (Alz, Mg3. Fey', Fe:')(OH)z[Sixs A10.40101 } 0.4- + 0.4 M+ 
= { (Mg3, Alz, Fey, Fe~')(OH)~[Si3.,,Alo.650,01> 0.65- + 0.65 Mi 
octahedral layer tetrahedral layer 
The structure of halloysite is discussed in Section 1.4. 
The formulas of the clay minerals are summarized in 
Table 1. Table 2. 
which there are no cations between the silicate layers, 
to the micas. An extract from this table is shown in 
Intracrystalline swelling is observed in clay minerals 
having a medium cation density between the silicate 
layers. I t is caused by the hydration of these cations, 
which draw the water in between the silicate sheets. 
All clay minerals contain exchangeable cations. If there are no cations between the silicate sheets, the 
Weiss [I41 found that in very finely cleaved micas and mineral cannot swell. Moreover, if the cation density 
in illite, only the potassium ions on the free basal planes between the sheets is too high, as e.g. in muscovite, 
are exchangeable. Within the limits of accuracy, the the Coulomb forces between the cations and the silicate 
quantity of exchangeable cations agrees quantitatively layers are too great; these forces hold the sheets 
1.3. Exchangeable Cations 
Table 2. Intracrystalline swelling of layer silicates (after A. Weiss 1161. 
Mineral 
Pyrophyllite, Talc 
Montmorillonite 
Vermiculite 
lllite 
Muscovite 
Equiv. cations 
per formula 
unit [a] 
0 
0.24-0.5 
0.65 
0.7 
0.8-1.0 
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 improb- 
able, 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 no substitu- 
tion 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 there- 
fore 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 
1141 Armin Weiss, 2. anorg. allg. Chem. 297,257 (1958). 
[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). 
Layer distance (A) with 
Alkaline earth ions K+-Ions 
dry in HzO 1 dry in Hz0 \ dry in HzO Na'-Ions 
constant 9.2 
10 co 10 03 
10 10 10 15 
11.5 20 
11.5 14.5 
- - - 
- - li: :8 11 - 
[a1 (M:'. 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 vermicu- 
lite. Sodium and potassium ions give infinite swelling 
Fig. 8. Electron micrograph of a montmorillonite having exchangeable 
alkaline earth ions. 
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 mont- 
morillonite 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 montmoril- 
lonite gel having exchangeable sodium ions gives a 
picture of the type shown in Figure 9. Owing to the 
intracrystalline swelling, the silicate sheets had sepa- 
rated singly or 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. 
Kaolinite 
Chlorite 
Mite 
Montmorillonite 
Vermiculite 
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 
1-20 
3-40 
10-50 
60- 130 
100-170 
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 decreas- 
ing thickness of the crystal platelets. In montmoril- 
lonite and vermiculite, the cations between the silicate 
layers can also be exchanged. 
Table 3 . Content of exchangeable cations 
(M') in the clay minerals. 
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 
Fig. 1 I . Electron micrograph of halloysite. 
is vaporized onto a piece of halloysite, and the 
halloysite is subsequently dissolved out with hydro- 
fluoric 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. Miner- 
alogist 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. Ne- 
metschek and U. Hofmann, Z. Naturforsch. 166, 620 (1961). 
[22] U. Hofmann, S. Morcos, and F. W. Schembra, Ber. dtsch. 
keram. Ges. 39, 474 (1962). 
Angew. Chem. internat. Edit. Val. 7 (1968) J No. 9 685 
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 high- 
hydration 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. 
14.7 
23.1 
25.8 
1.5. Dimensions of the Crystals of the Clay Minerals 
22.3 
29.7 
36.6 
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 measure- 
mentS (BET, AREA^^^^^), better agreement being 
count 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 
penetrating into the gaps between the crystals of the clay 
Obtained if the inside diameter is taken into ac- obtained from gels by freeze-drying, the nitrogen evidently 
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 
47.9 
44.7 
51.4 
54.0 
36.5 
40.6 
20’6 30.5 I”: 
243 
252 
226 
213 
309 
256 
551 
381 j ‘1: 
I 66 39.2 1 31.1 1 342 
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 octa- 
hedral 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 areable to roll up into a spiral 
tube, taking with them a layer of water between the silicate 
layers as a result of the hydration of the 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. 
mineral. However, the determination is meaningful only if 
there is no 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 adsorp- 
tion 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 to be 2.6 g/cm3. It is assumed in this 
equation that the thickness h is proportional to 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 thick- 
ness 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). 
686 Angew. Chem. internat. Edit. / Vof. 7 (1968) / No. 9 
this comparison shows that the value obtained is approxi- 
mately correct, and is too high rather than too low. It can 
be seen from Table 5 that kaolins are coarser than clays. 
Very coarse kaolin 
Very fine kaolin 
Very coarse clay 
Very fine clay 
Table 5. Average diameter 2 and average 
thickness 6 of the crystals of kaolins and 
clays. 
7000 1500 
2500 340 
2000 500 
1000 I 50 
Material 
in the 
sparingly 
soluble 
compound 
in pure 
HzO 
on the clay 
mineral 
half 
covered 
with the 
cation and 
1.6. Activities of the Exchangeable Cations 
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 
When kaolin is liquefied with soda, calcium ions in 
the kaolinite are replaced by sodium ions, and spar- 
ingly soluble calcium carbonate precipitates out. A 
bentonite is activated in the same manner. 
The cation exchange involves an 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: 
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 
Ca-kaolinlte t- 2 Na- + CzO42- -$ 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 
Table 6. Effective activities of the exchangeable cations on clay minerals. 
Clay mineral 
K-kaolinite 
K-illite 
Mg-kaolinite 
Mg-illite 
Ca-kaolinite 
Ca-illite 
Ba-kaolinite 
Ba-illite 
La-kaolinite 
La-illite 
I 
1271 U. Hofmann and W. Burck, Angew. Chern. 73, 342 (1961); 
H. Friedrich and U. Hofmann, 2.anorg.allg.Chern. 342, 10 (1966). 
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 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. 
This method naturally cannot be used to find the 
activity, in pure water, e.g. of the Ca ions on a kao- 
linite whose exchangeable cations have been com- 
pletely replaced by Ca ions. However, the curves in 
Figure 14 show that even when 90 % of the exchange- 
able 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 spar- 
ingly soluble as K[B(C6H&], MgF2, CaC204.Hz0, 
BaS04, and La2(C204)39Hz0 respectively. 
Fig. 14. Activity uca of exchangeable calcium on clay minerals as a 
function of the covering ratio Na:Ca. ---- activity at a covering ratio of 
1 : l ; -o-n-: glauconite (R); -A-A-: illite; -x-x- : kao1inite;-0-0-: glau- 
conite (U.S.A.). 
The higher the activity of the cation, the more readily is it 
exchanged. The lower the activity, on the other hand, the 
more readily is the cation taken up by the clay mineral. This 
is clearly shown when e.g. an 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. 
N a g K > Mg > Ca > Ba > La 
It must be a sumed that, unlike the Na ions, K, Mg, Ca, Ba, 
and La ions simply form a diffuse cloud around the clay 
mineral crystal. 
I f 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 Ca 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 
Ca and Mg salts should be used, these salts then being 
washed out with the excess of sodium salt. 
Angew. Chem. internat. Edit. / Vol. 7 (1968) J No. 9 687 
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 
H y d r o g e n I o n s 
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 poly- 
ethylene containers under nitrogen with the quin- 
hydrone electrode. 
It can be seen from Figure 1 5 that the H-clay minerals 
react as acids, though they are less acidic than equiva- 
lent 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 un- 
doubtedly much greater immediately adjacent to the 
surface of the clay mineral.) 
3r 
8L 9 
0 1 2 3 L 5 6 
rn equiv NaOH/100gKaolmite- 
Fig. 15. Neutralization curves. -: 0.01 N CH&OOH i 0.1 N 
Na0H; -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 synthesisof clay minerals in the labora- 
tory [11,301 that these minerals are formed from 
aqueous solution. 
Kaolinite, A12(0H)4[Si205], is formed by the weather- 
ing 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 cristo- 
balite 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 
(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, Naturwissenschaften 54, 613 (1967). 
___- 
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 potas- 
sium 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 con- 
sumed 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. Potas- 
sium feldspar is less common in soil and is coarser 
(0.1 to 0.01 mm diameter) and so weathers more slowly. 
Montmorillonite results from the weathering of 
volcanic ashes in the presence of sodium ions, which 
promote the intracrystalline swelling. Bentonite de- 
posits 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 mont- 
morillonite 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 sub- 
stitution of magnesium for aluminum in the octahedral 
layer. 
3. Properties and Use of Kaolin and Clay 
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. 51,475 (1961). 
Angew. Chem. internat. Edit. J Vol. 7 (1968) 1 No. 9 688 
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 to 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. 
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). 
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. 
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 por- 
celain. 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-of- 
Paris 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 smallquantity 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 an 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 combin- 
ation of two layers. 
[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). 
Angew. Chent. internat. Edit. f Voi. 7 (1968) j No. 9 689 
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. 
43.5 
47 
46.5 
47.3 
50.6 
Na - 
30 
18 
16.3 
10.2 
8.4 
@m3 (a) (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. 
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 or 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). 
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 
Plastic clay mixes consist essentially of the solid par- 
ticles 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. 
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 bend- 
ing 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 parti- 
cularly high densities because of the parallel arrange- 
ment of the platelets; consequently, higher dry bending 
strengths are obtained by casting than by shaping of 
the same plastic material. 
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). 
Dry bending 
Pfefferkorn 
Exchangeable plasticity 1 strength 1 Solids cations ( g H z O i l O O g (kg,cm21 (Vol: %) 
dry kaolin) 
Na+ 
Kf 
CaZ+ 
Ba2+ 
La,+ 
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 . Damm- 
ler, ibid. 44,131 (1967). 
690 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 mont- 
morillonites 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). 
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 radio- 
active 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 intra- 
molecular swelling in illite or muscovite [14,421, though 
this takes some weeks to occur.) One such mont- 
morillonite 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. Mont- 
morillonite is therefore used to free wine and export 
ale from proteins and protein degradation products, 
which would otherwise flocculate on standing. Mont- 
morillonite absorbs up to its own weight of protein. 
It is probable that only one end of the protein mole- 
cule 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 con- 
sider 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 lb , 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, Grund- 
fragen Silikose-Forsch. 3, 45 (1958). 
Angew. Chem. internat. Edit. 1 Vol. 7 (1968) / No. 9 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 incor- 
porated as a cation into the layer lattice of mont- 
morillonite 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 incor- 
porated into montmorillonite. Montmorillonite carry- 
ing 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). 
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 am- 
monium 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 in- 
tracrystalline 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 potas- 
sium 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 involvedas catalysts in the 
creation of life on earth. Three or four billion years 
ago, the earth probably had a “primordial atmo- 
sphere” of hydrogen, methane, ammonia, and water. 
The ultraviolet radiation from the sun (which was not 
absorbed by oxygen as it is now) and electric dis- 
charges 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). 
692 Angew. Chern. intenrat. Edit. Vol. 7 (1968) / No. 9