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STATE OF ILLINOIS

HENRY HORNER, Governor

DEPARTMENT OF REGISTRATION AND EDUCATION

JOHN J. HALLIHAN, Director

DIVISION OF THE

STATE GEOLOGICAL SURVEY

M. M. LEIGHTON, Chief
URBANA

CIRCULAR NO. 54

SOME CONCEPTS OF THE RELATIONSHIP

BETWEEN THE CHEMICAL COMPOSITIONS

AND STRUCTURES OF CLAY MINERALS

BY
W. F. BRADLEY

Reprinted From the Transactions,
Illinois State Academy of Science,
VOL. 31, NO. 2, PP. 130-131. 1939.

PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS

URBANA. ILLINOIS
1939

(84345)

130 Illinois State Academy of Science Transactions

Some Concepts of the Relationship Between the Chemical
Compositions and Structures of Clay Minerals*

W. F. Bradley

State Geological Survey, Urbana, Illinois

The common clay minerals which have been investigated up to the
present time can be classified in one or the other of two general structural
types. The first type may be thought of as resulting from the condensa-
tion of an hexagonal net of silica tetrahedra with a layer of hydrargillite
giving rise to a double layer of composition (OH)4Al2Si205. The second type
arises from the condensation of two such silica nets, one above and one
below, with one hydrargillite layer, giving rise to a triple layer of compo-
sition (OH)2Al2Si4Oio.

Of these two types the first seems to be the less variable. It includes
kaolinite, nacrite, dickite, and the halloysites. The second type is subject to
many significant variations in chemical composition with concomitant ef-
fects on physical properties. In this group are found montmorillonite,
beidellite, nontronite, illite, and perhaps several others. It has been sug-
gested by several investigators, notably Linus Pauling1 and C. E. Marshall2
that the broader chemical freedom of the second type is due to the greater
stability of its symmetrical configuration. It is with the inferential rela-
tionships between the chemical composition and properties of the second
type that this paper is concerned.

In these two idealized types, each silicon ion is located in the interstice
between four oxygen ions in tetrahedral configuration, and each aluminum
ion in the interstice between six oxygen (or hydroxyl) ions in octahedral
configuration. On the basis of various proven structures for silicate min-
erals, the following substitutions have been suggested as possible: tetrahedral
positions may be occupied by Al+++ or by p+++++; octahedral positions may be
occupied by Mg++, Pe++ Fe+++, Ti++++, or Li+.

By far the most common, the best authenticated replacement, is that
of Al+++ for Si++++, as it occurs in muscovite.3 One fourth of the Si++++ posi-
tions are occupied by Al+++, and the excess ( — ) charge resulting is com-
pensated by the presence of one K+ ion in 12-coordination between triple
layers. Muscovite builds a rigid stable crystal with perfect cleavage be-
tween layers. Its rigidity is attributed to the attractive power of the K+ ions
for each of the adjacent triple layers. Illite, perhaps the most widely dis-
tributed clay mineral in Illinois, exhibits the same crystal structure as does
muscovite. It differs in having a lower K20 content, a higher relative con-
tent of Si02, and in the exhibition of moderate base exchange properties.
Whereas the ratio of Si++++ to Al+++ in the tetrahedral layers for musco-
vite is 3, and one K+ ion is required for each set of 4 such positions, the
similar ratio for illite may be 6 or 7 or higher, and only about one K+ ion
is required for two sets of 4 such positions. Thus the illite, being subject
to only about one-half the attractive forces responsible for the crystal
habit of muscovite, occurs as a clay rather than as macro-crystals. The
analogy between the two minerals is perhaps better illustrated by ex-
amination of "structural formulae" for each:

Muscovite (KAlSi3) Ala (OH)2Oi„
Illite (KxAlxSi4_x) (Al2 • Fe2 • Mg» • Mg,.) (OH)2010
where x < 1, usually about y2.

A second common, and significant, replacement is that of Mgf+ (or
Fe++) for Al+++ in octahedral positions. This replacement, presumably can
occur in either of two ways, as has been implied in the above formula.

* Published with permission of the Chief, State Geological Survey.

Chemistry — 1938 Meeting 131

Three Mg++ ions may perform the role of two Al+++ ions as has been de-
duced in the case of several biotite micas, and of talc, without the in-
troduction of any unbalanced charges, or one Mg++ ion may directly re-
place one Al+++ ion with the appearance of one excess ( — ) charge per
replacement. Such a ( — ) charge, however, differs from that observed in
the case of muscovite in that, whereas, the muscovite charge is localized
at the surface of a unit and can be readily balanced by an adjacent K+ ion,
this second sort occurs in the middle of a stable structural unit. Observa-
tions on specimens of montmorillonite have shown that the number of this
sort of charges approximates the base exchange capacity. Montmorillonite
is a soft, finely crystalline, and very readily dispersed clay with high base
exchange capacity.

Nontronite is a name applied to several clays with physical character-
istics quite similar to those of montmorillonite, but with more or less Fe,03
content. These are presumed to arise from substitution of Fe+++ ion for
Al+++ ion in octahedral positions, a substitution which does not effect elec-
trical neutrality. Both nontronite and another similar clay, beidellite, are
commonly distinguished from montmorillonite on the basis of lower SiO^R^O:.
ratios, but neither is fully characterized at present.

No natural clay has yet been observed where only one sort of replace-
ment could be shown to be active. However, these qualitative consider-
ations of the relations between physical properties, structure, and chemical
composition lead to the suggestions that in cases where the first sort of
substitution predominates one finds the illites, with stable lattice dimen-
sions, moderate plasticity, and moderate base exchange capacities; where
the second sort of substitution predominates the clays swell on addition of
water, disperse readily to extremely small particle sizes, and exhibit high
base exchange capacities.

Literature Cited

1. L. Pauling, Troc. Nat'l Acad. Sci. 16, 123-9, 578-82 (1930).

2. C. E. Marshall, Zeit. Krist. 91, 433-49 (1936).

3. W. W. Jackson and J. West, Zeit. Krist 76, 211-27 (1930).

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