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III. Crystal Structure of the Clay Minerals in Soil Clays

III. Crystal Structure of the Clay Minerals in Soil Clays

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assembled many diagrams showing the arrangements of the ions and the

coordination between neighboring ions for most of the known clay

minerals. Davidson et al. (1943), have illust,rated the montmorillonite

crystal by means of a diagram in which successive layers of the crystal

are cut away. The relationship of the layer lattice group of clay mineral

Fig. 1. Arrangement of the ions in two adjoining units of a montmorillonitic

clay mineral crystal. Oxygen ions are represented by white spheres, and hydroxyl

ions are represented by gray spheres. Sorbed ions and molecules are represented by

the spheres between the crystallographic units. Small silicon and aluminum ions reside in some of the interstitial spaces between the oxygen and hydroxyl ions (Courtesy of C. E. Marshall).

t o the other silicate minerals has been shown by Jackson et al. (1948).

From a very casual observation of a scale model of a clay mineral

crystal (Figs. 1, 2, 3 ) , it appears that the crystal is composed of layer

upon layer of a mixture of 0-- and OH- ions, both of which have the

same size. It will also be noted that there are two kinds of layers. I n

the one layer there are only 0-- ions and the center of each 0-- ion is

a t the corner of two adjoining hexagons, each hexagon being formed by

six 0-- ions. This leaves a space in the center of each hexagon equivalent to the size of an 0-- ion. This layer will be referred to later as the

perforated oxygen layer. The other type of layer is composed of either



a mixture of 0-- and OH- ions or in some cases it is composed entirely

of OH- ions. These latter layers will be referred to as the solid oxygenhydroxyl layers.

A closer examination of a scale model of clay mineral crystals will

show that the sinall S i + + + + ions and the slightly larger A l + + + ,

F e + + + , or M g + + ions are found in the interstitial spaces left between

t,he regularly spaced large 0-- and OH- ions. Consequently, these

small positive ions do not affect the overall dimensions of the clay mineral

crystals. It will also be noted that these small positive ions are arranged

in definite three-dimensional patterns. The clay mineral crystals, therefore, are held together by nttractions beween neighboring positive and

Fig. 2. Arrangement of ions in an illitic clay mineral cryatal. Oxygen ions are

rcpresented by white spheres, hydroxyl ions by gray spheres and potassium ions by

black spheres. Silicon and aluminum ions reside in interstitial spaces between oxygen

and hydroxyl ions (Courtesy of C. E. Marshall).

negative charges on the ions and by multiple charged ions sharing their

influence with two adjoining layers.

This interlacing of forces between neighboring ions and layers of ions

is interrupted at the edges and sides of all crystals and this unbalanced

condition becomes the basis for some of the physicochemical reactions

of t,heir surfaces. As will be discussed in more detail in Sections VI and

VII-1-a, these unbalanced forces a t the edges of crystals are magnified

to enormous proportions in the clay mineral crystals due to the shape

and finely divided nature of these minerals.

1 . The Montmorillonitic Minerals

The 0-- ions of the inontmorillonites are stacked as follows: One

perforated layer, two solid layers, another perforated layer followed by a,



variable spacing in which is sorbed exchangeable ions, water molecules,

or other polar molecules. This whole sequence is then repeated over and

over until the thickness of the montmorillonite crystal is attained. The

small S i + + + + ions fit into the interstit.ia1 spaces between the outside

perforated layers of 0-- ions and the adjoining internal solid mixed

layer of 0-- and OH- ions. A l + + + or some other substituting positive

ion fills two-thirds of the interstitial spaces between the two solid 0-layers.

The perforated oxygen layer contains 0-- ions arranged in groups

of threes. The centers of each of these 3 ions form three corners

of a regular tetrahedron. The Si+ + + + ions reside in the centers of these

tetrahedra, which are completed through sharing with an 0-- ion in the

adjoining solid oxygen layer.

Two 0-- ions and 1 OH- ion, of one of the solid oxygen-hydroxyl

layers, form a reguIar octahedron with 2 0-- ions and 1 OH- ion in the

adjoining solid oxygen-hydroxpl layer. The 4 0-- ions in these octahedra share corners with their respective adjoining tetrahedra as pointed

Fig. 3. Arrangement of the ions in a ksolinitic clay mineral crystal. Oxygen

ions are represented by white spheres and hydroxyl ions are represented by gray

spheres. Silicon ions are represented by the smallest lower layer of ions in the

interstitial spaces left between the tetrahedral oxygen and hydroxyl ions. Aluminum

ions are represented by the upper layer of small spheres in the interstitial octahedral

positions. The aIuminum ions are somewhat larger than the silicon ions (Courtesy

of C. E. Marshall).



out in the preceding discussion on the oxygen tetrahedra. The interstitial

space a t the center of the oxygen-hydroxyl octahedra is somewhat greater

t,han the space at the center of the oxygen tetrahedra. Al+++ predominates in the interstitial space at the centers of the octahedra, but there

is also sufficient space in this position for F e + + + , Mg++, or Li+ ions.

If these latter ions are present when the montmorillonite crystal is

formed, some of them may occupy this octahedral position. Starting with

one flat side of the montmorillonite crystal, therefore, there is a silica

tet.rahedra1 layer upon which is stacked, and with which is interlaced, a

sesquioxide octahedral layer. The top side of the octahedral layer is


- 21.4 i*

n H20

MONTMORlLLONlTE tOHLAl. S 1 * 0 ~ ~ h. n0

Fig. 4. The crystal structure of montmorillonite as presented by Hofmann e t al.

(1933) (Courtesy of R. E. Grim).

interlaced with another silica tetrahedral layer. Upon this part of the

crystal lattice there is a variable space which will accommodate exchangeable ions, water, and other polar molecules. All these foregoing

layers taken as a unit are repeated over and over again until the flat

sides of the crystal are reached.

The smallest unit of the ideal montmorillonite crystal, or the unit

crystal cell, according to Hofmann et al. (1933), must contain the following layers with the following numbers of ions (see Figs. 1 and 4 ) :

(1) Variable spacing containing sorbed ions and molccules.

(2) 6 0--.

(3) 4 Si++++


(4) 4 0-- and 2 OH-.

( 5 ) 4 A1+++(insterstitial),

(6) 4 0-- and 2 OH-.

(7) 4 Si++++


( 8 ) 6 0--.



Edelman and Favejee (1940) suggest that the following structure would

more nearly account for the properties of the montmorillonitic minerals

(see Fig. 5 ) :








4 (OH). 2 0

2 56


2 5,






S1.0,a'n Ha0

Fig. 5 . The crystal structure of montmorillonite as presented by Edelman and

Favejee (1940) (Courtesy of R. E. Grim).

Variable spacing containing sorbed ions and molecuIes.

2 OH- (protruding).

2 sit+++(interstitial).

6 0--.



(6) 4 OH- and 2 0--.

(7) 4 Al+++(interstitial).

(8) 4 OH- and 2 0--.

(9) 2 Si++++(interstitial).

(10) 6 0--.

(11) 2 Si++++


(12) 2 OH- (protruding).

The montmorillonites are built, up by three dimensional extensions of

these units.

The montmorillonitic minerals found in nature seldom, if ever, contain

the exact ions mentioned above, but instead there is replacement of the

positive ions in the tetrahedral and octahedral layers, according to Marshall (1935b) by other ions having approximately the same size and

coordination. I n the tetrahedral layer Al+ + + can substitute for Si++ + +.

This leaves a deficiency of one positive charge which must be supplied

by a charge from an exchangeable cation on the clay mineral crystal or,

as will be pointed out in Section 111-2, by addition of a potassium ion

to form a micaceous mineral. In the octahedral layer F e + + + , Mg++,



or Li+ can substitute for A l + + + . When M g + + and Li+ substitute for

A l + + + , there is also a deficiency of positive charges which must be satisfied by exchangeable cations or by extension of the lattice into another

mineral, such as the micas. These replacements have been called isomorphous replacements by Marsha11 (1935b) due to the fact that they

occur without any overall changes in the dimension of the crystal lattice

units. Isomorphous replacements provide an important mechanism by

which clays acquirk cation exchange and polar sorptive capacities.

2. The Illitic Minerals

The crystal structure of the illites was proposed by Grim et al.

(1937'). These minerals like the montmorillonitic minerals are composed






( S I . ~ A I ~ ) O ~ ~

Fig. 6. The crystal structure of illite as presented by Grim et al. (1937) (Courtesy of R. E. Grim).

of a silica tetrahedral layer, an aluminum octahedral layer, and another

silica tetrahedral layer, but in place of the variable spacing of montmorillonite, there is a K + ion. This combination of layers comprises

the smallest crystallographic unit along the c axis and the illitic minerals

are formed by repeating these units over and over again until the flat

edges of the crystals are reached. The K + ions balance deficiencies in

positive charges which, according to these workers, arise from the isomorphous substitution of A l + + + for S i + + + + in the silica tetrahedral

layers. Substitution of M g + + for A l + + + in the octahedral layer is



responsible for the cation exchange capacities of these minerals. The

ratios of the ions in the various layers of the illites as given by Grim

e t al. (1937) are as follows (see Figs. 2 and 6) :

6 0--.

4-y Sit+" and y Al+tt (insterstitial).

4 0-- and 2 OH-.

4 Al"' (interstitial) with possible substitutions by Fe+++or Mg++.

4 0-- and 2 OH-.

(6) 4-y Si"" and y Al+++(interstitial).

(7) 6 0-(1)





(8) Y K'.

The illite crystals are built up by three dimensional extensions of these


3. The Kaolinitic Minerals

The unit crystal of the kaolinites is composed of one silica tetrahedral

layer and one alumina octahedral layer. The octahedral layer in






Fig. 7. The crystal structure of kaolinite as presented by Gruner (1932) (Courtesy of R. E. Grim).

kaolinite contains 4 more OH- ions and 4 less 0-- ions per unit crystal

than the montmorillonites and the illites. The ratios of the ions in the

various layers are as follows (see Figs. 3 and 7) :

(1) 6 0--.

(2) 4 Sit+" (interstitial).

(3) 4 0-- and 2 OH-.

(4) 4 Al"' (interstitial).

( 5 ) 6 OH-.

The kaolinite crystals are three-dimensional extensions of t.hese units.

From the numerous data available on the clay minerals, it appears that

isomorphous replacements in the kaolinite minerals are uncommon.



4. The Interstratified Clay Minerals

Pauling (1930a, 1930b) showed that t.he layer lattice tninerals containing silica tetrahedral layers and sesquioxide octahedral layers have

unit crystal faces of approximately the same size. He suggested that

this should permit different clay minerals t o form mixed interst.ratified

crystals. He also showed that the chlorites had a mica structure which

had the potassium layer removed and the structure expanded sufficiently

a t this point to accommodate a positively charged hydrated magnesia or

brucite octahedral layer in place of the K+ ions. Chlorites have been

found by Jeffries and Yearick (1948) in a number of Pennsylvania soils.

Chlorite-mica interstratified minerals are known and according to Pauling

these are intermediates in which only part of the K + ions of the micas

have been substituted by posit,ively charged brucite layers. Hendricks

(1939) showed that faratsihite is an interstratification of layers of kaolinite and nontronite ( a montmorillonite in which Fe+ + + substitutes for

Al+++ in the octahedral layer). Bradley (1945a) has shown conclusively that montmorillonitic-illitic interstratifications are present in


Since the clay minerals formed in soils crystallize in extremely

heterogenous systems and under conditions which are constantly changing, i t appears reasonable to assume that these clay minerals should

show numerous combinations of interstratifications of different minerals.

This concept and the work of Pauling and Hendricks have resulted in

feasible yet inconclusive suggestions by Bray (1937), Hendricks and

Alexander (1939), and Grim (1942) that soil clays commonly contain

interstratified clay minerals. As far as the aut.hor is aware, the only

interstratified clay mineral conclusively shown to be present in soil clays

is chlorite.

Other Imperfections in Clay Mineral Crystals. Certain imperfections

in the clay mineral crystals contribute to the unique properties of these

minerals. The substitution of an ion with one valence for an ion with

another valence and the influence of these substitutions on the properties

of the clay mineral crystals has been ment.ioned in Section 111-1. Substitutions involving whole layers have been mentioned in the discussion

on interstratified clay minerals, Section 111-4.

From electron micrographs and from sorption-desorption studies, it

appears that tthere are other physical imperfections in the clay mineral

crystals which probably involve relatively large spaces in the crystal

lattice which have been left vacant. Wiegner (1935) has shown that ions

sorbed in vacant spaces in the crystal 1att.ices of kaolinitic and bentonitic

clays c m actually blockade incoming ions. These vacant spaces or voids



in clay mineral crystals would be expected to offer a variety of possibilities with respect to shape. They could be channels, wide angled coneshaped pores, narrow angled cone-shaped pores, bottle-necked pores,

crevices, cracks, or they might be ragged edges on crystals. Since pores

offer possibilities of directing attractive forces from many directions

toward sorbed molecules or ions, it is evident that these internal surfaces

can be responsible for the sorption and fixation of ions and molecules

which may not be easily removed once bhey become sorbed.





Clays are most commonly composed of mixtures of one or more of

the clay minerals with free oxides of silicon and free sesquioxides. Much

of the material in these mixtures is composed of crystalline particles too

small to be observed by the best optical microscope. Such mixtures are

difficult to analyze. Any of the known qualitative procedures for the

identification of minerals has very definite limitations when applied to

mixtures of finely divided crystals of the types found in clays. When

attempts are made to devise quantitative met.hods for the estimation of

the individual minerals in the clays, still other limitations arise. It is

often necessary to use data obtained from several procedures to identify

and quantitatively estimate the minerals in these mixtures. When the

limitations of the various methods do not overlap, i t is possible to obtain

fairly conclusive evidence concerning the kind and amount of certain

of the minerals in the clays.

The methods used in the identification and estimation of the clay

minerals are: (1) chemical, (2) optical, (3) thermal dehydration, (4)

electron microscopic, and ( 5 ) x-ray diffraction.

1. Chemical Methods

Total chemical analyses have long been used in the study of the clays

and the clay minerals. They have been very useful in studies on deposits

of clay minerals which contain chiefly one component, but they are

limited in their usefulness when applied to the mixtures of minerals found

in the soil clays.

The total amount of potassium in clays has been widely accepted to

be an index of the amount of mica-like minerals in soil clays. While

the illites vary considerably in their potassium content, they have been

generally assumed to contain 6 per cent potassium. Pearson and Ensminger (1948) and Buehrer e t al. (1948) calculated the percentage of

the illites in clay samples by assuming that these minerals contain 6 per

cent potassium. This method of determining the amounts of the illites



in clays is based on the assumptions that the illites always contain 6 per

cent of potassium and that no other potash-bearing minerals are to be

found in the clays. These asfiiimptions are not strictly valid but this

method appears to be the best met.hod available for the estimation of

the illites.

Numerous attempts have been made to estimate the amounts of free

silica in clays by treating them with weak NazCOs solutions. These

methods are not widely accepted as specific for free silica.

Nascent hydrogen, from a mixture of oxalic acid, potassium oxalate,

and magnesium ribbon, has been used by Jeffries (1946) to remove free

iron oxides from soil fractions in preparing these fractions for mineralogical examination. The amount of iron removed from clays has been

considered to represent the amount of free iron oxides in the clays. This

method, however, is open to question for use on soil clays. Iron-bearing

clay minerals lose their crystalline structure, as revealed by x-ray diffraction methods, when they are treated with oxalic acid. Even though

other soluble materials may not be removed from t.he clay minerals by

oxalic acid solutions, iron may be removed from these minerals by this


Cation or base exchange capacity measurements often provide valuable supplemental evidence concerning the nature of the clay minerals

in clays. It is pointed out in Section VII-2-a, that the montmorillonitic

clay minerals have high base exchange capacities, the illitic minerals have

intermediate capacities, and the kaolinitic minerals have low capacities.

When the base exchange capacity bf an organic free clay is high, a high

percentage of montmorillonitic minerals is expected in the sample. Low

base exchange capacities do not necessarily indicate, however, the

presence of kaolinitic minerals. The illitic minerals, when combined with

high percentages of the miscellaneous oxides and hydrated oxides, would

also give a low cation exchange capacity.

2. Optical Methods

Optical methods are among the oldest methods applied to the study

of the clay minerals. These methods were especially valuable in the

early investigations on the more or less pure kaolinitic minerals. The

kaolinitic minerals often occur in relatively large crystals to which the

early optical methods could be adapted. Adaptations of optical techniques were made by Marshall (1935s) and by Bray et al. (1935) which

permitted the use of these methods on fine fractions of the clay minerals,

The optical methods are not especially well adapted to the study of the

clay minerals in soil clays since they give average refractive indices for

mixtures of minerals. These methods are often useful, however, in sup-

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III. Crystal Structure of the Clay Minerals in Soil Clays

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