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VI. The Configuration of the Clay Mineral Crystals as Related to their Properties

VI. The Configuration of the Clay Mineral Crystals as Related to their Properties

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THE CLAY MINEBALS IN SOILS



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hined surface areas which can be most conveniently expressed in terms

Thiv explains why there are no other inorganic substances

which have water-holding capacities that approach the water-holding

capacities of the clays. It explains why 6 t o 8 feet of silty clay soils

usually have sufficient water-holding capacity to hold the water equivalent to 40 inches of rainfall, It explains why the negligible sorptive

capacity of the original l-centimeter cube of mica can be changed by

subdivision to a capacity which is great enough to have an important

role in sorbing inorganic ions, organic ions, sugars, starches, and other

polar compounds from solutions.

Clay mineral crystals are to inorganic chemistry what the proteins

are to organic chemistry. Both groups of substances can be considered

to be large charged molecules which can be extended on and on. As a

result, they sorb ions and polar molecules. Due to their configurations,

they have enormous surfaces and since their surfaces are charged,

they can orient and sorb enormous quantities of water. Certain interparticle arrangements allow them to sorb, orient, and confine water

molecules in inter-particle three dimensional associations t o form gels.

The proteins are amphoteric and have both cation and anion sorbing

capacities. The clay minerals have well-known cation sorbing capacities, and recently Dean and Rubins (1947) and Coleman and Mehlich

(1.948) have shown that they have anion sorbing capacities. The work

of Schofield (1940) shows that the clay minerals are amphoteric and that

their crystals have both positive and negative spots.

There is a great variety of molecules, called polar molecules, in which

one end or one side of the molecule is weakly positively charged and

another end or side is weakly negatively charged. The oxygen and

divalent sulfur-bearing compounds belong to this group of compounds.

These molecules, when dispersed in liquids, tend to form associations

among themselves in order to neutralize these weak charges, or if clay

minerals are in contact with the polar molecules, associations can form

around the charged spots on the clay mineral crystals. I n these associations the positive end of one polar molecule will orient towards the

negative end of a neighboring molecule or clay mineral crystal. Exchangeable ions will also tend to orient these polar molecules in which

case the negative end of polar molecules will be oriented towards cations,

and the positive end of the molecules will be oriented towards anions.

I n water suspensions of clay mineral crystals, water would be expected to be oriented around the charged spots which holds exchangeable

ions and also around the exchangeable ions themselves. Furthermore, in

these thin film-like crystals it would not be expected that all charges

would be exactly balanced by an opposite charge especially since the

of acres.



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surfaces of the crystals are covered with large oxygen and hydroxyl ions.

There would be expected, therefore, weak widely distributed residual

charges over the surface of the clay mineral crystals. These charges

would be expected to sorb and orient polar molecules, and they would

account for the capacity of the clay minerals to sorb polar liquids and

gases, as will be pointed out in Section VII-1-a.

The clay mineral crystals are extremely flexible, flimsy; and fragile.

To appreciate this the thinnest clay mineral crystals might well be likened to wet sheets of paper with torn edges and with haphazard holes

punched in them. The thicker crystals could be likened to cardboard

with similar imperfections. When wet with thin films of water or other

polar liquids, these crystals tend to orient themselves and stick t o other

clay mineral surfaces or t o other surfaces. Since these crystals are

charged, t.hey also attract, orient, and organize the molecules of thin

films of liquid on their surfaces. These oriented semi-rigid films of

liquids serve as lubricants, and they become responsible for the high

degree of plasticity exhibited by large masses of wet finely divided and

highly charged clay mineral crystals.

If clay mineral crystals are dispersed in water and allowed to flow

through a porous medium, such as a soil profile, they gradually clog

the pores and channels of the soil even though these voids are many

times larger than the greatest dimension of the clay mineral crystals.

Jenny and Smith (1935) have shown that clays flocculated by electrolytes or hydrated iron oxide sols or clays aggregated by dehydrat.ion are

effective in clogging columns of coarse sand. The sheet-like nature of

the clay mineral crystals enhances their tendency to form claypans in

soil profiles.

The clay mineral crystals that associate large amounts of water with

their crystal surfaces tend to form gels in rather dilute systems. The

swelling bentonites, which are composed of montmorillonitic clay minerals, form gels in concentrations as low as 1to 2 per cent. This tendency

of certain clays to form gels is enhanced by the sheet-like nature of the

clay mineral crystals. Hydrous crystals of this type form more tenacious

cells in which oriented water molecules are confined.

The clay mineral crystals would be expected to show many structural

and voided imperfections due to pressures exerted upon them and due

to the fact that they are formed from extremely dilute solutions. Voids

could, therefore, result from a lack of sufficient ions a t the time the

crystals were forming. Mechanical breakage and frayed edges of the

frail clay mineral crystals certainly result from such forces as biological

activity, alternate freezing and thawing, and from the cultivation of

soils. Oulton (1948) has found that activated montmorillonite cracking



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catalysts are extremely porous. He found that the pores in these catalysts are completely interconnecting without “dead” ends and without

cylindrical sides. It must be rernembered, however, that the activation

process may‘ be responsible for rendering the catalyst more porous.

The voids in the clay mineral crystals should exhibit the same type

of reactions that are characteristic of the edges of the crystals, but in the

case of small voids, they should enter into these reactions with more

vigor. These voids should be especially favorable for the energetic

sorption of ions which have the proper size and coordination to fit in the

vacant places in the crystal. They could well be the places in which the

plant nutrient elements are fixed in difficultly available forms.

The sheetlike crystals of the clay minerals have a marked influence

on the physical properties of soils. When they are randomly oriented

throughout soil structure part.icles, they are efficient binding agents for

the structure particles because each crystal extends its influence over a

relatively great distance along its a and b axes. I n this way the randomly oriented clay mineral sheet.s provide a three-dimensional interlacing of binding forces for soil structural aggregates but still do not

greatly interfere with the movement of air and water when there are

channels and pores between aggregates.

When clayey masses are caused to flow either in the moist plastic

state or in suspension, the individual sheetlike clay mineral crystals tend

to orient in a preferred direction with the longest axis parallel to the

direction of flow. This occurs on a wet clayey furrow slice while i t is

slipping over the moldboard of a plow. If the water content of the mass

is high during the time that the clay mass is flowing, organic materials,

hydrated sesquioxides, and extremely small grainlike mineral crystals will

be gradually worked from spaces between the sheets of the clay minerals.

This will result in an arrangement of multiple sheets of overlapping clay

mineral crystals. If the water content of the clay mass is then reduced,

the sheets of clay mineral crystals will adhere toget,her or to faces of

other crystals much the same as moist bits of paper adhere together. The

moist clay mass will now be much more plastic and the overlapping of

layers of clay mineral sheets will interfere with the movement of water

and air. If the clay mass is dried, it will be hard and consistent. The

overlapping crystals will tend to clog channels and pores in clayey masses

much like bits of paper will clog N wet sieve. These are some of t,he essentials of a puddled condition in clayey soils.

Parallel-oriented clay mineral sheets can most easily be returned to a

condition of random orientation when they are surrounded by large

amounts of water containing organic materials or other charged colloidal

substances. The formation of ice crystals bet.ween the layers of clay



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mineral crystals during alternate freezing and thawing is also very effective in rearranging these crystals in a condition of random orientation.



VII. THE PHYSICOCHEMICAL

REACTIONS

OF THE CLAYMINERALS

It has been pointed out in Section VI that the clay minerals possess

extremely large surface areas. These surfaces provide soils with vast

capacities t o store sorbed water, organic compounds, and plant nutrient

elements. The clay minerals hold these valuable substances against the

forces of nature in forms which later may be used by microorganisms and

plants. For convenience of discussion, the sorption reactions of the clay

minerals are divided into two types. The one type of sorption involves

associations with polar molecules. The other type of sorption involves

the attraction of ions oppositely charged from the spots upon which they

are sorbed on the clay mineral crystals. Fixation of some of the plant

nutrient elements and the dispersion, flocculation, and gel-forming properties of clay minerals are closely associated with these sorption reactions.

I . Polar Sorption Reuctions

According to Debye (1929) many iionionic inurganic and organic

molecules are dipoles due to a lack of symmetry of electron distributions

within individual molecules. These molecules act as if they carried both

centers of positive charges and centers of negative charges. Clay mineral

crystals are also polar. When these polar crystals are in contact with

liquids, solutions or suspensions containing other polar substances, the

negative centers on the clay mineral crystals attract the positive centers

on polar substances in surrounding liquid phases, and positive centers

on the clay mineral surfaces attract negative centers on surrounding

polar particles. These attractions provide the forces by which polar substances are sorbed by the clay minerals.

Water is by far the most important polar compound which is sorbed

and conserved by the soil clays. The oxygen atom in water, according

to the work of Bernal and Fowler (1933), and Cross et al. (1937) tends

to direct its attractive forces toward the four corners of a regular tetrahedron. I n the water molecule, most of the attractive force of the oxygen

atom will be directed towards two corners of the tetrahedron where the

hydrogen atoms will reside, but a small residual force will be directed

towards the other two corners of the tetrahedron. The total positive

attractive force on the 2 hydrogen atoms should exactly equal the total

negat.ive attractive force on the divalent oxygen atom. For steric reasons,

according to Bernal and Fowler, the two hydrogens cannot exactly

neutralize the four spots on the oxygen atom so that the spots where the



THE CLAY MINERALS IN SOILS



185



hydrogen atoms reside will carry a slight reeidual posit,ive charge and

the other two corners of the tetrahedron should carry an equally small

residual negative charge. Water molecules, therefore, tend to attract or

sorb each other. I n this way a molecule of water is subjected to less

strain if the residual positive side of the molecule has this posit.ive

charge neutralized by the negative side of a neighboring molecule. This

process, whereby residual positive hydrogen-rich spots of one molecule

neutralize residual negative spots on neighboring molecules, is called

hydrogen bonding. According to Bernal and Fowler, hydrogen bonding

does not stop with the union of only 2 molecules of water in the liquid

phase, but from x-ray diffract.ion patterns of water, Fowler and Bernal

(1933) have concluded that a few tens or hundreds of water molecules

tend to be hydrogen bonded. The x-ray data indicate that 4 water

molecules are grouped around a fifth molecule, thus forming a tetrahedron. These units are assumed to be propagated on and on until

broken by the kinetic nature of the water molecules. The edges of

oriented, organized, pseudocrystalline clumps of water molecules would

still not have all the residual charges neutralized, but there would be

both negative and positive spots remaining on the sides and edges of the

pseudocrystal.

a. Montmorillonitic Minerals. The residual positive and negative

spots on water molecules or clumps of molecules are important in the

clay mineral-water sorption relationships. The oxygen ions in the surface of the clay mineral crystals (Figs. 1, 2, 3 ) , like the oxygen atoms in

the water molecules, will also be weakly charged. These layers of oxygen

ions will direct most of their forces backward into the crystal towards

t,he positive silicon or metallic ions in the interstitial spaces of the clay

mineral crystal. Weak residual negative charges will result on the outside layer of oxygen atoms in the clay mineral crystals, which will be

free to take part as donors in hydrogen bonding. Each oxygen atom,

if free from other sorbed substances, should attract the hydrogenated or

positive side of a water molecule. Hendricks et al. (1940), however,

have suggested that water molecules on hydrated clay layers arrange

themselves in a hexagonal network in which the individual molecules are

three angstroms apart. Such an arrangement would allow each water

molecule to reside in the depression of the base of each tetrahedron (a

ratio of 1 water molecule to 1% oxygen ions), which is composed of 3

oxygen ions in the surface of the silica tetrahedral layers.

Hendricks (1941) gives four distinct steps for the hydration of t,he

surface layers of dry montmorillonite. They are: (1) hydration of the

exchangeable cations on the external and on the internal surfaces of the



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