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V. Distribution of the Clay Minerals in Soils

V. Distribution of the Clay Minerals in Soils

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controversy over the conditions which are favorable for the formation of

the various clay minerals. The work of Schachtschabel (1938), Alexander et al. (1939) Kelley et al. (1939), Kelley, Dore (1939), Sedletzky

(1939a, b, c, 1940), Sedlet.zky and Yussupova (1940), Russell and Haddock (1940), Hosking (1940), Nagelschmidt et al. (1940), Kelley et al.

(1941), Sideri and Liamina (1942), Whiteside and Marshall (1944),

Coleman and Jackson (1945), Peterson (1946b), Jeffries and Anthony

(1948), Jeffries and Yearick (1948), Pearson and Ensminger (1948),

Buehrer et al. (1948) gives certain indications concerning the relationships between some of the soil-forming factors and the formation of the

clay minerals.

Clay minerals with high exchange capacities are widely distributed

throughout the humid temperate regions of the world, This would indicate that these minerals are montmorillonitic minerals except for the

fact that they often fail to show the characteristic basal x-ray diffraction

spacings of montmorillonitic minerals. Montmorillonitic minerals interstratified with other minerals or other imperfections in crystallographic

organization could account for these resu1t.s. I n soils where montmorillonitic minerals have been definitely shown to be a n important component, it appears that slightly weathered parent material having relatively high p H values and, in some cases, large amounts of organic

matter have been favorable for the formation of this clay mineral. The

author has obtained numerous unpublished data on the loessial soils of

the Mississippi valley which generally show the presence of considerable

montmorillonite. Erickson and’ Gieseking (unpublished) have recently

obtained data which show that the dark-colored grassland loessial soil

types contain more montmorillonite than the corresponding c l o d y

associated timbered types.

The illitic clay minerals are very widely distributed in soils. Jeffries

and Anthony (1948) give 20 per cent as the average mica content of the

sediment of the eart.h. It appears that much of the illitic fraction of clays

i e derived from the micas of parent materials, but there is no conclusive

evidence to show that these minerals cannot be formed as a result of soil

developmental processes.

There is general agreement that the kaolinitic minerals occur in

highly weathered, leached, well-drained, and acidic soils. While these

may be the ideal conditions for the formation of kaolinitic minerals, it

is probably not necessary for all of these conditions to be fulfilled in

order to have the kaolinitic minerals formed.

Chlorite has been found in a number of Pennsylvania soils by Jeffries

and Yearick (1948). Pearson and Ensminger’s (1948) description of

an unidentified clay mineral in a number of Alabama soil clays fits that



of chlorite. It seems probable that chloritic minerals have been overlooked in many clays.

No11 (1932,1935,1936) has been able to synthesize a number of d a y

minerals and related minerals in the laboratory. Among these have been

montmorillonite, sericite mica, and kaolinite. These syntheses were carried out by heating water suspensions of A1203 and SiOz under pressure

and under various conditions. He found that montmorillonite was

formed when slightly alkaline suspensions contained small amounb of

sodium, potassium, magnesium, and/or calcium ions. Sericite mica was

formed when the suspensions were strongly alkaline and when they contained sufficient potassium ions to form the mica. Kaolinite was formed

when the suspensions were acid. These experiments are of interest because they parallel some of the natural soil conditions under which these

minerals appear to be formed.

There is evidence to show that the illitic and montmorillonitic minerals are not resistant to intensive weathering. According to Grim

(1942)montmorillonitic and kaolinitic minerals may form from the illitic

minerals and according to Kelley, Woodford, et al. (1939), kaolinitic

minerals can be formed from montmorillonitic minerals. The author has

some unpublished studies on the stability of the montmorillonitic minerals in some very young calcareous loessial clays developed under grass

vegetation. These studies were made by comparing the x-ray diffraction

patterns of the clays after electrodialyzing for several months with the

x-ray diffraction patterns of these clays before electrodialysis. Before

electrodialysis these clays gave intense characteristic basal spacings for

the montmorillonitic group of minerals, but after electrodialysis no sharp

intense characteristic basal spacings were obtained. The electrodialysed

clays gave x-ray diffraction patterns similar to those obtained from

similar but older more highly weathered clays. They still retained

relatively high cation exchange capacities after electrodialysis. From

these experiments it seems that the more perfect montmorillonitic crystals

are rather unstable, and it may be postulated that they partially decompose to form a type of pseudo-montmorillonitic crystal under the acidic

destructive forces of electrodialysis. These rapid destructive forces of

laboratory methods should not necessarily be assumed to duplicate the

extremely slow orderly reactions occurring in nature. They may be

indications, however, of the trends of weathering reactions when somewhat similar conditions are met in nature.

The distribution of the various clay minerals in soils has not been

adequately studied. The meager available data, therefore, do not permit

definite conclusions concerning the factors which influence the formation

of the various clay minerals.









It has long been known from optical and x-ray investigations of the

clay mineral crystals that these crystals are plate-like and that they

readily assume a position of preferred orientation in which the flat-face

of one platelet tends to rest upon the flat-face of an adjoining platelet.

Recent developments in the field of electron microscopy have enabled

soils investigators to obtain more definite information on the configuration of the clay mineral crystals. Humbert and Shaw (1941),Humbert

(1942),Marshall et al. (1942),Shaw (1942),and Jackson et al. (1946)

have shown that various clay mineral crystals may exist in extremely

thin flimsy, film-like crystals. Ardenne et al. (1940) and Shaw (1942)

have reported clay mineral crystals which have a thickness spproximating the unit cell height (1 millimicron). This configuration of the clay

mineral crystals gives them very high specific surfaces (surface per unit

weight) which in turn has a profound effect on the physicochemical

properties of these minerals.

The amount of surface in a few grams of finely divided clay can best

be appreciated by considering the increase in surface if a one-centimeter

cube of massive mica were split into sheets 10 millimicrons in thickness.

This operation would increase the surface of the mica cube from 6 sq. cm.

to 2,000,004sq. cm. or an area of approximately 1/20 of an acre. If

these thin sheets of mica should be cut into pieces a few hundred millimicrons in length and width, we would have essentially a size fraction

of a mineral that we would expect to find in a fine illitic clay, Since the

original 1 centimeter cube of mica has essentially the same crystal structure as the illitic clay minerals, the large oxygen and hydroxyl ions will

occupy the bulk of the volume of the cube with the other ions filling

interstitial spaces in the same way as in the clay minerals. The original

mica cube will have 0.00015 per cent of its oxygen and hydroxyl ions in

iix surface layers. After the cube is reduced to sheets 10 millimicrons

in thickness, 5 per cent of the oxygen and hydroxyl ions of the original

cubic centimeter of mica will be in the surface area of the crystals. If

the mica crystals were divided further into sheets 1 millimicron in thickness, 60 per cent of the oxygen-hydroxyl framework would be in the

surface of the crystals. This same trend would follow for the kaolinitic

clay minerals. The montmorillonite clays, however, always have 50 per

cent of the oxygen-hydroxyl framework in either external surfaces or in

the internal surfaces of the expansible layers irrespective of the size of

the crystal aggregates.

The crystals in a few grams of finely divided clay minerals have com-



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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V. Distribution of the Clay Minerals in Soils

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