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IV. Qualitative Identification and Quantitative Estimation of the Clay Minerals

IV. Qualitative Identification and Quantitative Estimation of the Clay Minerals

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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-



plementing and confirming the results obtained by other methods. The

various optical properties of the clay minerals have been tabulated by

Grim (1939).

3. Thermal Dehydration Methods

Thermal methods are the oldest and most widely used methods for

the identification of the clay minerals. Kelley et al. (1936) found that

the clay minerals and related minerals lose water at critical temperatures

which are characteristic of the type of mineral. Orcel (1926), Orcel and

Caillere (1933), and Agafonoff (1935) used the differential thermal

method of LeChatelier (1887) in devising a simple method of determining

the nature of the loss of water from clays at various temperatures and the

nature of the reactions involved in the loss of the water from the crystal

structure. In this procedure an unknown sample and an inert standard

anhydrous aluminum oxide sample are placed in separate wells in a

furnace which is heated at a constant rate to 1000°C. The difference

in temperature between the unknown sample and the sample of inert

aluminum oxide is continuously measured by a recording device attached

to two opposing thermocouples, one of which is inserted in the unknown

and the other in the inert standard. Loss of water or any other crystallographic change in the unknown will either lower the temperature of the

unknown or raise its temperature with respect to the inert standard.

In case the temperature of the unknown is raised, an exothermic reaction

has occurred, but if the temperature is lower, an endothermic reaction

has occurred. The temperature of the inert sample is also continuously

recorded so that the temperature ranges can be determined over which

t.he endothermic and exothermic reactions occur. Detailed descriptions

of the apparatus and procedures for differential thermal analyses of clays

and typical differential dehydration curves are given by Norton (1939)

and Grim and Rowland (1942).

Allaway (1948) has used the differential thermal decomposition and

oxidation of piperidine sorbed on clay minerals to characterize these

minerals. He has obtained characteristic differential thermal oxidation

curves for the piperidine combinations with the various clay minerals in

soil clays. Allaway (1948) and Bradley and Grim (1948) have found

that amines sorbed on clays are oxidized in a step-wise fashion. The

hydrogen of the amine is oxidized first leaving carbon, which requires

a higher temperature for oxidation. This method appears to offer promise

in supplying additional confirmatory evidence concerning t.he nature of

the clay minerals occurring in soil clay mixtures.

Buehrer et al. (1948) have devised a method whereby they can obtain

differential thermal curves of clay samples and a t the same time weigh



the sample to determine the amount of water lost a t various stages of

dehydration. These weighings can be made at any predetermined temperature ranges. Buehrer and his associates consider the crystal lattice

water (water lost above 300°C.) of t.he montmorillonitic and illitic clay

minerals to be 5 per cent and that of the kaolinitic minerals to be 14

per cent. This is a refinement in differential thermal techniques that can

be very valuable in working with certain clay mineral mixtures.

The differential thermal method is usually suitable for qualitative and

rough quantitative estimations of the components of soil clays, if the

clays contain little or no organic matker and if the various mineral component,s of the clay do not give coincidental endothermic and/or exothermic reactions. The thermal methods of clay mineralogical analysis are

very valuable supplements to the other methods of analysis.

4. Electron Microscopic Methods

The electron microscopic methods have been extremely useful in showing the sizes and shapes of the crystals of the clay minerals found in

relatively pure deposits. The electron micrographs of the soil clays have

not shown the regularity of crystalline form that has been exhibited by

specimens of the clay minerals from geological deposits. This finding

alone is probably the most valuable contribution made by the electron

microscope in elucidating the structure of t.he soil clays. This confirms

the suggestion that the soil clays are heterogeneous mixtures of minerals

formed in a more or less fortuitous manner.

5. X - R a y Diffraction Methods

X-ray diffraction methods have been responsible for the decided

progress in t*he study of clay minerals since 1930. Pauling (1930a,

1930b) published the structure of the layer lattices of the micas and

the chlorites which led to further work and publication of the crystal

structure of the other clay minerals, as related in Section 11. The progress through the use of x-ray diffraction methods has not come as a

result of the lack of limitations in these methods but because of the

fact bhat good diffraction patterns from these methods can usually hc

interpreted in a conclusive manner. Furthermore, x-ray diffraction

techniques, while they are usually most effective on single crystals, are,

nevertheless, more effective than other physical methods of studying

finely divided crystalline particles.

Preferentially oriented aggregates of the flat sheet-like crystals of

the clay mineral crystal have been used by Clark et al. (1937) as a

substitute for single crystals. In these preferentially oriented aggregates

the individual clay mineral crystals may be rather perfectly oriented



along their c axes or in a parallel, flat-face to flat-face manner. Orientat>ionof the crystals along the other axes, however, will be random. The

arrangement of clay mineral crystals in these aggregates can be compared to a deck of cards thrown in a heap on a table. The cards will

preferentially orient along an axis perpendicular to their faces, but the

corners will not match and they will be randomly oriented except for

the preferred orientation along a single axis. Fortunately, the clay

minerals differ only in the nature and order of stacking of t,he various

units within each crystal along the c axis (Section 111-1,2,3,4). Preferential orientation along the c axes of the clay mineral crystals in aggregates, therefore, provides a valuable aid in the identification and

quantitative estimation of these minerals by x-ray and optical methods.

The ions in crystalline substances are arranged in definite threedimensional patterns. Upon inspection of a model of a crystal, i t will be

observed that the ions in the crystal can be placed in numerous planes

of ions. It will be observed that these planes may be thickly or sparsely

populated with ions and that they cut across the crystal in many directions. It will be further observed that every plane of ions of a given

composition will be repeated over and over again forming a series of

equally distant parallel planes. Any parallel series of planes of ions

always bears the same angular orientation with any other series of parallel planes of ions irrespective of the part of the crystal selected, and

these same constant relationships hold for all crystals of any substance

as long as it exists in only one crystalline form. Two different substances

which have exactly the same interplanar relationships described above

are said to be isomorphous. Isomorphous substances usually arise from

the substitution of one ion by another ion having the same coordination

number, approximately the same size, and carrying a charge of t.he same

sign but not necessarily an equal charge. The two end members of such

an isomorphous series are represented by crystals in which either one

or the other of the two ions completely fills the spaces allotted to these

ions. Int*ermediate members of the isomorphous series may have any

degree of replacement between the two end members of the isomorphous

series. Examples of isomorphous replacement were given in Section

111-1, 2.

The interplanar distances and planar orientations are constant and

specific for any given crystal and its isomorphous relatives, in case it

belongs to an isomorphous series. Measurements of these constant distances and planar orientations of the ions, therefore, form an excellent

basis for the identification of crystalline substances.

The distances between the more prominent planes in a crystal can be

measired by the diff rnction of monochromatic x-rays in s way similar



to the measurement of distances between closeIy spaced fine parallel lines

on a transparent or reflecting plate by diffracted monochromatic light.

Since constant interplanar distances are characteristic for a given crystalline substance or its isomorphous series, if such arc formed, x-ray diffraction methods are useful for the qualitative estimation of crystalline substances. Rough quantitative estimations of the components of mixtures

of crystalline substances have been made by measuring the intensities

of diffracted x-radiation from the various components and by comparing

these results with the results from suibable standards.

When fine crystalline powders are irradiated with a sharply collimated

monochromatic beam of x-rays, the reinforced diffracted rays come out

as sides of hollow cones with the various sized cones having a common

vortex a t the point where the undiffracted collimated beam hits the

sample. The angles, a t which the sides of the various cones of reinforced

diffracted x-rays come out from the sample, depend on the wave length

of the x-rays and the interplanar distances of highly reflective denser

planes of ions in the crystal. The edges of t,he bases of the cones of the

diffracted x-rays will form concentric rings. The position of these concentric rings can be recorded on a photographic film or by means of a

Geiger-Muller counter. The undiffracted x-ray beam will strike the

common center of the concentric bases of the various diffraction cones.

By knowing the distance of the recording device from the sample and

the distance of the sides of cone-bases of the various diffraction cones

from the undiffracted x-ray beam, the various angles between the common center of the cones and the sides of the cones can be calculated.

This angle is twice the diffraction angle 8. The edges of the base of each

diffraction cone form the lines on a common powder diffraction x-ray

pattern, when a strip of photographic film is used across the bases of the

cones to record the diffraction effects. Each line is characteristic of a

single value for the distance between the members of a series of parallel

ionic planes in a crystal lattice. The value for this distance d can be


calculated from tlie Bragg cquation: d = 2 sin 8

I n this expression .n represents the order of reflection. It is always a small

whole number, and it can be determined after all calculations are made

by using 1 for its value. Lambda is the characteristic wave length of

the most intense x-rays generated by the x-ray tube. This wave length

is characteristic of the target in the x-ray tube. The characteristic wavc

length from a copper target is 1.54A and from iron i t is 1.93A; these

being the most common wave lengths used in x-ray diffraction studies

of clay minerals. The d values for montmorillonite, illite, and kaolinite



are given by the American Society of Testing Materials, Philadeplphia,

Pennsylvania, August, 1945.

Under carefully controlled conditions with proper standardization

the intensity of x-ray diffracted radiation can be used to estimate the

amount of the various clay minerals in clay samples. Detailed procedures for such estimations are given by Favejee (1939a, 1939b), Hellman and Jackson (1944), Aldricli et al. (1944), White and Jackson

(1946), and MacEwan (1944, 1946).

The x-ray spectrometer recently developed by the North American

Philips Company, New York, has attracted much attention among clay

mineralogists. This instrument permits the measuring of diffraction

angles and intensity of diffracted x-radiation by means of a Geigerf i l l e r tube. The intensity of the diffracted lines can be determined by

manual counting or intensities can be recorded automatically on a Brown

recorder. Further details on this procedure are given by Jeffries and

Anthony (1948).

Like all other methods for the identification and estimation of the

clay minerals in soil clays, the x-ray diffraction methods have certain

limitations. The characteristic diffraction lines for the clay minerals arise

from the basal spacings along the c axes of the minerals. Diffraction

patterns of the clay minerals in samples of soil clays often show diffuse,

weak, or no basal diffraction lines which arise from spacings along the

c axes of the crystals. These spacings for the montmorillonitic, illitic,

and kaolinitic minerals are 13&, 10, and 7.2A., respectively. On the

other hand, the diffraction lines arising’ from the prismatic spacings in

these clay mineral crystals coincide. These latter lines from soil clay

mixtures are usually well defined. This can result from a mixture of

small percentages of the clay minerals with amorphous or poorly organized sesquioxides and oxides of silicon ; from poor organization or random

interstratification of minerals along the c axes of the crystals; from

crystals which are split into sheets which are too thin to give well defined

basal x-ray interferences; or from any combination of these conditions.

It must, therefore, be concluded that all methods of identification and

estimation of the complex mixture of clay minerals in the soil clays invite

the confirmation of results by other methods.




Methods for the identification and quantitative estimation of the

clay minerals are somewhat inadequate for the complex mixtures which

often occur in the clay fractions of soils. Consequently, there is insufficient data from which broad accurate generalizations can be drawn

concerning the occurrence of the clay minerals. There is also considerable



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

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IV. Qualitative Identification and Quantitative Estimation of the Clay Minerals

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