Some Colloidal Properties of Pleistocene Clays and Their Bearing on

SOME COLLOIDAL PROPERTIES OF PLEISTOCENE CLAYS AND THEIR BEARING ON T H E CHEMICAL THEORY O F T H E FORMATION O F T H E GUMBOTIL BY J. N. PEARCE AND .,I

B. MILLER

In a paper on “The Origin of the Gumbotil,” Kay and Pearcel have presented the geological evidence for and the chemical theory of the formation of the gumbotil from the original glacial drift material. The present paper deals with some of the colloidal properties of the gumbotil and of its related strata. In order to define clearly the object of this work and to bring out the relations involved it will be necessary to discuss briefly both the geological evidence and the chemical theory proposed in the original paper. A study of the glaciated regions of the United States shows that during the Pleistocene age five great glaciers have moved southward over what now constitutes the North Central States. These are known as the Nebraskan, the Kansan, the Illinoian, the Iowan, and the Wisconsin. Since all five of these are exposed to view in Iowa, this state offers an exceedingly favorable place for’ the study of glacier drifts, their orientation, their materials. As each glacier advanced from the North it gathered up and carried southward vast amounts of rock material which varied in nature from huge granite boulders to rock flour, some clays and former soil. This southward movement was halted at those points where the melting of the ice kept pace with the advancing glacier. In time the temperature of the whole region rose, the ice melted, the glacier receded and this rock debris was deposited in the form of broad, deep, and comparatively level plains. Between the recession of one ice-sheet and the on-coming of the second there were long interglacial periods during which Jour. Geology, 28,89 (1920). NOTE:G. I?. Kay, Professor of Geology and Dean of the College of Liberal Arts, The State University of Iowa: Director of the Iowa Geological Survey.

J . N . Pearce and L.B. Miller

2

a soil was formed, a flora developed and a fauna appeared. I n time erosion began and continued with the development of a new topography. The succeeding glacier bearing down upon this surface, razed the hills and filled the valleys, and on melting laid down another deep deposit of rock material. The process of soil formation and the development of plant and animal life began again. An extensive field study of these glaciated regions by Kay* has shown that the three older drifts, viz., Nebraskan, Kansan and Illinoian, are uniformly alike in structure and conformity. These similarities are best observed, in section, in deep recently e made railroad cuts. When erosion has not been too severe and the parts remain in place these drifts show five distinct strata. The top stratum, which lies just below the soil, consists of a loess, a wind-blown material, or of a loess-like clay, possibly related to the drift. The second stratum is that commonly known as the gumbo. This is a “gray to dark colored, thoroughly leached, non-laminated, deoxidized clay, very sticky when wet, very hard and tenacious when dry.” Below the gumbo is a narrow stratum of a yellow to chocolate colored clay, containing a few siliceous pebbles and sand lenses, but leached with respect to calcium and magnesium. This narrow, oxidizedleached stratum is always found just below the gumbo and, except for the color, they appear to be closely related. The fourth stratum is a yellow oxidized-unleached clay. It contains many pebbles and small boulders, and also many calcareous concretions due to infiltration from above. The bottom layer is the unoxidized-unleached, unweathered original drift material. The two yellow clays have, I believe, always been considered a part of the drift. Opinion as to the origin of the gumbo has been varied. I,everett2 favored the view that Geol. S O ~Amer., . 27, 115; Iowa Geological Surv., 26, 215 (1917). U.S.Geol. Surv., Monograph, 38, 28 (1899).

1 Bull. 2

Colloidal Properties of Pleistocem Clays

3

the gumbo is the result of aqueous deposition following the submergence of the region. Bainl presents the view that the gumbo suggests a quiet water deposit which has been compacted or puddled by water. A somewhat similar view is held by Arey.2 Udden3 states that it is probably an old loess which has been clogged by the interstitial deposition of fine ferruginous material through the agency of ground water, or perhaps, a fluviatile deposit made a t a stage of semistagnant drainage. As the result of an extensive field study Kay has been lead to the conviction that this so-called gumbo is in reality a part of the original drift,-that it is the residuum left after the complete weathering and leaching of the drift materials. In order therefore that the name might the more explicitly suggest both its origin and its relation to the drift, he has preferred to call it the “ G ~ r n b o t i l . ” ~ This new theory regarding the formation of the gumbotil involves the assumption that, following the laying down of each original drift a very long period elapsed during which chemical weathering and leaching took place; that the upper layers are the residua left by these processes. After these processes had continued for a long time diastrophic movements occurred, erosion began and then continued until the general topography existing a t the ends of the interglacial periods had been developed. Field Evidence Supporting t h e Theory I n any cut showing all of the five strata in place these points may be noted. The lines of demarkation between any two strata do not conform to the shape of the hill, but run directly and almost horizontally to the edge of the hill upon both sides. Considering any two adjacent cuts, it will be observed that the lines of demarkation are symmetrically placed, the Iowa Geol. Surv., 18,292 (1907). Ibid., 20, 611 (1909). Ibid., 11, 258 (1900). Science, 44, 637 (1916).

4

J . N.Pearce and L. B . Miller

continuity being broken only by the valley between. Except at the boundary between the oxidized-leached and the oxidizedunleached strata, the surfaces of separation are not as distinct as they would be, if one stratum were laid down directly upon another. They dove-tail, so to speak, into each other, and in these dove-tailings we may follow the processes of weathering and oxidation. The variation in the size of the pebbles offers strong evidence in support of the theory. The pebbles found are quartz, chert, flint, quartzite, granite, basalt and greenstone, feldspar, sandstone, etc., the proportions of each in any pebble count decreasing in the order named. This, however, is practically the reverse of the order of their hardness and solubility. I n the upper loess-like clay the pebbles are exceedingly rare and very minute. Passing downward through the gumbotil and the oxidized-leached strata the pebbles gradually increase not only in frequency, but also in size. In the oxidizedleached stratum we frequently find the disintegrated remains of huge granite boulders. They consist chiefly of fine sand grains; they are the residue left after all of the soluble weathered material has been leached away. . In the oxidized-unleached stratum and in the original drift material they are often massive in size, but they still retain their original form and structure. The Role of Water in Geo-Chemical Changes The dominant factor in all the geo-chemical changes here involved is water,-more especially the aerated rainwater. When rain falls upon the ground one part, the “runoff,”l flows over the surface and escapes. This is the cause of erosion. A second part, the “fly-off,” immediately evaporates into the air. A third part, the “cut-off,” enters the soil and, moving downward with considerable speed, carries with it the dissolved mineral matter. Of these the cut-off water is the only form which is directly effective in soil formation and in the theory of the formation of the gumbotil. Cameron; Jour. Phys. Chem., 14,340 (1910).

Colloidal Properties of Pleistocene Clay

5

When a relatively small quantity of water is added to a dry solid or soil the water spreads over the surface in the form of a film. With addition of water the apparent volume of the solid increases until a maximum is reached. Still further additions of water will not increase the thickness of this water film, but will produce free-water in the soil interstices, and in some cases this often leads to puddling. The film-water is held tenaciously by the soil particles. I n dry seasons it is practically a saturated solution of the dissolved rock and soil materials. When the surface becomes flooded, as in wet seasons, or during heavy rains, the freeor cut-off water in its downward movement extracts and carries away a part of the mineral content of this film-water solution. Once the free water is removed, the saturation of the film repeats itself. In this water are the dissolved rock materials, the carbon dioxide, oxygen, nitrogen, humus material and the soil bacteria. Between these are evolved all the processes leading to the disintegration of the rocks and the formation of the soil and its sub-strata. The rock materials transported by the glaciers were of igneous origin, viz., feldspars, amphiboles, pyroxenes, micas, quartz, some clay and other previously weathered materials. These are silicates, salts of a very weak acid,-silicic acid, with various base-forming elements, the alkalies, calcium, magnesium, iron and aluminum. Like all salts of strong bases and weak acids, these silicates when dissolved hydrolyze to form the free ionized bases,-the hydroxides of sodium, potassium, calcium and magnesium, some iron and either the free un-ionized silicic acid, or some simpler silicates. The latter will in turn continue to hydrolyze and there will result still other products, and in the end simpler silicates, perhaps kaolin, or even silica as sand or quartz. If the reaction products are not removed by leaching, a state of saturated solution equilibrium is attained and the solution process ceases. Under these conditions the decomposition products from different sources may react with each other to form more or less complex silicates of a secondary

6

J . N.Pearce and L. B. Miller

origin. Let the saturated solution be removed and fresh water added, the various solution equilibria are disturbed and the solution processes begin again. In the soil solution thus formed, the dissolved materials will a t the proper concentrations react with the carbon dioxide of the soil atmosphere to form the soluble, highly ionized, carbonates of sodium and potassium, and the soluble acid carbonates, or the insoluble carbonates of calcium and magnesium. The insoluble carbonates crystallize out as the calcareous concretions so frequently found in the clay subsoils. In the presence of a sufficient excess of carbon dioxide the insoluble carbonates pass into the form of the soluble acid-carbonates and are leached away by the downward moving free-water. Ferrous silicates upon hydrolysis give ferrous ions. These react with the ions of the soil solution to form the relatively slightly soluble ferrous hydroxide or carbonate, or, as is more likely the case, they may be oxidized immediately to ferric ions and then precipitated as the insoluble ferric hydroxide. The aluminum remains for the most part in the form of silicates. The soil solution contains other elements and compounds, but their proportions are relatively slight and subject to considerable variation. The consideration of these is not needed in this paper. In summarizing, we may divide the mineral matter of the soil solution into two classes. The more easily diffusible are the soluble alkali bases and salts, the soluble acid-carbonates of calcium and magnesium, the slightly soluble ferrous compounds and the semi-colloidal sodium and potassium silicates. The less easily diffusible are the colloidal forms of gelatinous silicic acid, the hydrated silicates and the hydrated ferric oxide. According to Julienl the solvent action of the alkaline soil solution and of the humic acids, aided by the abrasive effects . of the earth’s displacements, slowly but surely transforms the quartz pebbles into colloidal silica. Under the influence of Proc. Am. Assoc. Adv. Sci., 1879, 311.

Colloidal Properties of Pleistocene Clays

7

the decaying organic matter the ferric compounds are reduced, temporarily at least, to ferrous compounds. While the existence of ferrous compounds in contact with the soil atmosphere must obviously be a short one, the alternate oxidation and reduction permits the slow downward transportation of the iron. The decomposition of the aluminum silicates leads ultimately to the formation of the colloidal hydrated aluminum silicates. These are the most complex, most resistant, the least soluble and the least mobile of all of the decomposition products produced by the disintegration of the silicate rocks. It is obvious, therefore, that the aluminum compounds will not be subject to appreciable removal by capillary flow. Hence, in the leaching of the weathered products of the original glacier drift material we should expect to find a gradual increase in the proportion of the soluble diffusible materials as we pass downward from the surface. On the contrary, conditions permitting, we should expect to observe a gradual decrease in the proportion of alumina. These relations are exactly what we do find from the results of a series of chemical analyses of a series of strata in any single complete cut.

Chemical Analyses of the Clays The samples of clay used for analysis were taken from deep and recently made railroad cuts. Each sample was taken from the middle of the stratum and from the bottom of deep holes dug horizontally into the side of the cut. This latter precaution was taken in order to avoid as far as possible any contamination due to surface water. The air-dried samples were carefully powdered, without grinding, and sifted through a clean dry 20-mesh copper sieve. Since we are chiefly concerned with the weathered material only, the arbitrary choice of a mesh of this size gives us a uniform standard for the uniform fineness of the glacier flour for all clays. The sifted material was then ground in an agate mortar until every trace of it passed through a 100-mesh sieve. It

8

J . N . Pearce and L. B. Miller

was then thoroughly mixed on a mixing cloth and preserved in glass-stoppered bottles. All of the analyses were made exactly as prescribed by Hillebrand in his classic work on “Carbonate and Silicate Analysis.” Since we are concerned only with the less mobile constituents no attempt was made to make a complete analysis of any stratum. The results are given in two forms: (a) the percentage composition with respect to the less mobile constituents, ( b ) the parts by weight of these constituents per 100 parts of the more resistant Al2O3. Only the analyses of the samples taken from the localities a t “A,” “B,” “C” and “D” will be reported in this paper. They represent the three older drifts and are typical of the many analyses made. The three localities “A,” “B” and “D” show the gumbotil underlying the loess or loess-like clay and covered by a thin layer of soil. At “C” the Nebraskan gumbotil lies below the soil with no intervening loess or loess-like clay. The loess or loess-like clay are of great interest but are not being considered except incidentally in this paper. I n places these materials are clearly eolian in origin. I n other places the loess-like clay may be closely related in origin to the gumbotil. The presence of loess or loess-like clay above the gumbotil might be expected to have some slight effect upon the present chemical composition of the gumbotil, and in fact may explain the few percentages in the analyses which might seem to contradict the theory proposed. It is assumed that the composition of the flour of the unoxidized and unleached till is now the same as when laid down by the glacier. The possibility exists, however, that it may have received a small amount of leached material from above, or that it may have lost to the strata above by capillary flow slight quantities of the more easily diffusible dissolved materials. It is not to be expected that its composition will be similar t o that of the overlying materials which have been subjected to marked chemical changes, to leaching, or to infiltration.


9

TABLE I A. Chemical Analyses of Samples of Kansan Drift Materials taken in Monroe County, Near Foster, Iowa ( a ) Percentage composition

Gumbotil(Kansan) Glacial till, oxidized-leached

I

1

SiOz

I

72.03

1

73.11

Fez03

I

I

Gumbotil (Kansan) Glacial till, oxidized-leached

4.18 4.62

1

A1203

I

12.27

I

11.57

I

I

CaO

I

MgO

1.33

I

2.29

1.06

I

2.56

( b ) Parts per 100 parts A1203

587.0 631.9

1

34.10

100.0

39.92

100.0

I

I

10.84

I

14.35

I 22.18

18.68

TABLE I1 B. Chemical Analyses of Samples of Kansan Drift Materials taken in Clarke County, near Murray, Iowa ( a ) Percentage composition Si02

Gumbotil (Kansan) Glacial till, oxidized-leached Glacial till, oxidized-unleached

70.46

71.M

GS.56

I

Fez03

I

4.17

I

1 I (a)

Gumbotil (Kansan) Glacial till, oxidized-leached Glacial till, oxidized-unleached

1

AbOs

12.04

4.62

10.86

4.40

11.13

I

I ~

CaO

1.21

1

1

MgO

0.55

1.29

0.72

4.48

0.79

Parts per 100 parts

At103

585.2

34.7

100.0

10.05

4.57

661.5

42.5

100.0

11.87

6.66

616.0

39.5

100.0

40.30

7.16

10


TABLE 111 C. Chemical Analyses of Samples of Nebraskan Drift Materials taken in Carroll County, Near Manning, Iowa (a) Percentage composition

I

Si02

Gumbotil(Nebraskan: 71.59 Glacial till, oxidized-leached 66.85 Glacial till, oxidized-unleached 66.52

Fez03

I

1

A1203

I

CaO

MgO

4.35

12.29

1.26

0.93

5.92

11.65

3.67'

0.78

4.80

11.18

4.25

1.43

( b ) Parts per 100 parts A1203

Gumbotil (Nebraskan; Glacial till, oxidized-leached Glacial till, oxidized-unleached

:li: 1 1 50.7

100.0

42.9

100.0 138.30

31.51

I

6.71 12.81

TABLE Iv D. Chemical Analyses of Samples of Illinois Drift Materials taken in Lee County, near Fort Madison, Iowa (a) Percentage composition

1

SiO',

Gumbotil (Illinoian) Glacial till, oxidized-leached Glacial till, oxidized-unleached

71.07 72.24 72.30 ,

Si02

Gumbotil (Illinoian) Glacial till, oxidized-leached Glacial till, oxidized-unleached

Fez03

1

4'24 7.43

3.47

1

I1

A1203

I

CaO

0.79

I

1

MgO

0.85

1

14.91 ll.G5 8.59

1

0.61 4.13

I

1

0.95 1.28

( b ) Parts per 100 parts AlzOs

I

Fez08

A1203

CaO

MgO

476.0

28.44

100.0

5.34

5.68

620.1

63.80

100.0

5.26

8.22

841.2

40.39

100.0

48.10

14.95


11

A .study of Tables I to IV will bring out several interesting facts. In all of the series here represented the percentage of A1203 decreases downward from the gumbotil through the oxidized and leached stratum. Except in the case of “B” (Table II), this decrease continues also through the oxidized and unleached stratum. TABLE V Comparative Analyses of Strata from Various Localities and Drifts Locality

Gumbotil Glacial till, oxidized-leached Glacial till, oxidized-unleached

72.03 73.11

1 1 1 1 1 i::; I 1 ;?:E 1 70.46 71.84 68.56

.....

71.59 66.85 66.52.

71.07 72.24 72.30

Fez03


4.18 4.62

4.17

4.35

4:;

....

4.24 7.43 3.47

A1208

Gumbotil Glacial till, oxidized-leached ,Glacial till, oxidized-unleached

12.27 11.57


1.33 1.66


2.29 2.56 ,...

_ ,

.....

... .

’

12.04 10.56 11.13

11.18

1 1 I 1.21 1.29 4.48

1.26 3.67 4.28

1 I 1 0.55 0.72 0.79

0.93 0.78 1.43

14.91 11.65 8.59

0.79 0.61 4.13

0.85 0.96 1.28

Perhaps the most important evidence in favor of the leaching theory is to be gained from a study of the relative proportions of CaO and MgO in the various horizons. In

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J . N . Pearce and L. B . Miller

practically every series the proportions of these two constituents show a pronounced increase downward. Apparent contradictions for both might be considered for MgO in Table I11 and for CaO in Table IV. Field relations will show in these instances either that the gumbotil is overlaid by material containing a higher proportion of these constituents, or that erosion began before the leaching process in the gumbotil was completed. Assuming that the loess or the loess-like clay is a subsequent formation, it also will have been leached of some of its CaO and MgO. This means a slight increase in the proportions of these two in the stratum below, the leaching of which has not been completed. The silica and the iron are less diffusible and hence less subject to leaching than are the carbonates of calcium and magnesium, and a much longer time is required for complete leaching. In every instance the proportion of iron in the gumbotil is less than it is in the oxidized and leached stratum just below. Except for the case of the Nebraskan (Table III),the proportion of Si02is greater in the oxidized and leached stratum than in the gumbotil. It should be observed that the Nebraskan a t the locality “C” underlies the Kansan, and the apparent discrepancy may be accounted for in the transfusion of the alkaline silicates from the Kansan into the upper strata of the Nebraskan below. On the basis of parts per 100 parts of A1203not only CaO and MgO but also Si02 show distinct evidence of leaching even in the oxidized and leached stratum, and this is true for practically every series. On the same basis the evidence points to a leaching of the iron into the oxidized and leached stratum from gumbotil, but the time allowed was not sufficiently long for the subsequent leaching of the iron from the oxidized and leached stratum into the one below.

The Leaohing Process We are now in a position to picture the weathering and leaching processes as they occurred long ago. The oxygenated and carbonated water falls upon a uniformly level,


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more or less uniformly constituted, blue to blue-black drift. Percolating downward it dissolves a portion of the rock material. Hydrolysis follows, and there are liberated successively the hydroxides of sodium or potassium, then of calcium or magnesium, and finally the more or less difficultly soluble hydroxides of ferrous and ferric iron, depending on the nature of the iron in the original silicate. The ferrous iron throughout the depth penetrated by the dissolved oxygen is immediately oxidized and the deposit assumes the typical iron color of the yellow clays. The calcium and magnesium hydroxides combine with the carbon dioxide of the soil atmosphere to form the insoluble carbonates which crystallize out as calcareous concretions. The soluble alkalies and their salts and the alkaline silicates are carried downward by the moving free water. The negative colloidal silicates and the silicic acid are coagulated and rendered motionless by the electrolytes of the soil solution. As hydrolysis proceeds the mass of the insoluble material thus formed increases and probably does continue to increase until all of the easily available, hydrolyzable materials are used up, or removed. Obviously those insoluble materials which are most easily attacked will be the first to be leached away. These are the carbonates of calcium and magnesium. Although only very slightly soluble, the dissolved portions of these combine with the carbonic acid of the soil solution to form the soluble acidcarbonates. These are carried downward to lower levels where in fissures and crevices they again crystallize as irregular concretions. In this way were formed all of those concretions which are found in the oxidized stratum. According to the Law of Mass Action, the activity, or the solvent effect of the carbon dioxide will be greatest a t those points where its concentration is a maximum. This obviously will be a t the upper level of the initially unleached calcareous zone. Owing to its diffusion power some of the carbon dioxide may escape combination at the upper level only to combine at a slightly lower level. Ultimately there will

14

J . N . Pearce and L. B. Millcr

be a lower limit beyond which the carbon dioxide entering from the atmosphere will not penetrate, or its concentration in the soil solution will be too slight to produce any appreciable chemical effect. These limits of maximum and minimum activity represent the boundaries of the dynamic zone of carbonic acid activity-the oxidized and leached stratum. As time goes on the concentrations at the upper level disappear, and the levels of maximum and minimum activity move downward simultaneously. This dynamic zone has played an important r61e in all drift transformations. It spreads horizontally like a thin sheet of more or less uniform thickness. It is found always directly upon the oxidized-unleached stratum and always directly below the gumbotil. In the Nebraskan drift it is thin, less than two feet to somewhat more than four feet. The oxidized-leached stratum of the Kansan drift averages about five feet, attaining in a few places a thickness of seven feet. That of the Illinoian has an average thickness not to exceed six feet. In the Iowan it has reached a maximum of three and one-half feet, yet in those areas where the Iowan has been exposed it has not yet left the surface. The Wisconsin drift shows no leached stratum at any point. The original granite boulders of this glacial drift lie directly upon the oxidized-unleached material. Our assumption is that the time which has elapsed since the laying down of the Wisconsin has not been sufficiently long to permit these changes. The areas of Iowa covered by the Wisconsin are so flat and level that even the drainage channels already formed are insufficient to carry away the excess surface water. There is, therefore, a relatively large excess of cut-off water: the existing conditions favor leaching. Hence, we may safely assume that if the present existing conditions continue sufficiently long, this drift also will show not only the oxidized-leached stratum, but also, possibly, the gumbotil lying on top of it. This downward movement of the oxidized-leached zone did not cease until after disatrophic movements had occurred. Then erosion began and, instead of percolating downward, the


15

falling aerated rain-water escaped over the surface. After the leaching of the calcium and magnesium-perhaps simultaneously-there follows a second step. When the concentrations of the precipitating ions have been leached below their critical coagulating values new processes occur within the leaching zone. The coagulated iron passes into solution either as colloidal ferric hydroxide by peptization or deflocculation by the emulsoidal humus material, or as ferrous compounds through reduction by organic matter. Thus either by colloidal flow, by alternate reduction and oxidation, or through the medium of its slightly soluble salts, the iron is leached and slowly passes downward. The silica either in the form of the colloidal silicic acid or as the soluble alkaline silicates also moves downward. Likewise, through various peptizing influences the colloidal clays begin to swell and deflocculate. Ultimately some of these pass into suspension of colloidal particles; they are also caught in the downward current and carried by it to lower levels where they are again coagulated. Only a slight amount of this material is leached away.l The properties of the gumbotils are largely those which one might predict from a knowledge of the colloidal chemistry of clays. The characteristic color changes of the gumbotil are those imparted to it by the colloidal clays, perhaps by the kaolin contained in it, the color of the colloidal material being sufficiently strong to mask the reddish yellow color of any oxidized iron which may be present. This doubtless is responsible for the belief held by some that the iron in the gumbotil is deoxidized or reduced, a condition which could hardly be possible in the presence of the oxygenated soil solution. The chemical analyses of the gumbotils from different drifts and localities show, with respect to certain constituents, a striking similarity. This is especially true for the iron and the silica and, as we might expect, for the calcium. Slight Note: The writer is well aware of the extremely slow diffusion speeds of colloids. To transport appreciable quantities of such material would require long periods of time. These we have when we count time by geological periods.

16


fluctuations may be expected due to differences in the original rock materials, to the amount of rain-fall, or to leaching from above. It may be concluded, therefore, that all gumbotils have a common origin, viz., “the chemical modification by weathering of the glacial till.” Furthermore, the chemical analyses, as arranged in Table V, show a slightly less striking similarity between the gumbotil and the yellow oxidized and leached clay just below. Naturally one should expect to find a slightly greater concentration of the diffusible material in the leached zone. One should expect also to find a slight variation in the proportion of any one constituent between the top and the bottom of any single zone. Each level in any single stratum is still slightly unleached with respect to another level close to and above it. The proportions of most of the constituents present in the oxidized-leached and gumbotil strata differ in most cases by only a few tenths of one percent. When greater deviations than this occur it can be shown by field observations that one or more of the upper strata have been removed before the leaching process was completed. The distinguishing features between these two strata are, therefore, due primarily to differences in the physical properties, and these properties are chiefly the colloidal properties of the clay itself. It is possible that two forms of the same material are here being dealt with, namely, the gumbotil, a highly colloidalized form, and the oxidized-leached material, the non-colloidalized form, which in the presence of certain electrolytes is incapable of assuming certain colloidal properties.

Some Colloidal Properties of These Glacial Clays In what has preceded we have attempted to give a chemical explanation of the origin of the gumbotil. The theory proposed has been substantiated by the results of chemical analyses, According to this theory we should expect to find a more or less pronounced increase in the colloidality of the clay of any drift sheet as we pass upward from the lower strata.


17

To test this point Mr. Miller has made a rather extensive study of certain colloidal properties. Since the properties of colloids are primarily surface properties any process which would determine the extent of surface development may be considered a measure of the colloidality of the material. Kepplerl has found that the hygroscopicity of clays varies in the same order as their plasticity. Ashley2states that when clays are classified according to their colloidal content they are also classified according to their plasticity. He has also studied the adsorptive power of clays for certain dyes and he has found that the adsorption of dyes by clays furnishes a means of measuring the relative specific surface of the clays. We have used the measurements of hygroscopicity and adsorptive power for dyes as a means of determining the relative surface development of the various strata of Pleistocene clays produced by the weathering and leaching process. The clay samples used throughout this work were from the same original samples as those used for the analyses above. Unfortunately some of the original samples were entirely used up in the previous work, thus leaving a lack of continuity in the data from some localities. It was thought better however to work with the incomplete set of samples rather than to obtain fresh samples of the missing strata upon which no chemical analyses had been made. Preparation of the Clay Sampies I n order to reduce the clays to a condition of uniform fineness and homogeneity about one pound of each clay was stirred into a large volume of water; the lumps were broken up by gently rubbing between the fingers and the whole mass was then washed through a 200-mesh sieve. For all clays used only a very small amount of each sample failed to pass through the sieve during this treatment. An excess of 1 N hydrochloric acid was then added to destroy any carbonates Sprechsaal, 46, 445 (1912). Bull. U. S. Geol. Survey, No. 388.

18

J , N . Pearce and L.B. Miller

which might be present. After stirring vigorously for some time the clay suspension was allowed to stand for twenty-four hours, during which sedimentation became complete. The supernatant liquid was then removed as completely as possible by means of a siphon filter, more water was added, the clay was vigorously stirred and then allowed to stand for another twenty-four hours, when the liquid was siphoned as before. This procedure was continued until the filtrate gave only a very slight opalescence when tested for chlorides. At this point the spontaneous dispersion of the clay made it impossible to wash further without the loss of some suspended colloidal material. The clay was then filtered with the aid of suction through a hardened filter, the small portions of the material which passed through in the first filtrates being refiltered through the mass until the filtrate became clear, The clay was then sucked as dry as possible and the drying completed in an electric oven a t 110". The dried material was carefully reduced to a fine powder without rubbing and then carefully mixed on a mixing cloth to insure homogeneity.

Hygroscopicity Measurements As here used hygroscopicity is a measure of the power of the clay to absorb water from the atmosphere. Three fivegram samples of each clay were accurately weighed into tared glass-stoppered weighing bottles and heated a t 110O unti1 successive weighings of each sample checked to =t0.0015 gram. This corresponds. to an error of about *0.03 percent on the basis of the weight of clay taken, a closer check being impossible since the hygroscopic tendency of the clays varied greatly from day to day with changes in the humidity of the atmosphere. The weighed samples were then placed in the vapor from a large volume of 10 percent sulphuric acid contained in a large sealed jar immersed in a large Freas water-thermostat and kept constantly at 25 =t0.005. The dilute sulphuric acid was used since its lower vapor pressure prevents the formation of droplets on the inner walls of the vessels containing the clay. The sample was then allowed to take up water vaO


19

por until the change in the weight of the sample between successive readings taken on every second day became negligibly small. The results of the hygroscopic determinations are given in Table VI. In the first column are the various drift strata arranged in the order of their occurrence in the drift. The remaining columns represent the hygroscopic water for the various samples taken from localitied A, B, C, etc., each expressed in percentage by weight of the dried clay.

TABLE VI The Hygroscopicity of Certain Pleistocene Clays C

D

.....

11.6

10.63 9.05 6.63

S.8

13.05 11.10 9.86

Localitv.

Gumbotil Oxidized-leached Oxidized-unleached Unoxidized-unleached

11.7 7.6

.... ....

....

....

.....

E 12.33

... ..... .....

The blank spaces occurring a t the bottom of the vertical columns indicate that for these localities these strata were not exposed. When the blank space occurs a t the top, as in B, that stratum has been entirely eroded away. The remaining samples in E were entirely used in the former research. A survey of the data obtained for any locality shows a gradual decrease in the hygroscopicity of the clays as we pass from upper to lower strata. This would indicate for the gumbotil, as predicted, a greater surface development, i. e., a greater specific surface. A, B and C represent three widely separated localities in the Kansan. The gumbotils a t A and C show nearly identical hygroscopic powers. These values are slightly less than the corresponding values for the Nebraskan (D) and the Illinoian gumbotil (E). The high hygroscopicity of the oxidized-leached stratum (B) is to be accounted for by the fact that the upper stratum has been removed and colloidalization has proceeded even after erosion has taken place. The samples of the oxidized-unleached material from different localities exhibit very similar hygroscopic properties.


20

Locality


A -

73.9 36.3

. .. . . . ..

B

C

D

....

65.0

68.9 63.2 51.5

51.2

79.3 64.4 44.1

....

....

....

E ~ 70.5

.... .... ....


A

Locality

B

C

....

53.4

58.3 42.2 44.9

54.4

’

21

D

~~~~~


1

62.2 46.5

.... ....

.... . ...

“Principle and Practice of Agricultural Analysis,” p. 331.

66.3 53.3 34.2

....

‘E 58.2

....

....

....

22

J . N . Pearce aizd L. B. Miller

in a twenty-five gram sample of the clay. This would indicate that in these strata the humus material has been completely oxidized by the soil atmosphere.

The Adsorption of Dyes by Glacial Clays On the basis of the leaching theory for the formation of the gumbotil the colloidality of the clays should decrease from the surface downward. These clays in their colloidal form are negatively charged. They should, therefore, show a pronounced adsorption power for positive dyes. On the other hand, we should expect but slight adsorption for negative dyes. To test this point we have studied the adsorption of the positive dyes, methylene blue and methyl violet, and of the negative dye, eosin. These were carefully purified by several crystallizations from the proper solvents. Various solutions of the dyes of the desired concentration were carefully prepared in accurately calibrated flasks a t 25 '. Two exactly one-gram samples of each clay were weighed into separate 100-cc glass-stoppered bottles. To each pair of samples were added exactly 100 cc of one of the dye solutions. The bottles were then sealed with paraffin, placed in a shaker immersed in a large water thermostat and rotated for twentyfour hours a t 25". The bottles were then taken from the shaking apparatus and allowed to stand in the bath until the clay had completely settled. Accurately measured volumes of the supernatant dye solution were withdrawn from each bottle, accurately diluted to a convenient color intensity and the amount of dye in one cc determined colorimetrically by means of a Dubosc colorimeter. From the data obtained the amount of dye adsorbed by one gram of the clay is easily determined. In order to obtain an idea of the relative adsorption powers of the different clays in any one drift sheet a duplicate series of adsorption experiments were made involving all of the various samples a t our disposal. To each gram of clay we added 100 cc of a solution containing exactly one gram of dye per liter. The mean weights of dye adsorbed by one


Gumbotil Oxidizedleached Oxidizedunleached Unoxidizedunleached

Locality-

Gumbotil Oxidizedleached Oxidizedun1eached Unoxidizedunleached

C 0.09945

0.09994

.......

0.09991

.......

. . . . . . . 0.06684 0.007G1 0.05717

.......

0,09989

.......

0.04399

0.08672

. . . . . . . 0.06900

I

I

E 0.09597

A

Locality

23

D -__

.....................

0.09957

. . . . . . . 0.09937 0.09995

E 0.09673

0.05643

0.09094

.......

0.099G5

.......

. . . . . . . 0.09275 0.09857

0.08080

.......

A

B

. . . . . . . 0.07926

C

D

.....................

a

24

J . N . Pearce and L.B. Miller

in the gumbotils from different drifts or from different localities in the same drift. According to Kay and Pearce (loc. cit.), however, the gumbotils, being the residuum from the weathering and leaching of more or less similar igneous drift material, should be similar from whatever drifts or localities they are obtained. The data herein exhibited support the theory proposed. Physical Chemistry Laboratory The State University of Iowa

Some Colloidal Properties of Pleistocene Clays and Their Bearing on

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