26. MIXED-LAYER ILLITE/MONTMORILLONITE CLAYS FROM SITES 146 AND 149

Herman E. Roberson, State University of New York, Binghamton, New York

INTRODUCTION

The purpose of this report is to describe the clay mineralcomposition of twenty-four samples from Sites 146 and149, Leg 15, DSDP. As the predominant clay mineral inthese samples is a mixed-layer illite/montmorillonite, aconsiderable part of the report is devoted to discussing andapplying recently proposed methods for characterizingthese types of clay minerals. It is hoped that in futureinvestigations of clay minerals from cores of deep-seasediments, attempts will be made to describe mixed-layerclays whenever they are present, as it is believed they mayyield clues concerning the origin and diagenetic history ofthe sediment.

PROCEDURE

In order to separate the clay mineral fraction for X-raydiffraction studies, samples were prepared as follows:

1) Washed free of salts by centrifugation;2) Treated with an ultra-sonic probe for 10 minutes to

disaggregate completely;3) <2µ fraction separated by centrifugation; and4) Oriented clay aggregates prepared by allowing clay-

water suspensions to dry at room temperature on glassslides.

X-ray diffraction data were obtained with a Philipsdiffractometer. Filtered Cu K α radiation (40 kv, 20 ma)was used; scanning speed was 2 degrees per minute. Eachsample was scanned in the air-dry state, after glycolsolvation by the vapor technique and after heating to550°C.

RESULTS

Figures 1 and 2 show diffractometer tracings or glycol-solvated oriented clay aggregates from samples taken fromSites 149 and 146. These representative patterns illustratethe most obvious change in clay mineral composition:kaolinite and illite decrease in abundance with depth.Figure 3 shows a plot of the ratio of the 10Å peak height(001 illite) to the 17Å peak height (001 expandable clay)against core depth in meters. There is a very abruptdecrease in the amount of illite with respect to montmoril-lonite at a depth of about 90 meters from the sedimentsurface. The kaolinite/illite ratio stays rather constant in allthe samples studied down to a depth of about 400 meters.Thus, kaolinite sharply decreases in abundance relative tomontmorillionite at about 90 meters also.

There has been no attempt to make precise quantitativedeterminations of the amounts of the individual clayminerals for each of the samples because of the numerousproblems involved, especially when mixed-layer clays arepresent, as is the case here. Quantitative estimates of claymineral abundances based on a scheme similar to that

Oriented Clay Aggregate-Ethylene Glycol

72Å IO.OÅ I70Å

149-2

149-5

149-19

149-22

149-36

30 25 20 15 10 5 3Degrees 2 θ

Figure 1. X-ray diffraction patterns of clays from Site 149.

proposed by Johns, Grim and Bradley (1954) were made.Samples from the uppermost 170 meters of Site 149contain on the average 40 to 60 per cent mixed-layerillite/montmorillonite, 20 to 30 per cent illite, 15 to 25 percent kaolinite, and 5 to 10 per cent chlorite. Samples belowabout 170 meters in Sites 149 and 146 contain 80 to 90 percent mixed-layer illite/montmorillonite, approximatelyequal amounts of kaolinite and illite, and sometimes, smallquantities of chlorite. The presence of chlorite was deter-mined after heating to 550°C. Figures 4 and 5 show that

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H. E. ROBERSON

Oriented Clay Aggregate-Ethylene Glycol

IO.OÅ72Å I7.0ÅJ

146-1-3

146-12

146-15

30 25 20 15 10 5 3Degrees 2 θ

Figure 2. X-ray diffraction patterns of clays from Site 146.

small peaks at 14Å and 7Å still persist, indicating thepresence of chlorite. The decrease or apparent loss ofintensity of the 7Å peak after heating indicates thatkaolinite, which is destroyed at 55O°C, is generally muchmore abundant than chlorite in these samples. The 10Åpeak after heating to 55O°C shows that the expandablelayers in the mixed-layer ülite/montmorillonite are fullycollapsed. That none of the 17Å expandable phase in anyof the samples collapsed to spacings below 10Å on heatingindicates the presence of a mixed-layer clay mineral ratherthan a pure montmorillonite phase which would collapse toa sharp 9.4Å spacing on heating.

The rather broad and diffuse diffraction maximum atapproximately 17Å for many of the glycolated samplesstrongly suggested the presence of a mixed-layer phase.Heating to 55O°C showed there were few or no interstrati-fied chlorite layers in the mixed-layer clay. Thus, themixed-layer clay appeared to be a simple random inter-stratification of illite and montmorillonite layers. Mixed-layer illite/montmorillonite is a common constituent ofpelitic sediments and sedimentary rocks (Weaver, 1956;Perry and Hower, 1970), and a method recently proposedby Reynolds and Hower (1970) can be used to characterize

these kinds of minerals. However, in order to apply theirmethods, it is necessary to obtain several orders of the basalreflections. This presented a problem because the <2µfractions from these samples did not give enough basalreflections. Very fine fractions (<0.05µ equiv. sphericaldiam.) were collected from some of these samples in orderto obtain better preferred orientation of the thinner (moreplate-like) clay particles. Size fractionation has been shownto be an effective means of attaining excellent preferredorientation of montmorillonite particles, which then afforda much higher number of basal diffraction maxima than isnormally the case for particles settled from <2µ sizefractions (Roberson, Weir, and Woods, 1968). The qualityof the X-ray data is also improved because the fine fractionscontain considerably smaller quantities of non-clay mineralsand coarser clay particles, such as kaolinite and illite, thando the <2µ fractions.

In most cases, oriented clay aggregates from the veryfine fractions gave X-ray diffraction patterns with severalbasal orders. Figures 6 and 7 show differences betweenX-ray diffraction patterns of <2µ fractions and thoseobtained from the very fine fractions. It can be seen thatthe relative intensities of all basal reflections are muchincreased for the clay aggregates from the finer sizefractions. The broad diffraction maximum at 16.3 to 16.5degrees (5.4Å) which precedes the 5.0Å maximum (002 fordiscrete illite) in samples 149-14 and 149-16 shows that themixed-layer material has about 50 to 60 per cent expand-ability. That is, 50 to 60 per cent of the layers in themixed-layer crystallites are fully expanded to 17A withethylene glycol; the other 40 to 50 per cent do not expandwith ethylene glycol and have a 10Å periodicity. Table 1shows peak positions (Cu K^) and d-values for somereflections of illite- (glycol) montmorillonite. The broaddiffraction maximum, referred to above, represents acombination of the 002 reflection for illite and the 003reflection for the interlayered glycol-montmorillonite. Thed-spacings and relative intensities of all the basal reflectionsfrom these two samples are in very good agreement withthe values calculated by Reynolds and Hower for mixed-layer illite-montmorillonite with 50 to 60 per cent expand-ability shown in Perry and Hower's study. The d-spacing ofthe 001 peak in both these samples is slightly lower thanthe 17Å d-spacing of Reynolds and Hower's calculated 001diffraction profiles of mixed-layer illite/montmorillonite.This lower d-spacing may indicate the presence of a smallnumber of interstratified chlorite layers or possiblyvermiculite-like layers.

Only a few of the samples were fractionated because thisis such a time-consuming operation, and also because manyof the samples were still partially flocculated even afterthorough washing and addition of deflocculating agents.However, the broad and diffuse shape of the 001 peaks inthe diffraction patterns of the samples taken from theuppermost 170 meters in Site 149 is a strong indicationthat the clays are mixed-layered with an expandabilityprobably in the 50 to 60 per cent range. About 150 to 200meters below the sediment surface at Site 149, themixed-layer illite/montmorillonite becomes about 80 percent expandable. Except for one or two exceptions (e.g.149-39 shown in Figure 8) the mixed-layer clays appear to

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MIXED LAYER ILLITE/MONTMORILLONITE CLAYS, SITES 146 AND 149

IOÅ peak height

O.I 0.2

I7Å peak height

0.3 0.4 0.5 0.7

100-

200-

Depth(meters)

300-

400-

500Figure 3. 10A (001 Illite)jl 7A (001 Expandable Clay) versus depth.

show about the same degree of expandability to the bottomof the hole.

Mixed-layer illite/montmorillonite makes up 90 per centof the clay fraction from all the samples studied from Site146 (with the exception of 146-1-3 which is at a depth of106 meters). The clay fraction of seven samples from Site146 between 476 meters and 575 meters below thesediment surface is composed of illite/montmorillonite,which has a uniform expandability of about 80 per cent.Figure 9 shows a diffraction pattern of a typical mixed-layer illite/montmorillonite of 80 per cent expandability.

uppermost 100 meters of sediment (Early to Mid-Plioceneage and younger) indicates that terrigenous sedimentationwas a much more dominant feature than during the earlierpart of the Tertiary. There is a rough parallelism in timebetween this change and the change in the character of themixed-layer illite/montmorillonite. The mixed-layer illite/montmorillonite in the uppermost 170 meters has only 50to 60 per cent expandability. The mixed-layer illite/montmorillonite in samples below 170 meters in Sites 149and 146 has a uniform expandability of about 80 per cent.Perhaps this suggests that some or all of the clays of lower

DISCUSSION

Whether the pattern of changes in clay mineral composi-tion with depth for these cores is representative of, orsimilar to, the clay mineral distribution for other cores inthe Caribbean is still not known. Studies of clays fromother cores from the Caribbean as well as further studies ofclays from DSDP cores in adjoining areas, must be carriedout before it is known how much significance can beattached to the observations reported here. Nevertheless,the changes in the clay mineralogy observed in this studycan be tentatively evaluated in terms of their possiblegeological significance.

Kaolinite and discrete illite in these samples (notincluding biotite) are most likely detrital clay minerals. Thesharp increase in the abundance of these minerals in the

TABLE 1Peak Positions (Cu Kα) and d-values for some Reflections of

IUite- (glycol) Montmorillonite3

Percentexpandable

0102030406080

100

Randomly interstratified

RefL(001)10/(002)17

20

8.688.808.92

9.439.91

10.1710.40

d(A)

10.1810.04

9.91

9.378.928.698.50

(002)10/(003)17

20

17.6517.5017.19

16.5816.1515.8715.78

d(Å)

5.025.065.15

5.345.485.585.61

Calculated by Reynolds and Hower (1970).

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H. E. ROBERSON

550βCIO.OÅ I4.0Å

149-

149-14

149-28

149-33

30 25 20 15 10Degrees 2 θ

5 3

Figure 4. X-ray diffraction patterns of samples heated to550° C for 1 hour.

expandability in the younger sediments is of terrigenousorigin.

The chalks, oozes, and siliceous muds below 100 to 150meters are interbedded extensively with volcanic ashdeposits. It would appear that the illite/montmorillonite of80 per cent expandability is a product of a devitrificationof volcanic ash that was laid down in this deep-watermarine environment.

It is interesting to compare these results with the resultsof Perry and Hower from their study of pelitic sediments(Pleistocene to Eocene) in the Gulf Coast. Mixed-layer claysof 50 to 60 per cent expandability overlie clays of higherexpandabilities in Site 149. This would not have beenpredicted on the basis of observations made by Perry andHower. They observed a monotonic decrease in expand-ability from samples 120 meters to 5,200 meters below thesurface and interpreted these changes in expandability to bea result of burial diagenesis, mainly as a function ofincreasing temperature with depth.

The change in these cores from mixed-layer illite/montmorillonite of 50 to 60 per cent expandability tomixed-layer illite/montmorillonite of 70 to 80 per centexpandability with depth is most likely unrelated todifferences in temperature, which are probably quite small.If both the illite/montmorillonite of 50 to 60 per centexpandability in the uppermost beds and illite/montmoril-lonite of 70 to 80 per cent expandability in the underlyingbeds has formed from the devitrification of ash, it is con-

146-2-5

55O β C

IO.OÅ I4.0ÅI

146-7-1

146

20 15 10Degrees 2 θ

Figure 5. X-ray diffraction patterns of samples heated to550°Cfor 1 hour.

ceivable that the differences in the mixed-layer illite/montmorillonite composition are related to differences incomposition of the original volcanic ash. One could alsoenvisage, assuming again that all of the mixed-layer illite/montmorillonite regardless of expandability has formedfrom the devitrification of volcanic ash, that the post-depositional chemical environment of beds of Miocene ageand older was somewhat different than that of beds of lateMiocene age and younger. Further chemical studies of theclay fraction and pore fluids should help to clarify theserelationships.

REFERENCES

Johns, W.D., Grim, R.E. and Bradley, W.F., 1954. Quantita-tive estimations of clay minerals by diffraction methods.J. Sediment. Petrol. 24, 242.

Perry, E. and Hower, J., 1970. Burial diagenesis in GulfCoast pelitic sediments. Clays and Clay Minerals 18, 165.

Reynolds, R.C. and Hower, J., 1970. The nature ofinterlayering in mixed-layer illite-montmorillonites.Clays and Clay Minerals 18, 25.

Roberson, H.E., Weir, A.H. and Woods, R.D., 1968.Morphology of particles in size-fractionated Na-montmorillonites. Clays and Clay Minerals 16, 239.

Weaver, C.E., 1956. Distribution and identification ofmixed-layer clays in sedimentary rocks. Am. Mineralo-gist 41,202.

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149-14

Oriented Clay Aggregate - Ethylβne Glycol

Mont. Mont. Mont.(003) (002)

MIXED LAYER ILLITE/MONTMORILLONITE CLAYS, SITES 146 AND 149

149-39Oriented Clay Aggregate

I70Å

20 15 10Degrees 2 θ

< 2 µ fraction2000 cps

<.05 µ fraction5000 cps

< .05 µ fraction1000 cps

Figure 6. X-ray diffraction patterns of mixed-layer illite/montmorillonite from two different size fractions ofsame sample.

Ethylene GlycolTreated

30 25 20 15 10 5 3

Degrees 2θFigure 8. Poorly crystallized mixed-layer illite/

montmorillonite which appears to have less than 80 percent expandability.

149-16

Oriented Clay Aggregate-Ethylene Glycol

Mont. Mont, i(003)

^ J <*->

20 15 K)Degrees 2θ

<2µ fraction1000 cps

< .05 µ fraction1000 cps

5 3

Figure 7. X-ray diffraction patterns of mixed-layer illite/montmorillonite from two different size fractions ofsame sample.

146-15Oriented Clay Aggregate-Ethylene Glycol

0 0 3 002 0015.54Å 8.60Å I70Å

40 35 30 25 20 15

Degrees 2 θ

5 3

Figure 9. Typical mixed-layer illite/montmorillonite ofabout 80 per cent expandability.

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