Clay Minerals (1984) 19, 67-76

I N T E R S T R A T I F I E D X R D C H A R A C T E R I S T I C S OF P H Y S I C A L M I X T U R E S OF E L E M E N T A R Y C L A Y

P A R T I C L E S

P. H. N A D E A U , J. M. T A I T , W. J. M c H A R D Y AND M. J. W I L S O N

The Macaulay Institute for Soil Research, Craigiebuckler, Aberdeen AB9 2Q J, UK

(Received 11 July 1983; revised 28 October 1983)

ABSTRACT: Transmission electron microscopic (TEM) examination of the <0.1-/zm fraction of montmorillonite and regularly interstratified illite-smectite (I-S) shows that these clays, when dried from suspension, consist primarily of particles 10 A and 20 A thick respectively. However, X-ray diffraction (XRD) examination of sedimented aggregates of montmorillonite indicate that the effective number of unit cells that are diffracting coherently is ~9. This discrepancy can be reconciled by postulating an interparficle diffraction effect from the sedimented aggregates of oriented particles. The interfaces of these particles are capable of adsorbing water, ethylene glycol etc. so that on this basis smectite is composed of elementary silicate particles 10 A thick, and regularly interstratified I-S is primarily composed of elementary 'illite' particles 20 A thick, values which are in agreement with the TEM observations. This concept is confirmed experimentally by XRD examination of sedimented aggregates from mixed suspensions of both materials; the resulting patterns are identical to those of randomly interstratified illite and smectite layers, which indicates that the layer sequence examined by XRD has been entirely rearranged. It is concluded that the use of XRD peak breadth to determine mean crystal thickness cannot be reliably applied to these systems. Standard XRD data from sedimented aggregates may not be able to distinguish between true interstratification and interparticle diffraction effects of intimate physical mixtures.

A fundamental problem in clay mineralogy has been the understanding of the structure of clay particles and how that structure is perceived by X-ray diffraction of sedimented aggregates-- the common analytical technique employed. Sedimented aggregates, consist- ing of clay suspensions dried onto a flat surface, are useful because the preferred orientation of the sheet-like clay particles enhances the d(00/) diffraction maxima, thus resolving the diagnostic c, or more precisely the c sinfl, dimension of various clay minerals. Although it is generally accepted that clay particles are of the order of 103 A, in length and width (i.e. in the a and b dimensions), controversy exists as to the thickness of the particles, which places constraints on the number of unit cells or layers along the c direction (M+ring & Oberlin, 1971). Many TEM studies have demonstrated the presence of thin clay particles of the order of 10-20 A thick, corresponding to only 1-2 unit cells thick. Examination of the same material by X R D , however, indicates that the number of unit cells, N, per 'crystallite', which is generally equated to the mean particle thickness, is 5-15. This is particularly true for smectites (Tettenhorst & Roberson, 1973), interstratified clays (Weir et al., 1962) and illites (Gfiven et al., 1980; McHardy et al., 1982). The controversy arises from the relationship between the shape and intensity of the X R D maxima and N as defined by the interference function (James, 1948; Reynolds, 1980), which states that for large values of N, say N > 20, diffraction maxima are very sharp and intense. The maxima broaden and their intensity decreases as N decreases. Because the intensity is equal to

�9 The Macaulay Institute for Soil Research, 1983

68 P . H . Nadeau et al.

unity at all diffraction angles when N = 1, Bragg reflection is not possible from a single scattering layer. It has been generally accepted that if particles of elementary thickness are present within a sedimented aggregate, they contribute little if anything to 00l diffraction. In other words, 001 diffraction is produced mainly by thicker particles which have a relatively large number of scattering layers, i.e. 15 < N > 5. This interpretation has been questioned by the proposal that interparticle diffraction by sedimented aggregates of very thin particles may explain the interstratified I-S XRD results from an ethylene glycol-solvated clay which, by TEM, was shown to be composed of very thin illite particles (McHardy et al., 1982). The interstratification XRD character was considered to be caused by ethylene glycol adsorption on the interfaces between adjacent thin illite particles in the sedimented aggregates.

The investigation described here concerns a regular I-S (52% illite layers) with nearest neighbour R1 type ordering (see Reynolds & Hower (1970) and Reynolds (1980) for a more detailed account of interstratified clay mineral terminology) isolated in the <0-1 #m suspension of a bentonite clay. A TEM study of the <0.1-#m material showed that it had morphological similarities to the filamentous illite described by McHardy et al. (1982), particularly with regard to the abundance of thin particles. Experiments were carried out to determine whether the <0-1/~m material was an ordered I-S with particles 5-15 unit cells thick (50-150 ]~), or a relatively homogeneous population of illite particles approximately 20 fi, thick, which would correspond to two layer-silicate sheets coordinated by a single layer of K ions. The interfaces of these particles would be capable of adsorbing ethylene glycol, thus giving rise to the 27/~ superlattice spacing by interparticle diffraction. It was reasoned that if interparticle diffraction occurred, it could be modified by making physical mixtures of particles of elementary layers, thus altering the sequence of layers in the sedimented aggregates. Conversely, if the XRD pattern were produced by interstratified clay particles 5-15 unit cells in thickness, the resulting XRD pattern of the sedimented aggregate of the mixture would simply be that of the separate components superimposed on one another. Because of a similar discrepancy between XRD and TEM data that exists for smectites (Tettenhorst & Roberson, 1973), the two components chosen were a <0.1 grn regularly interstratified I-S and montmorillonite Clay from the Permian

"Rotliegend aeolian sandstone of the Southern North Sea Basin (Rossel, 1982) was included in the study as an illite standard.

E X P E R I M E N T A L P R O C E D U R E

Dilute Na-saturated suspensions of the <0.1 #m fractions of bentonite sample MB235 (analogous to sample MB670--Fig. 8, Nadeau & Reynolds (1981)--from the Greenhorn formation, Canon City, Colorado) and Wyoming bentonite (montmorillonite 25, from Upton, Wyoming) were prepared. The final washings of the suspensions were achieved by dialysis to ensure retention of very fine material. Sedimented aggregates of the resulting suspensions were prepared on glass slides, solvated by ethylene glycol vapour and examined by XRD.

For TEM examination, several drops of diluted clay suspensions were spread over a piece of freshly cleaved mica and allowed to air-dry. The particles on the mica were shadowed with Pt at a low angle (~ 10 ~ the exact angle being recorded for each sample) and carbon coated. The carbon film with the attached shadowed clay particles was picked up on electron microscope grids for TEM observation. Thickness measurements on 48 to

XRD of mixtures of elementary clay particles 69

85 particles for each sample were recorded by measuring the shadow length relative to the shadowing direction and angle. Care was taken to ensure that all representative particles were measured.

Four-millilitre aliquots of the clay suspensions were oven-dried and weighed so that the concentration of clay per ml of suspension could be determined. Mixtures of the two suspensions were prepared in 1:1, 2:1, 4:1 and 8:1 ratios by weight of MB235 to montmorillonite. Sedimented aggregates of the mixtures were dried onto glass slides, ethylene glycol solvated and examined by XRD. Mixtures of the <0-2 #m fraction of the Rotliegend illite and montmorillonite were prepared in a similar manner.

Small amounts of the three suspensions were Ba-saturated and washed, utilizing an ultra-high-speed centrifuge (130000 g for 1.5 h). Several drops of the Ba-saturated suspensions were dried onto graphite discs mounted on an aluminium stub, carbon coated and analysed by a scanning electron microscope (SEM) equipped with a Link energy-dispersive X-ray analysis system. By this method the chemical composition and cation-exchange capacity (CEC) of the samples could be determined despite the small amounts of material available.

,3o , .~ . ~.o . . ~ ~ 16.9

b

5~62 8-49

d 5'42 9"01/

9.09

~ ' 3 "35 14 "3

f 5-~ 9.14

, , , , , ~ ~ 30 20 10 3

CoKQ: FIG. 1. X-ray diffraction patterns of ethylene glycol-solvated sedimented aggregates on glass slides. (a) Montmorillonite <0.1 #m; (f) MB235, <0.1 #m; (b)-(e) physical mixtures of MB235

and montmorillonite in the following ratios by weight: (b) 1:1, (c) 2:1, (d) 4:1, (e) 8:1.

70 P.H. Nadeau et al.

R E S U L T S

The XRD results for the ethylene glycol-solvated sedimented aggregates are shown on Fig. l (a-f) . The montmorillonite (Fig. la) gives a typical smectite diffraction pattern. The XRD pattern of MB235 (Fig. lf) is that of a regularly interstratified I-S, with 52% illite layers, showing nearest-neighbour R1 type ordering as indicated by the 27/k superlattice maximum. The XRD patterns of the mixtures (Fig. lb-e) show a strong resemblance to those of ethylene glycol-solvated randomly interstratified I-S with 10-50% illite layers (see Fig. 4a in Reynolds & Hower, 1970). The percentage of illite layers increases with the relative proportion of MB235 in the mixture. The familiar peak migrations from 8.49 to 9.14 ~ and 5.62 to 5.38 A (see table 3 in Reynolds & Hower, 1970; or table 4.5 in Reynolds, 1980) also demonstrate this point. Another feature of the diffraction patterns is the broadening of the 17 .A maximum as the proportion of montmorillonite decreases, a feature which continues to dominate the low-angle region even at the dilution factor of 8:1.

30 20 10 3 0 '~ r ' - - ~ # I I - - a , I

16"9

5"62 8 '49

b 9 . 9 3 / 4-99 8"50

C 10.0

5 "0 8-56 ,~ 5.64 LJ

I0'0 d

4-95 8 . 5 6 : I : I', 5 " 6 3 ' / v ' J ~-~o-~'" / !Jl~/

e

L 8,49 t , 5 " 0

I 0 " I i

f 3,33 ' 5-~ o I

30 20 10 3 C o K e '

FIG. 2. X-ray diffraction patterns of ethylene glycol-solvated sedimented aggregates on glass slides. (a) Montmorillonite <0.1 /~m; (f) Rotliegend illite <0.2 gin; (b)-(e) physical mixtures of Rotliegend illite and montmorillonite in the following ratios by weight: (b) 1:1, (c) 2:1, (d) 4:1,

(e) 8:1.

XRD of mixtures of elementary clay particles 71

Similar XRD data for the mixtures of montmorillonite and the <0.2 pm Rotleigend illite are shown in Fig. 2. These patterns are those of simple mixtures with no indication of interstratification.

Transmission electron micrographs of the particles are shown in Fig. 3. The montmorillonite particles exhibit thin-plate and curly-flake morphologies (cf. Roberson et al., 1968). The particles of MB235 comprise thin laths and sub-equant plates. Electron diffraction shows that b = 9.06 A, and that the laths are a-axis elongated. The Rotliegend particles are on average much thicker than those of the other two samples, but exhibit some morphological similarities to those of MB235, particularly in their lath-like habit. Particle-thickness data are shown in Fig. 4. The number of particles analysed for the montmorillonite, MB235, and Rotliegend illite were 53, 84, and 48 respectively. Given the

FIG. 3. Transmission electron micrographs of platinum-shadowed samples. (a) and (b) montmorillonite; (c) and (d) MB235; (e) and (f) Rotliegend illite.

72 P. H. Nadeau et al.

7.5

�9 . t

5 21 ~ 37

i

53

2;i21 %1

5 21 37 53

:I c 12

y,

5 85 165 245

FIG. 4. Particle-thickness distribution data in A. (a) Montmorillonite; (b) MB235; (c) Rotliegend illite.

4 A thickness of the shadowing platinum grains, the accuracy of the determinations is somewhat reduced, particularly for the very thin montmorillonite particles. If one considers N to be the number of 2:1 silicate layers per particle and that such particles have an integral number of layers, the data can be divided into N = 1, 2, 3 etc. Using this approach, the montmorillonite has 83% of its particles N = 1, and 17%, N = 2. Similarly, MB235 has 15% N = 1, 79% N = 2, 3% N = 3 and 3% N = 5. Values of N for the Rotliegend illite range from 2 to 16 with a mean value o f N = 7.

The X-ray-spectrum microchemical analysis data for Ba-saturated samples are shown in Fig. 5. An approximate elemental oxide analysis for each sample derived from this data is also shown in Fig. 5 but should be considered for comparison only. The Ba-CEC (Fig. 5) is calculated from the elemental weight percent of Ba in terms of mEq/100 g, and, for montmoriUonite, is 13% lower than the NH4-CEC determined independently on a dry-weight basis.

D I S C U S S I O N

The XRD results of the sedimented aggregates of mixtures of MB235 and montmorillonite suspensions (Fig. lb -e ) do not resemble a simple superposition of the diffraction patterns

XRD of mixtures of elementary clay particles

SiO 2 57

AI203 30

I Fe O 4-2

a II MgO 3-4 I I BaO 5.9

Si 02 54

AI203 34

t FeO 1-0

I[ MgO 3-8

b JI K20 3-4

Si 02 49

AI203 38

Fe O 2-5

MgO 1.4

BaO 0.6 CEC 7

�9 ~ ~ a F~ I 1 i I I I I I I

2 4 keV 6 8

FIG. 5. X-ray spectra and calculated wt% oxide composition for Ba-saturated samples. (a) montmorillonite; (b) MB235; (c) Rotliegend illite. Cation exchange capacity (CEC) is in

mEq/100 g.

73 �84

of the two components (Fig. la, f). The 27 A 00l maxima of MB235 (Fig. l f) are easily destroyed by relatively small amounts of montmorillonite (Fig. le), which would not be expected if the regular I-S XRD pattern were the product of intraparticle diffraction, i.e. from particles 5-15 unit cells (50-150/k) thick. Furthermore, the breadth of the 16.9/k glycol-solvated montmorillonite 001 peak is not preserved in the mixtures, as would be expected if its shape were the product of large crystaUites (N = 10). The Scherrer equation (Brindley, 1980), which is generally applied to determine the relationship between N and the peak breadth, B, measured in radians 20 at half-height, is:

B = K2/Nd cos 0

where K is a constant (0-91), 2 the wavelength of radiation (for Co, 2 = 1.79/k), d the layer spacing (for montmoriUonite, d = 16.9/k). /9 = 6.1~ (= 3.05 ~ and B = 0.6 (= 0.0105 radians). Solution for the <0.1 pm Wyoming montmorillonite gives N = 9. Acknowledging the errors inherent in determining N by this method, it is clear from the experimental results that the effective number of 2:1 silicate layers that scatter coherently, here designated N e, is much larger than the number of layers per particle, Np. Although Ne

74 P.H. Nadeau et al.

= 9 for the Wyoming montmorillonite, TEM data show that for 83% of the particles Np = 1. Similarly, TEM analysis of regularly interstratified I S MB235 shows it to be composed of a somewhat less homogeneous population of particles, for 79% of which Np = 2 and are thus similar to the 19 A thick particles in dispersed samples of allevardite reported by Weir et al. (1962).

The discrepancy between the XRD and TEM data, where N e > Np, can be reconciled by postulating an interparticle diffraction effect. The 27 A 00l maximum of MB235 would arise from interparticle diffraction of elementary 'illite' particles 20 A thick whose interfaces adsorb ethylene glycol. Similarly, the 16.9 A maximum of the montmorillonite results from interparticle diffraction of elementary silicate layers. The randomly interstratified I-S XRD character of the mixtures (Fig. lb-e) results from interparticle diffraction by the random arrangement of elementary particles of 'illite' and montmorillonite within the sedimented aggregate.

The XRD patterns from mixtures of the montmorillonite with the Rotliegend illite (Fig. 2) are, conversely, those of simple mixtures. There is no indication of peak migration or fundamental change in the overall diffraction character of the individual components. The series of 10 A 001 maxima of the Rotliegend illite are due to conventional intraparticle diffraction, as would be expected from the particle thickness measurements that show Np = 2-16. Peak-breadth measurements from XRD of the Rotliegend illite give ATe = 10-13.

Previous workers have discussed some of the difficulties in the interpretation of XRD data from interstratified clays (Tettenhorst & Grim, 1975; Mills & Zwarich, 1972). The latter indicated the possibility that physical mixtures of smectite and illite may produce XRD results similar to those of interstratification, but did not consider that the XRD of illite may be dependent on particle size or, more precisely, particle thickness, as has been shown here.

The phenomenon of interparticle diffraction may open new lines of experimental research. It has already been utilized to explain the XRD results of coagulated colloidal suspensions of smectites (Frey & Lagaly, 1979). Experience has shown that to manipulate its effects experimentally the suspensions must be free of clay particle aggregates, a requirement usually achieved in the <0.1 #m fraction of Na- or Li-saturated material. Quantification of this effect is essential, however, to ensure accurate interpretation. A powerful technique may be to analyse by TEM a statistically valid population of clay particles to determine the fundamental dimensions of the particles and the distribution of types of particles within a given sample. From these data a simulated sequence of randomly sedimented particles could in turn be converted into a sequence of layer spacings--if the assumption is made that the particle interfaces are capable of adsorbing water and ethylene glycol, etc. and that the silicate layers within the particles are coordinated by potassium. The probability data could then be incorporated into theoretical XRD calculations such as those described by Reynolds (1980). This approach, coupled with experimental data for well-characterized materials, could prove invaluable in understanding the physical nature of clay mineral particles by virtue of their interparticle diffraction behaviour. Research is currently under way to determine the feasibility of such an approach.

Another important relationship with particle size may be the variations of chemical composition of clay materials. If one considers the chemical composition of Na-saturated illite, it is clear that the K content will be proportional to the number of silicate layers

X R D o f mixtures o f elementary clay particles 75

coordinated by interlayer K ions per particle. As Np --, 2, corresponding to particles composed of two silicate (2:1) layers coordinated by a single interlayer of K ions, the K content would approach half the K content of muscovite. Using 10.08% K20 as reference for muscovite (Deer et al., 1962), the microchemical data (Fig. 5) indicate that the Rotliegend illite and MB235 have 89% and 34% of the K20 content of muscovite respectively. In the case of MB235, this concept can account for some, but not all, of the K deficiency. More work is required to determine whether substitution by other cations such as Na + or N H + for K + is responsible for the remainder of the discrepancy.

C O N C L U S I O N S

TEM data demonstrate that the <0.1 pm fractions of montmorillonite and regularly interstratified I-S consist of elementary particles 10 A and 20 A thick, respectively. XRD examination of the sedimented aggregates of these materials demonstrates the importance of interparticle diffraction in determining their XRD characteristics. Mixtures of elementary particles of 'illite' and montmorillonite'exhibit diffraction patterns identical to those of randomly interstratified I-S. The layer sequence examined by XRD in the sedimented aggregate is the product of random arrangement of elementary particles. Interparticle diffraction is considered here to be the coherent diffraction of radiation by atomic planes of two or more adjacent, parallel particles. This phenomenon is thought to be important for very thin planar particles giving a high degree of preferred orientation when prepared for XRD analysis by the sedimented aggregate technique. The material identified as regularly interstratified I-S by XRD is primarily composed of elementary 'illite' particles whose interfaces are capable of forming complexes with water and organic molecules. Standard XRD data may not be able to distinguish between true interstratific- ation and interparticle effects of intimate physical mixtures.

ACKNOWLEDGMENTS

Appreciation is extended to British Petroleum and A. Tice for supplying the samples of Rotliegend sandstone and Wyoming bentonite, respectively, to D. Duthie, L. Forsyth, L. Mielewczyk, and Y. Bisset for technical and clerical assistance, and to the Atlantic Oil Corporation for financial support of this research.

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DEER W.A., HOWIE R.A. & ZUSSMAN J. (1962) Rock Forming Minerals, 3, Sheet Silicates. Longmans, Green & Co., London.

FREy E. & LAGALY G.H. (1979) Selective coagulation and mixed layer formation from sodium smectite solutions. Pp. 131-140 in: Proc. Int. Clay Conf. Oxford, 1978 (M. M. Mortland & V. C. Farmer, editors). Elsevier, Amsterdam.

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76 P. 1t. Nadeau et al.

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