SIZE AND SHAPE OF MONTMORILLONITE CRYSTALLITES

Transcription

1 SIZE AND SHAPE OF MONTMORILLONITE CRYSTALLITES by EDWARD C. JONAS and I~OB~RT M. OLIVER Department of Geology, The University of Texas and Clayton Foundation Biochemical Institute The University of Texas, Austin, Texas ABSTI~ACT SPRAY drying dilute suspensions of bentonitic montmoriltonite produces a powder that shows totally random orientation of the crystallites within a sample large enough to diffract X-rays. The powder is collected by an electrostatic precipitator and can be handled in the normal mounting processes without introducing preferred orientation. Electron micrographs show this powder to be composed on a small scale of thin, crumpled, and rolled films. The extremely small montmorillonite crystallites that make up the film are oriented with [001] directions perpendicular to the film surface. Orientation within the plane of the film is random as shown by selected area electron diffraction. Crumpling and rolling of the film is sufficient to make the orientation of [001] directions random in three dimensions in a large sample when X-ray diffraction is registered. The X-ray diffraction patterns all show diffraction maxima (both hk and 00/), and their relative intensities with respect to each other can be determined. The line broadening of the 06 and the 003 peaks was studied. The average crystallite size as calculated from the line broadening varied from six to eleven unit layers thick for four bentonitic montmorillonites. The average lateral dimension of crystallites varied from 140~ to 250/~. Ratios of lateral dimensions to thickness varied from 2.3 to 3.4. INTRODUCTION THE most popular method for determining the size and shape of montmorillonite particles is electron microscopy. Advantages of this method are that particles can be viewed directly and magnifications can be determined accurately. The main disadvantage that must be considered is related to the tendency of montmorillonite crystallites to aggregate. All techniques for preparing montmorillonite for electron microscopy involve drying suspensions in some way. During the drying process there is an ideal opportunity for montmorillonite crystallites to aggregate. Unfortunately, when the crystallites aggregate their orientations with respect to each other are partly crystallographieally controlled. It is often difficult to define a true crystal boundary, and it is still more difficult to recognize it. 27

2 28 15]IFTEENTH CONFERENCE ON CLAYS AND CLAY MINERALS Thickness measurements reported in the literature by electron microscopists do not vary over a wide range. The lateral dimension, on the other hand, has been reported by Kahn (1959) to be larger than 2800/~ for most of the montmorillonites he studied, while Whitehouse, Jeffrey, and Debbrecht (1960) report 400A for the lateral dimension. These differences of opinion are almost certainly caused by slight variations in the sample preparation techniques used or by the more important variations in the nature of the montmorillonite used. There are other techniques for measuring average particle size of montmorillonite. These techniques depend on the diffraction of X-rays by the erystallites. They produce dimensions that generally contrast strongly with the electron microscope measurements. It has been shown for many finegrained crystalline materials other than clays that the breadth of a diffraction line is related to the crystal dimension normm to the planes giving rise to that diffraction line (Klug and Alexander, 1957). Line broadening is not influenced by the state of aggregation of the erystallites. An individual particle, as seen by X-ray diffraction, must not only be a physical unit, it must, also have erystmline regularity within it. Greene-Kelly (1964) used this technique in studying the 001 diffraction of montmorillonite samples. He concluded that the thickness of the crystallites ranged from 8.3 to 47 units cells for the various samples. Another X-ray technique was used by Longuet-Escard, Mering, and Brindley (1960) who studied the shape and location of the h]c diffraction of montinorillonite. They concluded that the average lateral dimension of the erystallites was of the order of 100 A. EXPERIMENTAL TECHNIQUES It is ordinarily not possible to conduct line broadening studies to determine both the lateral dimension and the flake thickness of montmorillonite from the same diffraction pattern because random powder patterns are so difficult to record. Most X-ray sample preparations for montmorillonite introduce a considerable amount of preferred orientation which either favors the 00t diffraction or the h/c diffraction. Patterns from such preparations would p~ovide data to study either the thickness or the lateral dimension at the expense of the other. Recently, Jonas and Kuykendall (1966) have discovered that montmorillonites that are spray dried from a dilute suspension appear to produce a randomly oriented powder. The powder can be handled reasonably for specimen loading and forming without decreasing the randomness. Diffraction from this preparation offers a unique opportunity to study the size of montmorillonite erystallites in both dimensions. It is interesting to know the special physical state of this spray-dried powder that produces random orientation. Plate 1, B is an electron micrograph of the montmorillonite powder of Black Hills bentonite. It is composed

3 SIZE AND SHAPE OF MONTMORILLONITF (~RYSTALLITES 29 of a series of sheets that are rolled or crumpled into particles that are about equidimensional. These particles are randomly oriented because of their relatively spherical shape. Any preferred orientation within the sheets would be first made random by the crumpling and second by the equidimensional shape of the particles. The size of these crumpled masses appears to vary through a narrow range. The external dimension is of the order of 1 t~. ]['LATE 1. A. (Upper left) Electron diffraction pattern from selected area of spray dried montmorillonite from Black Hills, S.D, B. (Upper right) Electron mierograph of spray dried montmorillonite from Black Hills, S.D. C and I). (Lower pair) Stereoscopic pair set for direct viewing. Plate l, C and D are a stereoscopic pair viewing the spray-dried montmorillonite from Black Hills bentonite. The hollow rolls can be more easily seen stereoscopically than in the single micrograph. At least six different overlapping layers of the sheet can also be seen. Preparation of the sample began with disaggregation in distilled water. No dispersing agents were used.

4 30 FIFTEENTH CONFERENCE ON CLAYS AND CLAY MINERALS The suspension was allowed to settle and the fraction containing particles < 2 F e.s.d, was dra~l off. This suspension was diluted to g per ml and spray dried. The construction details of the spray drying apparatus and its operation have been reported by Jonas and Kuykendall (1966). AI L 122.Y~/7;42 ~ /.24 26;40 ~ 12g AI 33;06 ~a4 ~.. /43 14g /6g 0O5 lg2 AI 22j 04 ~ 2.22 AI ~ /3~20 ~ Z.S ~ ;02 ~ ~ % 00l g. 6 Fic. 1. X-ray diffraction pattern of spray-dried montmorillonite from Black Hills, S.D., bentonite. Figure 1 is the X-ray diffraction powder pattern registered from a spraydried specimen of the Black Hills bentonite. The sample was dehydrated and maintained in a dry atmosphere during registration of the diffraction. A slow scanning speed of 0.2 ~ 20/min was used with CuKa radiation and a nickel filter. Lines from metallic aluminum were used as an internal standard. Orders of the basal 9.6 A period are recognized out to a suggestion of 007. The hk, peaks are all well formed and relatively much more intense than ordinarily found in montmorillonite powder patterns. Not all of the 001 peaks were equally well suited to a line broadening study. The third order of 001 which occurs at 3.2 A was chosen for the erystallite thickness measurement. The 06 line at 1.49-~- was used for the lateral dimension measurement.

5 SIZE AND SHAPE OF ~V[ONTMORILLONITE CRYSTALLITES 31 Tile reasons for choosing these lines are: (1) they are relatively intense, (2) they occur in a part of the pattern that has a unitbrm background, (3) they are symmetrical, and (4) there are no interfering peaks. Their high intensity suggests that the two chosen peaks occur at an angle at which their respective structure factors are likely to vary only slightly with the angle 0. The minimal variation in structure factor near a diffraction peak is a requirement for its successful use in line broadening particle size determinations. Any broadening of the lines introduced by instrumental characteristics is corrected by comparing the breadth of aluminum lines to the breadth of the montmorillonite lines. The breadth was measured at half maximum intensity for all of the aluminum lines. There is no consistent measurable variation with diffraction angle. The average of all these measurements, 0.35 ~ 20, was therefore used as the line breadth for infinitely large crystals. The linebroadening caused by small crystal size, B, was then calculated from the relation suggested by Warren (1941), B 2 =BM 2 -- Bs2 where BM--~measured montmoriilonite peak breadth, Bs =average measured aluminum peak breadth in radians. The Scherrer (1918) formula relating peak breadth to crystal size was used. 0.9A.D =B cos OB A = 1.54, 0B = diffraction angle at maximum intensity. t~esults Table 1 shows the calculated flake width and thickness for four bentonitic montmorillonite samples. Except for the Twiggs CO., Georgia, sample the measurements seem to be rather uniform. The crystallite width range is TABLE ].--~)IMENSIONS O~ ~/[ONTMORILLONITE CRYSTALLITiES f Angstrom units Sample Width Thickness Width Thickness Black Hills, S.D Clay Spur, Wy Twiggs Co., Ga Gonzales Co., Tex

6 32 FIFTEENTH CONFERENCE ON CLAYS AND CLAY MINERALS , and the thickness is The two western bentonites, Clay Spur, Wyoming, and Black Hills, South Dakota, have similarly shaped crystallites but the Gonzales, Texas, bentonite has somewhat wider flakes for their thickness. In comparing these measurements with previous ones, it can be seen that the Clay Spur bentonite crystallites are only about Ii units thick whereas Greene-Kelly (1964) calculated 31 units thickness for his Wyoming bentonite. The width measurements are somewhat larger than the Longuet-Escard, Mering, and Brindley (1960) dimension of I00~_ but their sample was not Wyoming bentonite. Both Hofmann et al. (1955) and Mathieu-Sicand (1951) have published montmorillonite crystallite widths not greatly different from these, 380_~ and 300A. The suggested dimensions as indicated by X-ray diffraction methods for montmorillonite crystallites are unit cells wide and about 10 unit cells thick. The particles seen by electron microscopy of the spray-dried clay are then obviously sheet-like aggregates of crystallites of these small dimensions. An electron diffraction pattern of a 0.5 t~ diameter selected area, shown in Plate 1, A, demonstrates this. The two main diffraction angles correspond to the 4.6 ~ and 2.5~_ hk lines. There are only slight indications of 00/ diffraction from the aggregates. The parts of the sample that are effectively diffracting electrons are the thin flakes lying normal to the electron beam. The diffraction is made up of a large number of spots famy uniformly distributed along rings. This uniform distribution of spots indicates that the crystallites within the ~hin shee~s are randomly oriented around the [001] direction, the [00I] is normal to the film, and there are many erystmlites within the 0.5 t* selected area. DISCUSSION The measured dimension calculated from X-ray line broadening of the 003 appears to be the thickness of the film and not the thickness of individual crystal]ites. Many properties of montmorillonite-water suspensions can only be justified if the montmorillonite particles are much thinner than they are wide. If it is assumed that the measured width of montmori]lonite crystallites is reasonable, the thickness should be more nearly one unit cell rather than ten. From another point of view, the crumpled films seen by electron microscopy could hardly be built from aggregates of individual crystaltites with low ratios of width to thickness. If the film is 10 unit cells thick and is constructed of crystallites that are 10 unit cells thick and 20 unit cells wide the only bond giving strength to the film would be an edge-to-edge bond between crystaltites. It must be remembered that the crystallites have no regular orientation within the film except for the [001]. The random orientation of [100] precludes there being a very strong edge-to-edge bond. A more reasonable model of the film that is produced by spray-drying is that it is constructed of crystallites one unit cell thick and unit cells

7 SIZE AND SHAPE OF MONTMORILLONITE CI~YSTALLITES 33 wide. These crystullites are arranged in layers, and the film contains ten of these layers. Within each layer the crystmlites are randomly oriented around an axis parallel to [001]. Stacking of the layers provides overlap of eryst~llites in one layer with the erystallites in subjacent and superjacent layers. This overlap allows face-to-face bonding that gives strength to the film. The ten mosaic layers of erystallites are stacked parallel to each other with such nearly perfect crystallographic regularity to form the film that 001 X-ray diffraction occurs much as it would from a single crystal I0 unit cells thick. Line broadening of the 003 then measures the film thickness and not the erystallite thickness. The film thickness is somewhat dependent on sample preparation. During spray-drying, all of the crystallites contained in a single droplet of the suspension will aggregate to form the film. The forces of aggregation are not influenced by any supporting surface but are equally active in all directions as the water evaporates. The film thickness depends on the suspension concentration, droplet size, rate of evaporation, and many other factors. Because line broadening of the 003 is influenced by sample preparation it does not measure a fundamental characteristic of the clay crystallites. All of the montmorillonites studied appear to have an effective crystallite thickness of one unit cell. The width of the crystallites is variable within a range of approximately unit cells. I~EFERENCE S GRnE~E-KELLY, R. (1964) The specific surface areas of montmorillonites: Clay Minerals Bull. 5, No. 31, HOF~ANN, U., WEISS, A., Koch, G., I~EHLER, A., and SCHOLZ, A. (1955) Intracrystalline swelling, cation exchange, and anion exchange of minerals of the montmorillonite group and of kaolinite: Clays and Clay Minerals, Prec. 4th Conf., Natl. Aead. Sci. Natl. Res. Council Pub. 456, JoNAs, E. C., and/siuykendai, L, d. 14. (1966) Preparation of montmoritlonites for random powder diffraction : Clay Minerals 6, No. 3, KAHSr, A. (1959) Studies on the size and shape of clay particles in aqueous suspension: Clays and Clay Minerals, Prec. 6th Conf., Pergamon Press, New York, KLUO, H. P., and ALEXANDER, L. E. (1957) X-ray Diffraction Procedures: John Wiley, New York, LONOUET-ESCAnD, J., M]~ING, J., and BP~INDLEY, GEO. (1960) Analysis of hk hands of montmorillonite: C.R. Aead. Sci., Paris 251, :~r A., ME~INO, J. and PE]t.RIN-BENNET, C. (1951) Electron microscopic study of montmorillonite and hectonite saturated with different cations : Bull. Soc. franc. Mingral., 74, SCHEI~I~ElCg, P. (1918) AnMysis of external and internal structure of colloidal particles by means of X-rays : Nachrichten vonder Konig. Gesellsch. Wissensch. zu Gottingen-Math.- Phys. Kl., WARREN, B. E. (1941) X-ray methods: Jour. Applied Physics 12, WHITEtIOIJSE, V. Cx., JEFFREY, L. M., and DEBBRECItT, J. D. (1960) Differential settling tendencies of clay minerms in saline waters: Clays and Clay Minerals, Prec. 7th Conf., Pergamon Press, New York, 1-79.