Analysis of Mixed-Layer Clay Mineral Structures W. F. BRADLEY, Illinois S t a t e Geological Surcey, Urbana, Ill. .4mong t h e enormously abundant natural occurrences of clay minerals, many examples are encountered in which no single specific crystallization scheme extends through a single ultimate grain. The characterization of such assemblages becomes a n analysis of the distribution of matter within such
grains, rather t h a n the simple identification of mineral species. I t having become established t h a t t h e particular coordination complex typified by mica is a common component of man? natural subcrystalline assemblages, t h e opportunity is afforded to analyze scattering from random associations of these complexes with other structural units. Successful analyses have been made of mixed hydration states of montmorillonite, of niontmorillonite with mica, of \ermiculite with mica, and of montmorillonite with chlorite, all of w-hich are variants of the mica complex, and of halloysite with hydrated hallo? site.
N
-4TURBL occurrences of clays include both the moderately frequent instances in which a given species, of characteristic crystallization, exists alone, and the enormously preponderant masses of common clays, shales, and soils which consist of mixtures of species, and even of mixtures of assemblages in which adjacent units differ from each other within a coherently scattering domain. This short review is intended only to illustrate the application of x-ray diffraction methods to the latter group. Poorly crystalline materials afford considerably fewer diffraction data than are desirable. Progress is made only because the structural scheme of muscovite has been found to be the theme upon which most variations are developed. The coordination of this type of layer is illustrated in Figure 1. The considerable latitude which exists in the chemical identity of the positive ions actually present in a given example of such a layer conforms to the ordinary principles of isomorphous replacement, and variations in both the thickness and lateral dimensions of individual layers are small (4,11j. The nature of the chemical entities (ions, molecules, or complex structures) which may interleave 1iettTeen the standard muscovite-type layers is highly variable. and their volumes introduce large differences in the over-all t,hicknessof difFigure 1. Coordination Scheme of Mica-Type ferent cornposited layers. Several representative esaniples are Hydrous Aluminosilicate Layer According to S. B. Hendricks schematically illustrated in Table I for repetitive sequences which O x ) gen framework is of relative composition Oio(OH)?, with exist naturally or may be prepared, each of which has n different 4 tetrahedrally coordinated ions, usually mainly Si(’] ), and either nearly 2 or nearlr 3 octahedrally coordinated ions, of over-all layer thickness, each affording its own characteristic intotal valence near 6. .Neutrality is maintained b) interlayer tegral sequence of successive orders of diffraction along the pole t o populations the layers. The ability to modif!. natural “hydrates,” as the montmorilTable I. Schematic Projection of Layer Sequences for Several Common Layer lonite minerals. by converting Silicates with Different Interlayer Populations them to organic liquid ”solPyrophylNa Ca 3Iontmorillonite vates” with the consequent lite Muscovite ,110ntmorilMontmorillonite Glycol Talc Biotite lonite Chlorite Verniiculite Complex volume change. has afforded AIe+-or M e + + + .... .... .... .... .... .... a sensitive criterion of iden. . . . . . . . . . . . 40 + 20H .... .... .... tity for the montmorillonite 4 Si .... .... .... .... .... .... minerals (3.9 ) and has also 60 .... .... .... .... .... . . . furnished the key to interpre2K 60H 4Hz0 tation of assemblages in which .... lH20 6!?. component layers within co4 bl .... .... M e En(OH)z .. herently scattering domains 60H 4H20 40 4-20H .... are of two or more kinds. .... .... A l e + + or M e + + + .... .... Of the successful analyses .... .... (EnOHIz of this kind, one ha? been an .... .... instance of regular alternation .... .... .... .... .... of pairs of silicate layers sepa.... rated hy pairs of water layers .... ( 2 ) . The integral sequences .... of orders afford data for nor-
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ANALYTICAL CHEMISTRY
mal one-dimensional Fourier syntheses for both the natural and the glycol solvated state, as illustrated in Figure 2. A much more frequently encountered natural occurrence is the ease m which layers of two kinds, and of different layer thicknesses, are associated in random epitaetic growths on the unit
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Figure 2. Relative Electron Density Projection of Regularly Alternating Douhle-Layer Mineral One hslf of a period is shown. Upper = m e . water system; lower, ethylene g l y ~ ocounterpart l
level ( 1 ) . Such assemblages do not afford integral sequences of higher orders of diffraction. They produce instead a complex array of irrationally related scattering maxima first analyzed by Hendricks and Teller (7), and later by M6ring (10). Both methods depend upon the synthesis of theoretical scattering curves for assumed models, and experimental curves are matched with synthetic curves by trial and error. Three such synthesized cume8 are illustrated in Figure 3. Each of these is compiled from the Hendricks and Teller relationship and modified by the illustrated experimentally estimated fom-factor function (Figure 4) composited from the intensities of 8 number of observed sequences from varied layer periodicities for montmorillonite complexes. For the simplest case, a 1 to 1 mixture
where + ( I ) and are the phase angles of the respective layers, 5 is their average, and V is the scattering amplitude for one layer. The availability of the empirical curve (Figure 4) obviates the necessity far conventional intensity corrections. A large number of similar scattering sequences, including more general cases, have been compiled by Brown and MacEwan (6) and are reproduced by Brindley (4). These analyses can be considered highly reliable in cases where syntheses may be made for both the
A
Figure 3.
X-Ray Diffraction Diagrams and Theoretical Scattering Distribution Curves for '
Montmorillonite-Illite Systems in Random Intergmwth
A. 10 and 12.5 A. units. Specimen is not quartz-free.
B. 10 and 15 A. units
C. 10 end 17 A. units
V O L U M E 2 5 , N O . 5, M A Y 1 9 5 3
729
natural assemblage and its solvate and compared in turn nith the two experimental scattering sequences. In practice it is normally adequate to dispense with the actual synthesis of scattering curves and merely employ the device illustrated by 416ring (Figure 5). Reciprocal lattice nodes for the two assumed species are simply scaled colinearly along a line sin 9 proportional to --,x in XIhich case scattering is anticipated only a t positions of nearer coincidences and is sharper the nearer the coincidence.
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Composite Curve of Relative 001 Powder Diffraction Intensities
Figure 4.
Compiled from varied set of montmorillonite-organic liquid complexes.
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Figure 5 . Scale of Two Sets of Reciprocal Lattice Nodes along 001 Reciprocal Line as Used by RIbring
The inverse analysis is also possible and has been made for a few examples by MacEwan, by adaptation of the methods of radial distribution to the 001 reciprocal space line (8). Scattering data of high quality are readily availahle by suitable use of Geiger counter Epectrometers. The natural flaky habit of clays permits them to be dispersed a n d allowed to sedimentonto a microscope slide, assuming a high degree of preferred orientation. The spectrometer traverse then records the scattering distribution along the 002 reciprocal space line with only very minor contributions from extraneous diffraction. .os Sin e.IO Such a trace is illusA trated in Figure 6. AsFigure 6. Smoothed Traced Specsuming a knoxn geotrometer Record of 001 Scattering metrical form factor, Distribution from Well-Orieneuch a trace may be tated Aggregate interpreted by MacMixed 10 and 12.5 A. layers in ratio near 2 t o 1. Small extraneous feature near Ewan’s a n a l y s i s i n sin _E - 0.065 is from associated gypsum terms of layer thick-
nesses A and B and the frequency of neighbors of type A A , AB, BB, and A A A , AAB, ABB, BBB, etc. Each of the above types of analysis is dependent upon the approximation that form factors for the component layers be of similar shape. The fact that solutions have been made a t all confirms that this is nearly true in the analyzed assemblages. -4 far less satisfactory situation is encountered in those assemblages which include chlorite, and considerable evidence exists that the chloritic assemblages may be of so general occurrence that they are actually the ones of greatest significance (6). Of prime interest in sedimentology is the concept of geochemical cycles. The briefest possible outline of pertinent overall aspects xould be described by these general, but imperfect, trends: Primary rocks weather in time to clay minerals, one large group of which is composed of the three-layer structures discussed above; another abundant product is kaolinite. Kaolinite is based on a related but different coordination scheme of only two layel‘s of coordination polyhedra per unit. The three-layer structures are the more stable, can exist in finer states of disaggregation, and have wide latitude in chemical composition. Kaolinite units are asymmetric and exist only in particles composed of relatively large numbers of associated layers-that is, as much thicker crystals. Thus, the tendency is that kaolinite is formed in sztu (or is subject only to moderate transport), maintains a fixed chemical composition, and is characteristic of continental and stagnant water accumulations. I n contrast, the three-layer minerals may be transported long distances by weathering agencies, may lose or gain ions in changing environment (especially the interlayer associated ions), and eventually contribute to marine depositions. In transport by surface waters, which are acid, composition changes are in the direction of loss of ions like K + and Mg++. As rivers discharge into the sea, such ions are regained, first probably only as exchange ions, but later, under the alkaline environment of sea water, as actual interlayer precipitation of brucite -6 M9 layers. The affinity of -6 0 the three-layer structure for K + , but not 4 SI - 6 0 Na+, is reflected in the accumulations of N a + but not K + in sea water, and the level of -OH 01 HzO M g + + content in sea water is p r o b a b l y -Me++ maintained according -OH or HzO to the solubility of chloritic brucite a t the pH which prevails, usually about 8.0 to 8.5. -6 0 -4 Si This precipitation is, in effect, the gradual - 6 0 diagenesis of chlorite from montmorillonite. -6 M9 + 0 1 T h e d i s t i n c t i o n between kaolinite and Figure 7. Phase Relationships in chlorite in sediments, 14 A. Minerals however rudimentary First- and second-order net amplitudes the chlorite developare + with respect t o origin a t layer center and third-order amplitude is ment may be, is thereDeficiencies of interlayer scattering matfore an important criter strengthen first order and weaken second and third. Excesses have oppoterion of the environsite effects ment of deDosition. Chlorite ‘layers are of 14 A. thickness, and montmorillonite under laboratory conditions is often of about 14 A. thickness. Therefore the rudimentary chlorite intermediates include mixed layer assemblages of 14 -4.layers with 14 A. layers in varying proportions, and the form factor for chlorite differs significantly from that for montmorillonite. These intermediates may or may not have Essociated with them minor proportions of kaolinite (which is of i A. layer thickness) which contributes diffracted radiation a t angles coincident with those of their own even-order reflections, as well as admixtures of more typically developed chlorite or montmorillonite grains, and (extraneous to this discussion) illite grains.
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A schematic diagram for comparison of the relations of montmorillonite and chlorite form factors in the lowangle region is
ANALYTICAL CHEMISTRY
730
h
plex manner in which these features modify real sequences is not yet worked out. Currently the best distinctions are made by following the intensity trends in diffraction from complex mixtures with application of heat. In ranges up to 200" or 300' C., montmorillonite layers collapse to 9.5 or 10 A. either in monomineral or mixed layer assemblages; chloritic layers deviate in form factor and later collapse in ranges from 300" or 400" C. up to as much as 600" C. ; kaolinites become for practical purposes amorphous in a narrow temperature range at about 550" C. A supplementary diffraction diagram of the assemblage treated with a liquid, like ethylene glycol, which swells only montmorillonite layers, is often also helpful. Several specimen spectrometer traces are illustrated in Figures 8 and 9. Although these principles do not yet necessarily demonstrate that all analyses will be successful, they seem to indicate clearly that sufficiently detailed examinations of any natural suite can be depended upon to minimize sharply the number of doubtful characterizations. LITERATURE CITED
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Figure 8. Smoothed Traces of Typical Spectrometer Records for Natural Mixtures Including 14 A . Minerals Cpper. Illite. chlorite, and quartz Lower. Chlorite diffraction maxima altered i n intensity b u t not i n position by rapid firing u p to 550' C. Traces from intermediate temperatures showed intermediate intensity changes
sketched in Figure 7 . For each, the net summed amplitude contributions are normally positive for the first and second orders and negative for the third. The sketch relates the distribution of scattering matter in a cell with the cosine of the phase angle by which the scattering of various components lags an arbitrary reference at the layer midpoint. Algebraic sums of the p r o h c t s of relative electron densities by their respective phase angle cosines establish the scattering amplitudes of the respective orders of diffraction. Chemical accumulation of interlayer matter thus weakens a first order for niontmorillonite and strengthen8 the second and third orders. Expulsions of matter, as by heat from chlorite, have the reverse effect. In comparison with Figure 4, the chlorite form factor is generally much the same a~ montniorillonite, but includes an additional distinct minimum, near zero, sin e at __ values near 0.05,
Bradley, W.F., Am. Mineralogist, 30, 704-13 (1945). Ibid.,35, 590-5 (1950). Bradley, W.F., J;4Am.Chem. Soc., 67, 976-81 (1945). Brindley, G. W., X-Ray Identification and Crystal Structure of Clay Minerals," London, Mineralogical Society, 1951. . . Brown, G . , and MacEwan, D. M. C., J . Soil Sci., 1, 239-53 (1950). (6) Grim, R. E., Bradley, W. F., and White, W. A., A m . Minerdogist, 37, 293 (1952) (Abstract). (7) Hendricks, S. B., and Teller, E., J . Chem. Phys., 10, 147--66 (1942).
(8) MacEwan, D. 11,C., discussions at First Xational Conference
on Clays and Clay Technology, Berkeley, Calif., 1952. (9) MacEwan, D. M. C., Trans. Faradag Soe., 44,349-67 (1948). (10) Mbring, J., Acta Cryst., 2, 371-7 (1949). (11) Ross, C. S., and Hendricks, S.B., U . S.Geol. Survey, Profess. Paper 205B (1945). RECEIVED for review Xovember 19, 19.52. Accepted January 2 6 , 1953. Published with the permission of the Chief of t h e Illinois State Geological Siirvey.
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ues near 0.07, exact positions and magnitudes varying w i t h c h e m i c a l composition. The com-
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followed by a strong posisin 8 tive maximum at x val-
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Figure 9. Smoothed Traces of Typical Spectrometer Records Illite, chlorite-montmorillonite random mixture, and kaolinite Chlorite-montmorillonite maxima diffused and shifted i n position by rapid firing up to 450' C. Kaolinite maxima not influenced C p p e r r i g h t . Montmorillonite, chlorite, illite Lorcer right. Montmorillonite collapsed to approximate coincidence with illite, with small chlorite maxima retaining original positions after rapid firing t o 600' C. C p p e r left.
Lower l e f t .
