ILLITE Sally A. WENTWORTH Department of Soil Science

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Clay Science, Vol. 3, No. 6, pp. 140-155, 1970.

ILLITE

Sally A. WENTWORTH

Department of Soil Science, University of Guelph, Guelph, Ontario

Introduction `Illite`-much confusion has developed regarding the meaning of this term. The confusion originated mainly from attempting to attach more precision to the illite definition than was possible with the available knowledge about the nature of these clays. In view of the numerous investigations relating to the nature of illite, a review to summarize the understanding of illites that has resulted from the last 30 years of research seems appropriate. Prior to 1937, clay minerals characterized by 10A basal spacings were called such names as hydromica, hydromuscovite, glimmerton, sericite, sericite-like,.... GRIM, BRAY and BRADLEY (16)considered that all of these terms were unsatisfactory and that a new term was needed" to indicate, for example, that the clay mineral mica in argillaceous sediments is distinctive from previously named species." The new name proposed for" the mica occurring in argillaceous sediments" was `illite`. It was" not proposed as a specific mineral name, but as a general term for the clay mineral constituent of argillaceous sediments belonging to the mica group." It was hoped that later, when these clays could be better defined, a specific name could be applied. Difficulties arise because the clay mineral constituent does not consist of only one mica-like clay mineral. In many cases illite has been regarded as a specific mineral name, and the characteristics of particular mica-clay minerals have been regarded as those of a specific mineral, illite. Since these mica-clay minerals are not only extremely difficult

(15) 141 to characterize but also are not all the same, much confusion has been caused by treating them as examples of a specific mineral, illite. Through the years many additional qualifications have been added to the meaning of the term. YODER and EUGSTER 52) summarized various published statements more or less defining illite and drew attention to their mutual inconsistencies. Subsequently, BRA- DLEY and GRIM 2) re-emphasized the original description of illite, with the addition that the original definition of illite" might well have read.... those of the clay mineral constituents of argillaceous sediments belonging to the mica group." In the second edition of Clay Mineralogy, GRIM writes 15)".... the term illite has now been widely accepted for a mica-type clay mineral with a 10A c-axis spacing which shows substantially no expanding- lattice characteristics". The broadness of this description leaves open the much discussed question whether illite can be regarded simply as a fine-grained mica, or whether it of necessity contains interstratified layers of more hydrous composition, such as montmorillonite-type layers. Many" clay mineral constituents of argillaceous sediments belonging to the mica group" give X-ray reflections indicating such random interstratifications, but some illites may have a sequence of layers of uniform composition and structure, which possibly differ from the layers in muscovites and in other macroscopically crystallized micas. It seems that if illite is defined in the general sense given by GRIM, BRAY and BRADLEY, then one cannot specify that it is or is not an interstratified mineral. In a report on the status of clay mineral structures, BAILEY 1) has summarized his opinion concerning illite as follows:" The writer's interpretation of the consensus of recent studies on the nature of sedimentary illite is that it is a heterogeneous mixture of detrital 2M1 muscovite, detrital mixed layer micaceous weathering products, detrital weathering products partly reconstituted by K-adsorption or by diagenetic growth of chloritic interlayers, plus true authigenic 1Md and 1M micas.... some having mixed layering also." Two terms used previous to the introduction of the term `illite`

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still are found frequently in the literature. These are'sericite' and hydromuscovite'. Although these minerals may qualify to be ' included under the term illite, they are often used to emphasize a readily identifiable property. Sericite is a term used to describe any fine-grained white mica, usually muscovite or paragonite. Sericites are not necessarily chemically different from muscovite and appear to exhibit all the known polytypes of muscovite 7). However, SCHALLER") has described minerals which he refers to as " high-silica sericites," and thus to ac- commodate the additional Si, there is also a charge deficiency on the octahedral layer to be balanced by the interlayer cation. Hydromuscovite is a term used to emphasize a higher water content and lower potassium content than usually occurs in muscovite. Hydromica and hydrous mica are terms which are sometimes used with the same meaning as hydromuscovite. The source of this variation in composition, just as with illite, is open to question. It should be noted that understanding of mica structures has come mainly from single crystal analyses, whereas information concerning the clay micas must arise from powder diffraction data. Indexing of powder diffraction data useful in determining the different polytypic forms is given by YODER and EUGSTER 51, 52)and by SMITH and YODER41). Analysis of diffraction effects from samples with mixed-layering is given by MACEWAN et al. 26) who have compared calculated with observed scattering distributions for mixed-layered clays. Recent advancement in studying the nature of mixed-layering has resulted from development of a computer program to calculate one-dimensional diffraction profiles, REYNOLDS and HOWER36). The effects of particle size distribution, chemical composition, and convolution factors as well as proportions of layers and interstratification type are included in the calculated profile. Comparison of these calculated profiles with the X-ray diffraction patterns of natural illites and illite-montmorillonites of known chemical composition has led to the conclusion that there are three main types of interstratification: (17) 143

1) random, 2) allevardite-like ordering, and 3) superlattice units con,- sisting of one montmorillonite and three illite layers (IMII).

Characteristics of Illite

Following the introduction of the term illite, 16)many investigators examined particular minerals which they regarded as examples of illite. Since all illites are not the same, properties that are char- acteristic of a single mineral cannot be specified. From the many investigations describing illite samples, 6,11, 14, 25, 27, 28, 29, 46) general characteristics can be noted. Illites, minerals with 10A spacings that are predominately non-expanding with glycol, often occur in soils, clays and shales. They have a small particle size (roughly <2ƒÊ)

and a low degree of order such that their polytypic form is usually

determinable only as Md. The majority are dioctahedral, although

some instances of trioctahedral materials have been reported 42, 47).

Analytical difficulties resulting from the small particle size and the

abundance of inseparable mixtures of several clay minerals do not allow determining the . precise chemical composition of illites.

However, chemical data indicate a lower potassium content, higher

water content, and more variable tetrahedral and octahedral popu-

lations than for macro-crystalline micas.

Recent investigations have been directed toward questions con-

cerning the nature of the extra water, the low potassium content,

and whether illite of necessity is mixed-layered material.

Data of MEHRA and JACKSON") suggested that an illite with less

than 10% K2O (compared with 11.8% K2O for 'ideal' muscovite)

should contain some expanded layers with all the potassium concen-

trated between the contracted layers. This conclusion was based

on glycerol sorption measurements where assuming 10% K2O for the

10A, illite, layers and a planar sorption surface of 760-808 m2/g for the expandable layers gave a constant sum for the interlayer sorption surface and equivalent K2O (mica) surface for several illite, mixed- layer clays. In accordance with this concept, WEALER46) determined the potassium content of 249 illite samples by relative intensity

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measurements of the 001/002 reflections, and concluded that "strongly bonded illite-like layers commonly have 9-10 per cent K2O and any significant smaller values indicate the presence of expanded layers or weakly bonded 1Md....contracted layers." However, GAUDETTE, et al.12) have done detailed chemical and X-ray diffraction analyses for several illites concluding that although some illites are undoubtedly mixed-layer assemblages, others give X-ray diffraction data with no indication of mixed layering and consequently the lower potassium content of illite cannot be ascribed universally to mixed layering. Likewise, HOWER and MOWATT19) have examined 21 samples of illite and mixed-layer materials showing that the relationship between potassium content and per cent mica layers is excellent, but terminates at a potassium content significantly less than that of a true mica . They also point out that even the best illites do not approach the sericites in terms of tetrahedral charge and conclude that montmorillonites, mixed-layer illite/montmorillonites, and illites form a continuous sequence. It was believed that the " illite end of the sequence appears to be distinct from true dioctahedral micas." Further investigation18' of the relation of mixed layering to illites was done by comparing curves from observed data for per cent expandable layers vs. total lattice charge with curves obtained from theoretical models of interlayering sequences . The results led to the conclusion that there is an ordering of high-low charge 2:1 units in illites and illite/montmorillonites: "The amount of ordering is variable , accounting for the vari- ability in expandability and cation exchange capacities of illite- montmorillonites with the same structural formula . Ordering of 2:1 units also explains how illites can achieve a non-expandable structure at lower lattice charges and potassium contents than true dioctahedral micas."

Early attempts to explain the extra water of illites were centered around a theory proposed by GANGULY9)and developed by BROWN

(19) 145 and NORRISH5. With the assumption that (H3O)+ occupies the interlayer position along with K+, the calculated structural formulae of two illites yielded interlayer charges equal to that of muscovite and reduced the octahedral cation population nearly to the ideal 2.00/ 010(OH2). Further support of the existence of (H3O)+ came from infrared studies by WHITE and BURNS49'. However in general, support of (H3O)+ in illites has not been forthcoming. WARSHAW44)was unsuccessful in attempts to synthesize illites of the BROWN-NORRISH type. She always obtained mixed phase assemblages when the starting compositions had the same Si/Al ratio and layer charge as in muscovite, but with deficient potassium. Both HOWER and MOWATTI9) and GAUDETTE, et al.12' calculated structural formulae for several illites according to both the BROWN- NORRISH procedures and to those of MARSHALL30). It was found that assuming (H3O)+ in the interlayer positions resulted in octahedral occupancy less than dioctahedral, whereas the Marshall method gave octahedral populations closer to 2.00/010(OH)2. These authors sug gested that the excess water in illite is present as neutral H2O trapped in the interlayer space. GAUDETTE, et al.12' calculated one- dimensional Fourier syntheses for. several illites, but the results were not adequate to prove which assumption for the structural formulae was more nearly correct ; it could only be said that the samples were dificient in potassium when compared to macro-crystalline muscovite. NELSON34),however, concluded from one-dimensional Fourier syn- these that the interlayer sites in illites were essentially anhydrous. The validity of the BROWN-NORRISHhypothesis was explored by RAMAN and JACKSON35)through studies of pH changes during the course of exchanging the interlayer cations in illites for Na in sodium tetraphenylboron solutions. They interpreted an observed increase in the pH of the solution as the layers expanded to preclude the possibility of the release of (H3O)+ along with K+ from the interlayer space. In hydrogen-deuterium exchange studies by MOUM and ROSEN-

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QVIST,32) the BROWN-NORRISH formula was used. Hydrogen was found

present in at least three different states in the sample. For the clay

examined, 13% of the hydrogen content at 110•Ž was considered to

be adsorbed water and 17% of the hydrogen content at 110•Ž was

considered to be present as (H3O)+ .

However, further studies by ROSENQVIST ,37' refuted this earlier proposal. He found still that water existed in three different forms,

but that the form which was neither adsorbed water nor normal OH

groups of the mica structure could not be explained as being (H3O)+ . Infrared studies showed that the OD band for this unknown water

nearly completely disappeared by heating at 300•Ž for five hours , making it improbable that the hydrogen was there as (D 3O)+. If it were (D3O)+, the cracking of the (D3O) group would have resulted in

a loss of D2O leaving one D+ in the structure , thus only 2/3 of the

deuterium would have been lost by heating . Similar results were obtained with tritium exchange studies ,37) further supporting the position that the extra water of fine-grained micas is not present as

(H3O)+.

In many illite analyses, insufficient care was taken in determining

water content, so that some of the excess water contents reported

may be due to trapped interlayer water of the low charged regions , improper purification of the samples , or incomplete removal of physically sorbed water. In fact, the procedures and temperatures at which to determine water are questionable8,43) . However, in instances where precautions have been taken in

determining the water content , an excess of water as compared to muscovite still remains in illite samples and the nature of this wate r continues to be unexplained .

Experiments of GARRELS and HOWARD10) have indicated that

keeping a powdered silicate in water may result in heterogeneous

particles with a structurally disrupted and chemically modified ex - ternal surface. Such a disordered surface layer is considered to be

related to the presence of extra-water (which is released by about

300•Ž in vacuo) measured by ROUXHET and BRINDLEY38) for a fine - (21) 147

grained muscovite and for a synthetic fluarphlogopite. Extra-water measured for the muscovite amounted to 16 moles/m2 which would correspond to 2.9% water if the powder had a surface area of

100 m2/g.

These results support the suggestion by BRINDLEY and WENT-

WORTH4) that some of the 'extra'-water found in illites may be associated with chemically altered hydrous surfaces. Comparison of coarse and fine fractions of muscovites and illites has illustrated the importance of sample preparation and investigational conditions as well as the ability to distinguish between surface and volume characteristics when studying fine-grained minerals48). Lack of attention directed toward these factors has resulted in much of the confusion about the nature of illites. The following table shows the extent to which pre-treatment of samples has been considered in several published illite studies, and the water contents as reported in chemical analyses. In the references cited no one has stated whether or not the samples were dried to constant weight or for a specific time period at the designated temperatures. In most instances the reader probably assumes that samples have been dried to constant weight, however this is not necessarily the case. In GROVES, Silicate Anal- ysis,17) the procedure for determining hygroscopic moisture is to heat the sample in an air-oven at 110•Ž "for at least two hours, though

a longer period will do no harm ".

Only two of the illites, Fithian and Beavers Bend, have been

analyzed chemically by different investigators. The large differences

in water contents obtained illustrate well the dependence of these values on conditions of investigation. For Fithian illite, the following are given:

For Beavers Bend illite, the data are:

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Since many chemical analyses are done on representative bulk samples, it could well be that some of the high water contents reported in the literature are due at least in part to inadequate clean-up' treatments before examination in addition to' surface effects which make conventional low-temperature drying insufficient. The form in which this extra-water is associated with illites, or with fine-grained minerals generally and the difficulties of separating surface-sorbe d water from structural water are problems not yet answered. This writer's opinion is that any water contents reported for fine-grained minerals with high surface areas should be considered questionable unless pre-treatment of the materials is ex- plicitly stated. In addition, it should be remembered that some weight losses for fine-grained micas in excess of the theoretical value could possibly result from ammonium ions in the interlayer region as demonstrated by YAMAMOTO and NAKAHIRA50)for several Japanese sericites. Influence of particle size on the character of micaceous minerals has been stressed by JONAS22) and by JONAS and ROBERSON23). Particle size exerts a strong control over the binding force between two highly charged silicate layers, and hence control over expansion. Qualitative results of JONAS and ROBERSON23)demonstrated that mica layers with high surface charge density can be expanded if their particle size is sufficiently small. A similar relationship was shown between cation exchange capacity and particle size22). As the particles become smaller, edge area increases, becoming an appreciable part of the whole. Total cation exhange capacity increases because of the larger proportion of ions located at edges available for exchange. Easily exchangeable ions at edges could result in unusual compositions at crystal edges, which would be reflected in bulk chemical compositions. JACKSON20)has shown that the potassium content of micas de- creases with decreasing particle size ; this could result from the ease of exchange of edge ions. A weathering model for micas proposed

(25) 151 by JACKSON21)is based on the formation of the weathering product as a "frayed edge" around the mica particles. Breaking off of frayed margins from weathered mica may result in the formation of beidellite, as discussed by BRAY3). Radial change inward has been observed in laboratory alteration experiments by SCOTT and REED40) who found that mica particles retained an inner core of 10A mineral as the 'weathered' fringe increased. The influence of the alteration process (total alteration of a particle vs. breaking of frayed edges) on the product formed has not been explored. Extension of the procedures of Scott and Reed may allow study of this question. High Cs sorption by illites, with no apparent structure change as determined by one-dimensional Fourier syntheses with X-ray diffraction, led GAUDETTE et all13).to propose a "core-rind" model for illite. The model proposed a structurally coherent silicate core sur- rounded by an incoherent silicate rind. The Cs sorption behavior was considered to be due mainly to competition of Cs with other ions of the skeletal incoherent rind portion of the illite.

Summary

Since the term 'illite' was first proposed in 1937 as a general term for the " clay mineral constituent of argillaceous sediments belonging to the mica group", many investigations of these minerals have been reported. Confusion has developed from attempts to make the term more specific than originally was proposed. It is emphasized here that the term 'illite' remains a useful and convenient abbreviation for "clay-grade, mica-like material", if one does not make the mistake of regarding the characteristics of any one clay as being the characteristics of a specific mineral, illite. Any further definition of the term only limits its use to circum- stances where the particular characteristics can be proved. Micaceous clays in sediments are not all the same ; illite is a non-specific term which can be applied to these minerals which are

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variable in composition, in degree of crystalline order and in their related physical and chemical properties. Much published information on illites may be either wrong or misleading, in that insufficient attention has been given to clean-up treatments to remove contaminants, to fractionation into well-sized fractions, and to conditions under which low-temperature water is removed. Furthermore, possible inhomogeneity of the particles must be considered in examining such high-surface-area minerals as illites. When the surface becomes a considerable part of the whole, it cannot be neglected when trying to interpret the properties of the material. A disordered, potassium deficient external surface sur- rounding a structurally coherent core is a possible explanation for the low K2O and source of 'extra'-water in some illites, but certainly does not explain the properties of all illites. The nature and amount of water related to illites and to fine- grained micas generally is a complex problem not yet solved but the surface properties are clearly important. There is little hope of interpreting illite analyses unless the experimental conditions for determining water are greatly refined over what is customarily done very careful decisions will be necessary for distinguishing water originating from surfaces and from the volume.

Acknowledgments

The ma jor portion of this review constituted a part of the literature search for the Ph. D. thesis. Grateful acknowledgment is made to Dr. G. W. BRINDLEY for his guidance during the thesis program and to the American Petroleum Institute, Research Project 53, which sponsored the fellowship for the thesis research.

References

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