140 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 con- stituent 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) sum- marized 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 sedi- ments 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 mont- morillonite-type layers. Many" clay mineral constituents of argil- laceous sediments belonging to the mica group" give X-ray reflec- tions 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` (16) 142 still are found frequently in the literature. These are'sericite' and hydromuscovite'. Although these minerals may qualify to be ' in- cluded 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 chemi- cally different from muscovite and appear to exhibit all the known polytypes of muscovite 7). However, SCHALLER") has described min- erals 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 musco- vite. 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 con- cerning the clay micas must arise from powder diffraction data. Indexing of powder diffraction data useful in determining the dif- ferent 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-di- mensional 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 char- acteristics 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 (18) 144 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/mont- morillonites, 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