The Nature of Illite

INTRODUCTION THE NAME illite was proposed by Grim, Bray and Bradley in 1937 for the mica-type mineral occurring in argillaceous sediments. It was specifically stated that "the term is 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". Abundant research has shown that many soils, clays and shales contain a mineral with a lOA spacing that does not expand on treatment with glycol. Commonly, because of the complex association of the loA mineral with other clay minerals, its relatively low degree of order and its small particle size, the polymorphic form of the mineral may not be absolutely determined in all cases. Neither can its precise chemical composition be determined. However, chemical data indicate that this lOA mineral has a variable but substantially lower potassium content than well-crystallized micas and that its tetrahedral and octahedral cation population is variable. Experience, it is believed, has shown that it is desirable to apply a general name illite to such clay minerals rather than to call them simply micas or hydro-micas, since this does not distinguish them from well-crystallized micas that may have somewhat different chemical compositions and whose polymorphic form can be definitely determined. Some of the illites described in the literature are undoubtedly mixed-layer assemblages. Other illites give X-ray diffraction data that do not suggest any mixed layering. Consequently, the lower potassium content of illite cannot be ascribed universally to mixed-layer assemblages. 33 34 THIRTEENTH NATIONAL CONFERENCE ON CLAYS AND CLAY MINERALS The present paper presents analytical data for several clay-mineral assemblages ranging from illite without any mixed layering to illite-mixedlayer structure. In the case of the mixed-layer assemblages, the evidence indicates that the lOA component has less potassium that well-crystallized micas. Such assemblages may then be best described as illite-montmorillnoite, illite-vermiculite, etc, mixed-layered structures. E X P E R I M E N T A L Sample Description The samples of this study range from Ordovician to Pennsylvanian in age. They include Beavers Bend illite, from near Broken Bow, Oklahoma; illite from Fond du Lac County, Wisconsin, called Marblehead illite in this study; Rock Island Ulite, from near Geneseo, Rock Island County, Illinois; Fithian illite from Vermillion County, Illinois, which has been previously described as API reference clay mineral no. 35; and an illitic material from the Goose Lake area, Grundy County, Illinois, which is marketed at present under the trade name "Grundite". The Beavers Bend sample is composed of a well-crystallized illite of exceptional purity, with minor amounts of chlorite as a mechanical mixture reported from some samples. It is a fine-grained dark olive-gray clay that is found in layers in the Blaylock formation of Silurian age. This illite has been described in detail by Mankin and Dodd (1963) and, owing to its exceptional purity, has been proposed for use as a reference clay mineral. The Marblehead illite, a gray laminated clay, which occurs in pockets in a Silurian dolomite at the Western Lime and Cement Company Quarries near Marblehead, Wisconsin, is also an exceptionally pure illite containing no detectable quantities of any other clay minerals (Gaudette, 1965). The Rock Island illite was collected from pockets in an Ordovician limestone at the Cleveland Quarries near Geneseo, Illinois. This illite is light gray, well-crystallized, and, like the Beavers Bend iUite, contains a mechanical mixture of small amounts of chlorite in some samples. The description of this material is now reported for the first time. Both Fithian illite and "Grundite" are from Pennsylvanian underclays. Fithian illite from Vermillion County, Illinois, was first described by Grim, Bray and Bradley in 1937 and was one of the bases for the proposal of the term illite as a general term for the clay-mineral constituent of argillaceous sediments belonging to the mica group. More recently, Molloy and Kerr (1961) have described Fithian illite as Illite no. 35 in the A.P.I, project on reference clay minerals. The light-green, nonbedded, underclay from the Goose Lake area, Illinois, was first described by Grim and Bradley (1939). Production of this underclay at that time and up to the present has been made under the trade name "Grundite", the name by which this material will be designated in this study. THE NATURE OF ILLITE 35 The bulk material for each of these samples was disaggregated by gentle stirring in distilled water. Fractionation of the <2n material was accomplished by settling in distilled water, with the suspension containing the <2ix clay drawn off and consoUdated by drying. This <2/x fraction was then utilized for all the analytical procedures. Chemical Analyses The chemical analyses of the illite samples (Table 1) show a restricted range of silica and alumina contents, with the most prominent relative variation occurring in the iron and magnesia. The potash content, which ranges from 5.90 per cent for Fithian illite to 7.98 per cent for the Marblehead sample, is of special interest and is significantly below the theoretical potash content for the ideal muscovite structure. TABLE 1. CHEMICAL ANALYSES OF ILLITBS