Chapter VI CLAY MINERALOGY Chapter VI CLAY MINERALOGY Introduction Because of the importance of clay materials in ceramics and other industries, in agriculture, in geology, and elsewhere, their investigation goes back far into antiquity. From the very beginning, it has been observed by the various investigators, that clays and soils very widely differ in their physical and chemical properties. These variations are not in the amounts of the ultimate chemical constituents, but also in the way in which they are combined, or in the manner in which they are present in various clay materials. In the older literature, a considerable number of concepts were suggested to protray the fundamental and essential components of all clay materials and to explain their variations in properties. Until very recent years, there has been no adequate analytical tools to determine with any degree of certainity the exact nature of the fundamental building blocks of most clay materials. But during past few decades, considerable investigation of clay materials has been made by using the modern techniques such as spectrophotometric analysis, X-ray 104 diffraction, Infrared, Differential thermal analysis, Scanning electron microscopy etc. These investigations have thrown more light on the presence of clay minerals, their crystal structures, chemical characters, water content, impurities present etc. which are responsible for physical properties, of clay materials which are mainly considered for their suitability in various industries. In the present Chapter, the mineralogy of the clay samples from Sindhudurg district is presented. The results have been obtained by using X-ray diffraction, Infrared, DTA, TG, and Scanning electron microscopy. Definition of Clay : The term 'clay' is being used as a rock term and also as a particle size term in mechanical analysis of sedimentary rocks, soils etc. for years together. In general the term clay implies a natural, earthy fine grain material which develops plasticity when mixed with water. The essential chemical components of clay are silica, alumina and water. Iron, alkalis, and alkaline earths are nearly always present. The term clay is used for material that has a variety of origin, such as i) product of weathering, ii) formed by hydrothermal action on pre-existing rocks or iii) sedime­ tary deposition in river or lake or sea. Wentworth (1922) asssigned clay grade to material less than 4 microns in lOS size, whereas soil scientists have placed clay grade at 2 microns. But most of the clay fraction occurs with size less than 2 which appears to be a convenient limit. During the earlier days, Kaolinite was considered as the essential clay mineral and the other material occuring as impurity. Soil scientists held the view that the essen­ tial component of all clay materials was a colloid complex. Clay Minerals : Clay minerals are phyllosilicates or layer silicates. They consist of layers with two types of units involved in each layer, the tetrahedral and the octahedral. During the two decades 1920-1940, there were several contributions made towards the study of clay material on the basis of X-ray diffraction analyses. Hadding (1923) and Rinne (1924), regarded the clays as independent materials and not amorphous material or colloids. Microscopic and chemical study of clays by Ross and Shannon (1925, 1926) suggested that the components of clay materials are essentially crystalline which may be called as clay minerals. Ross (1928) proposed a classification of clay minerals. Ross and Kerr (1931) and Correns (1936) gave widely accepted concept of clay mineral. According to this concept, clays are considered to be composed of extremely fine crystalline 106 particles, w~ich are essentially hydrou:; aluminium silicates. AJ. kla 1 is and alkaline earths are present in some clay minerals wh ; l.!=: in some others, magnesiun1 or iron partly replace some of the aluminium. In addition to clay minerals, some of the non-clay minerals, organic matter and soluble salts also form important constituents of clays. Ross and Kerr (1931, 1934), Ross and Hendricks (1945) made detail study of kaolinite, halloysite and montmorillonite respecti­ vely. Hedricks and his co-workers (Hedricks 1938, 1941, 1942, 1945, Hendricks ~.al. 1936) made significant contri­ butions by way of giving the physico-chemical properties of different clays. Different aspects of clays have been studied by Grim, Bradley and their co-workers (Grim, 1947, 1948 , 1950, Grim e t. a l. 1935, 1937, Bradley, 1945). The credit of cre ation of s ynthe tic clay minera ls in the laboratory for the first time, goes to Noll (1935). Orcel (1933) was the first to apply the techique of DTA in the study of cla ys. Brindley and Robinson (1946a, 1946b) studied in g reate r details the properties of kaolin minerals. Glaeser (1948), Mering et.al. (1950) studied ad s orption of various lons on clay mine rals and the ir textures using electron microscope. Mukherjee et.al. (1946) and Chatte rj e e (1949) have studied the e l ectrochemistry and chemistry of clays from India. 107 Since 1950, there has been a considerable addition of literature based on the various aspects of clays. X-ray identification and crystal structure study by Brindley (1951) and Brown (1961). Carroll (1970) published a guide to clay mineral identification using X-ray techniques. Grim (1953), (1962, 1968) published books on 'Clay Mineralogy' and 'Applied Clay Mineralogy'. DTA investigations of clay edited by Mackenzie (1957), 'Geology of clays' by Millet, (1970). 'Atlas of infrared spectroscopy of clays and their admixtures' and 'Atlas of electron microscopy of clays and their admixtures' by Vander Marrel and Beutel­ spacher (1968 and 1976) and 'Clay Minerals' (Nemecz, 1981). Classification of clay minerals : Several workers have attempted classification of clay minerals, mainly based on the structural characteristics as there is a wide range of chemical composition not only within a single group, but also within single mineral. Caillare and Henin (1956) proposed a tabular classifi- cation of sheet silicates. This was later modified by Caillere in 1960 by Millet (1970). It consists of six tables, one for the general classification of hydrated silicates, and kaolinites and serpentinites with real basal spacing of 7 ~' micas and dioctahedral montmorillonites 108 with basal spacing of 10 ~' micas and trioctahedral montmori­ llonites with basal spacing of 10 ~' chlorites having basal spacing of 14 ~' and for mixed layer complexes having varying basal spacing, are the other five tables classifying major groups of clays. Deer, Howie and Zussman (1962, 1976) have classified clay minerals into five main groups, again on the basis of characteristic basal spacings. 1. Kandite group (7 ~) - kaolinite, dickite and nacrite, anaunite, halloysite, metahalloysite and allophane. 2. Illite group (10 ~) - illite, hyrdo-micas, phengite, brammallite, glauconite and celadonite. 3 . Smectite group (15 ~) montmorillonite, nontronite, hectorite, saponite and sauconite. 4. Vermiculite (14.5 ~). 5 • Palygorskite group palygorskite, attapulgite and sepiolite. The clay minerals attapulgite and sepiolite from the palygorskite group have chain like crystal structures and are less common. However, the mixed layered minerals which occur very commonly in nature have no place 1n this 109 classification. Though there are few more classifications available, such as those by Franke-Kamenotskii (1961), Jasmund (1955), the classification scheme for phyllosilicates proposed by AIPEA etc. The one proposed by Grim (1968) and reproduced below is more useful and takes into account not only structure but shape and also expandibility. Classification of the Clay Minerals by Grim (1968) I. Amorphous group Allophane group II. Crystalline A. Two layer Type 1. Equidimensional Kaolinite group - kaolinite, nacrite etc. 2. Elongate Halloysite group. B. Three layer types 1. Expanding lattice a. Equidimensional Montmorillonite group - montmorillonite, Sauconite etc,. Vermiculite b. Elongate Montmorillonite group - nontronite, saponite, hectorite 110 2. Non-expanding lattice Illite group c. Regular mixed layer types Chlorite group D. Chain - structure type Attapulgite Sepiolite Palygorskite The important clay mineral groups relevant to the present work are described in brief below The Kandite ·. gro~p : : The kandite group include a group of minerals characte- rised by a basal spacing of 7 ~. Compared to the clay minerals from other groups, the kandites have a very restri- cted range of chemical composition. Kaolinite, diokite and nacrite and halloysite are described from this group. Kaolinite : It is chemically a hydrated Al silicate Silica tetrahedra are linked with the aluminium at their centers by a group forming a single layer. Several such layers are stacked together with a periodic spacing of 7 ~' this is clearly reflected in the X-ray diffractobands. On heating 550°C to 600°C, the structure co 11 a p s e s and the 7 ~ peak is lost in the diffractograms. These two features were also observed in lll the present study in the clays from Sindhudurg district. Dickite and Nacrite : These are the minerals usually found in hydrothermal deposits. They show chemical affinity with kaolinite but crystallise in monoclinic system in contrast to kaolinite. There lattice structures are different. They show an intense pask at 7.1 ~but the structure remains stable after heating to 600°C. Halloysite : It is known to occur in two forms with basal spacing 10 ~and 7 ~ in two forms,