PHYSICO-CHEMICAL STUDIES OF SOME TRANSFORMED MONTMORILLONITE CLAYS

Abstract

Silicon and aluminum are the most abundant of the elements constituting the earth. Clay-the powder obtained from the disintegration of rocks,is,chemically,ferro-aluminosilicate, of varying composition,with some other elements like sodium,potassium,calcium,magnesium etc.thrown in.Physically it consists of particles of diameters ranging from 1.0 mm to 0.00001 mm in varying proportions. The definition of soil(l)as a mass of inorganic material that holds inorganic and organic colloids,dead and living plant and animal material,water and gases in variable but balanced proportions has lost much of its original significance. Soil is much more complex than what this definit ion conveys. Broadly speaking^it consists of four components, namely,mineral materials,organic matter,water and air.The most important and yet highly intriguing constituent,to which the soil complexity may be attributed,is its mineral matter, consisting of particles of varying size;those in the finer state of subdivision(<C0.002 mm) form the clay fraction of the soil and the coarse ones consist of mainly rock debris(2). Clay minerals study and Its importance: Clay minerals have been studied for a great many years, but within the last thirty years,great strides have been made not only by soil chemists,but by investigators from very many different disciplines.This has been possible mainly due to increasing importance of clay mineralogy in industry and technology(3),substantiated with the results in the field of -2- mineralogy and emergence of new instrumental techniques. In the ceramic industry,only some particular clays can be used to manufacture these ceramic products. Studies of the changes taking place when clay minerals are heated to elevated temperatures have greatly enhanced our knowledge in this field, (4,5). In the oil industry,certain types of bentonite clays are essential,for the preparation of the muds,required for the drilling of oil wells,while a different type of bentonite, forms the basis of many of the catalysts,used in the refining of petroleum products,(6,7). Certain types of kaolinites are used as fillers and coating materials in the paper industry. Research into structure and properties of kaolinite has resulted in improvements of such paper properties as acceptance to ink, rate of drying etc., (8,9). On the basis of the empirical laboratory data the engineer has tried to ascertain conditions for soil stabilization, (10). Similarly for the geologist and the agricultural research workers,the treatment of the subject depends to a very large extent on the proper knowledge,of the clay mineral composition, of the soil and rock debris, (2,11,12). Origin of clav minerals: All the various clay minerals,with the possible exception of attapulgite-palygorskite and vermiculite,have been authentically reported in hydrothermal bodies. In many -3- hydrothermal clay bodies a zonal arrangement of clay minerals has been observed. Often there is an inner halo of sericite (white mica alteration product),an intermediate zone of kaolinite,and an outer zone of montmorillonite and chlorite; chlorite being most abundant on the outermost fringe, (2). On the basis of their classic study at Montana,Sales and Myer(l3) concluded that alteration is essentially contemporaneous with ore deposition. Lovering (14) and Kerr (15) have described wall rock alteration,at numerous deposits, which seems to have developed in stages. Chloritic mica and vermiculite were reported as minor constituents,in a clay mineral analysis, of series of sediments from Eastern France and Germany by Millot(l6). The illitic and chloritic clay minerals found in slates have a higher crystallinity and are composed of layer units more regularly oriented. Grim et al(l7) identified chloritic clay mineral, in some samples from the Gulf of Qalifornia,and the Pacific Ocean off the California coast,and also concluded from their data that kaolinite is very slowly being lost in these marine sediments,probably by alteration to illite or chloritic mica. It seems certain that chloritic mica and illite also tend to form during marine diagenesis from other minerals. Laboratory synthesis of clay minerals: a. Synthesis from mixtures of oxides and hydroxides at elevated temperatures and pressures:- Noll(18) working with silica and alumina gels,between 250° - 500°C, claimed the formation of kaolinite with some other different phases. On the other hand,Roy(19) has shown that halloysite may be the -4- resulting phase in the system Al203~Si02~H2° rathQr than kaolinite. b. Synthesis from mixtures of crystalline minerals and chemical reagents at elevated temperatures and pressures:- According to Gruner(20) kaolinite,pyrophillite,muscovite and boehmite are the phases formed,when micrqline and albite are subjected to high temperatures and""pressures,in presence of Al(0H>3,Si02 and KC1 under acid conditions .Badger and Ally (2l) produced kaolinite,on heating potash feldspar for 24 hrs., at 225°C under a pressure of 1800 P.s.i., in presence of 5% hydrofluoric acid. c. Synthesis from mixtures of oxides and hydroxides at ordinary temperature and pressure:-Sedletsky(22)mixed sodium silicate and sodium aluminate,leached it with 1 N MgCl , washed the product and after four years of ageing,identified a product similar to montmorillonite. Caillere and Renin (23) reported the synthesis of clay minerals,by electrolysis of solution of silica,or alumina-silica mixture. The product depended on the nature of the anode used. Aluminum anode gives kaolinite while with a magnesium anode antigorite is formed. d. Transformations of clay minerals at ordinary temperature and pressure:- Many investigators including Caillere and Henin (24),Volk(25),Aleshin(26) and Barshad(27) have shown that a material substantially like illite is produced, from montmorillonite when all its exchange positions are occupied by potassium ions, and the material is dried at 110°C. Formation of kaolinite from montmorillonite,by its -5- treatment with 2C# CaCl2,0.5# NagAlOg and 10# A1(NG^)3 for 3-4 days and then with HC1 or NR"40H has been reported by Caillere and Henin(28). The latter authors(29) have further shown that the treatment of montmorillonite,under certain ++ conditions,with a solution containing Mg ions so that all the exchange positions are occupied by the magnesium ions, produces a product having the characteristics of chlorite. Synthesis of chlorite: Most of the work cited in the literature on chlorites has been done by Caillere and Henin. The widespread occurrence of different varieties of chlorites,in soils and sedimentary rocks,has been explained frequently as the result of precipitation of hydroxides of magnesium,iron,or aluminum between the unit sheets of expanding clay minerals. This explanation has been supported by reports of the laboratory preparation of chlorite like materials as a result of the precipitation of hydroxides in montmorillonite suspensions. Tne original description of this process was given by Caillere and Henin (loc.cit.). Subsequent papers by Caillere and Henin(30),Longuet Escard(31) and Youell (32) have indicated,that the hydroxides or hydrous oxides, of magnesium, aluminum,nickel,cobalt,zinc and ferrous iron, can form montmorillonite-hydroxide complexes. A wide variation exists under which these chlorite like structures have been prepared. Caillere and Henin (loc.cit.) used fairly concentrated suspension-solution mixtures, containing 6% clay and 1 N to 4 N concentration of MgClg. The precipitation of magnesium hydroxide was -6- carried out by dropwise addition of ammonium hydroxide solution accompanied by vigorous shaking. Longuet Escard (loc.cit.), on the other hand,reported the formation of aluminum hydroxide and nickel hydroxide complexes with montmorillonite,in systems containing 0.5 - 1.0 jS clay and less than 0.1 N concentration of metal nitrates.Youell (loc.cit),reported,without details,a successful electrolytic method of precipitating magnesium and zinc hydroxides within the interlayer space of montmorillonite. Slaughter and Milne(33) have prepared chlorite»like complexes of montmorillonite with magnesium hydroxide and other hydroxides,under a variety of physical and chemical conditions. Requirements for the preparation of these complexes have been simplified considerably by the use of rapid mixing techniques. A complex may be formed either by precipitating the hydroxide in a clay suspension,or by preparing the precipitate separately and mixing it with the clay suspension immediately. These studies had been under taken to assess the possible geological significance of this method of formation of chlorite. Structure of clay minerals: The older concepts that the clay minerals are particles more or less spherical,have now been replaced by newer one,Shaw(34). It is now well established that the particles are laminated,i.e., made up of layers of plates or flakes, the individual size and shape depending upon their mineralogical organisation and the conditions in which -7- they have developed e.g.hexagonal blades,rods or fluffy. For long,clays were considered to be composed of amorphous matter and all attempts to interpret their properties had to be based on the findings of strictly chemical methods of investigation. Extensive studies of X-Ray diffraction patterns have revealed that even the finest fraction of clays are crystalline in nature and composed of comparatively few and simple building units,(35,36). The difference in the properties of various clays is now believed to depend on the spatial arrangement of these units in the crystal lattice. The structure of the common clay minerals have been determined in considerable details,by numerous investigators, based on the generalisations of Pauling(37). Two structural units are involved in the atomic lattice of most of the clay minerals. The first consists of a silicon tetrahedron,in which a silicon atom is equidistant from four oxygens or hydroxyIs,the silicon being in the centre. The second unit consists of two sheets of closely packed oxygens or hydroxyls in octahedral co-ordination,so that they are equidistant from six oxygens or six hydroxyls. Combination of these structural «nits(held by chemical forces),with modifications, finally gives rise to the structure of clay minerals,that are found in the colloid clay fractions of soils, a. Structure of kaolinite:- It is a hydrous aluminum silicate of approximate composition Al203 2Si02 2H20. Its structure was first suggested by Pauling(loc.cit.) and worked out in -8- details by Gruner(38) and Brindley et al (39). In kaolinite, a single silica tetrahedral sheet is topped with a slightly distorted gibbsite sheet,both being formed by condensation and splitting off of water between adjoining hydroxy! group in vertex position. All the tips of the silica tetrahedrons point in the same direction and towards the centre of the unit made of silica and alumina sheets. The mineral can thus be described as,having a 1:1,non expanding lattice.Structural formula may be expressed as (0H)g Al4Si40l0 and the size of the unit cell is 7.2 A. The space lattice is such that there is little substitution of ions in its structure and there are no unsatisfied valencies on the cleavage surface. Cation exchange capacity (c.e.c.) is insignificant, b. Structure of montmorillonite:- The mineral was first studied by Le Chatelier(40) and has been assigned the formula (OH) Al4Sig020 x RgO. Ross and Hendricks(41) established the identity of montmorillonite as a definite clay mineral species. Structurally, the mineral consists of three layers,a gibbsite sheet enclosed between two silica sheets with their vertices pointing towards each other and towards the centre of the unit. X-Ray studies have shown stacking of siLica-alumina-silica units in the c-directlon, layers being continuous in a and b directions