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Relation of the Composition to the Properties of Clays

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Relation of the Composition to the Properties of Clays .tttfltlBffilt* »HH knlwtt HHi'JgRKfHUt rVI IIBtHfiW1MJ • t Mi FuH ! m.;....'. RttUj ililH iiii »B mmMWmmmmi '• i raoBRHB ••.•..••...'. L0GICAL ftSSft HlftyK m f,? SURVEY 3 3051 00004 306 STATE OF ILLINOIS HENRY HORNER. Governor DEPARTMENT OF REGISTRATION AND EDUCATION JOHN J. HALLIHAN, Director DIVISION OF THE STATE GEOLOGICAL SURVEY M. M. LEIGHTON, Chief URBANA CIRCULAR NO. 45 Relation of the Composition to the Properties of Clays By Ralph E. Grim Reprinted from Jour. Amer. Ceratn. Soc, 22 [5] 141-51 (1939). PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS URBANA. ILLINOIS 1939 [Reprinted from the Journal of the American Ceramic Society, Vol. 22, No. 5. May, 1939.) RELATION OF THE COMPOSITION TO THE PROPERTIES OF CLAYS* By Ralph E. Grim Abstract Most clays are composed essentially of minute particles of one or more of the clay minerals of which the kaolinite, montmorillonite, and illite groups are most important. The clay minerals occur in flake-shaped particles, possess base-exchange capacity, and exist in or are reducible to extremely small grain sizes on working with water. Different clay minerals possess these properties in varying degrees. The clay mineral component is the chief factor determining the properties of a clay. In general, plasticity and bond strength caused by the clay minerals decrease in the following order: montmorillonite, illite, and kaolinite. In many clays, the plasticity and bond strength mainly result from the presence of the montmorillonite minerals or some members of the illite group, although these constituents may compose only minor amounts of the clay. The influence of the clay minerals on other properties is con- sidered. The green properties of clays are also related to the character of the exchangeable bases carried by the clay minerals. The fundamental reasons for the differences in properties resulting from different mineral constituents and various exchangeable bases are considered. The properties of clays are related further to the effective size-grade distribution developed in actual use which frequently differs from the size-grade obtained by mechanical analyses. I. Introduction (1) Kaolinite The concept that most clays are essentially aggre- The chief member of this group is the mineral, kaolin- gates of very minute particles of one or more of a few ite, with the composition (OBQsAL^Oio. Dickite and minerals, known as the clay minerals, has been well nacrite with similar compositions but different crystallo- established in recent years. Because the clay miner- graphic forms are rare in sediments. Another member, als are crystalline, it follows that clays are composed anauxite, which differs from kaolinite by having a of material which is crystalline rather than amorphous. higher silicon and a lower aluminum content, is not a In addition to the clay minerals, some clays also con- common mineral. tain, usually in minor amounts, such constituents as quartz, limonite (ferric-iron hydroxide), and organic (2) Montmorillonite material. The so-called colloid content of a clay is the This group takes its name from the mineral, mont- percentage of these constituents, particularly the clay morillonite, with the composition (OH^AliSisC^o-x H 20. it smaller minerals, which contains in particles than a Although not normally written in the formula, mont- certain size. morillonite usually contains magnesium. Beidellite, in a few laboratories have lately yielded Researches reported to have a lower silicon and higher aluminum information the structure and certain proper- much on content, and nontronite, in which the aluminum has ties of the clay minerals. It is proposed here to con- been replaced by ferric iron, are usually placed in sider this information and to attempt to derive there- this group. Recent work has suggested that saponite picture of the of clays which will pro- from a make-up can be classed as a montmorillonite in which the alumi- understanding of such properties as vide a satisfactory num has been replaced completely by magnesium. plasticity, green strength, and shrinkage. It is pro- posed also to analyze the influence of specific clay (3) Illite minerals and specific exchangeable bases on certain This group includes the abundant and widely dis- properties of clay. Other factors, not considered herein tributed clay minerals which are similar but not identi- because they are of minor importance for most clays, cal with muscovite. So-called "hydromica" and "seri- also exert an influence on the properties of clay. cite-like" material belong in this group. The general formula of members of the group may be written II. Clay Minerals (OH)4K„(Ai4-Fe4 -Mg4-Mg6) (Si 8 -yAl„)O 20 . When y Mineralogical analyses of a large number of clays equals 2 and magnesium and iron are not present, this is have shown that there are three important groups of the formula for muscovite. In all illites which have been clay minerals. Clays, therefore, generally are composed studied, only a small amount of iron replaces the alumi- of a member or members of the three groups. num, and y is considerably less than 2, varying from * Presented at the Fortieth Annual Meeting, American about 1 to 1.5. After future work has indicated the Ceramic Society, New Orleans, La., March 29, 1938 (Gen- range of variation of composition within the illite eral Session on "Constitution of Clay"). Received August group, it may be desirable to give specific names to 1, 1938. Published with the permission of the Chief, Illinois State members. Geological Survey. Other clay minerals, such as halloysite, are known, 141 142 Journal of the American Ceramic Society, 1939—Grim but they will not be considered in the present paper be- Montmorillonite consists of structural units of one cause, in general, they are not important constituents of gibbsite sheet between two sheets of tetrahedral silica clays. groups (Fig. 1) stacked one above another in the direc- tion of the c-axis. The structural units are loosely held together with water present between them. The c A Si dimension varies with the H 2 content, and the mineral 60 is said to have an expanding lattice. Analytical data suggest that the A1+++ of the gibbsite layer may be + +++ ++ 9.6 -2I.4A xH2 replaced by Fe or Mg , in the former case yield- ing nontronite. The beidellite member of the mont- morillonite group, reported to have a lower silica-to- alumina molecular ratio, might suggest the possibility of a small amount of replacement of Si ++++ by Al +++ 4 + 2 (OH) in the silica tetrahedral sheets. The implications of the unbalanced character of the lattice resulting from some of these replacements will be discussed presently. C-AXIS Q'' 4 +2 (OH) CAS V CA£ 4-y Si y Al 60 4 Si ¥v¥\ To "SoS/MSj 6 O 10. OA b- AXIS- MONTMORILLONITE (0H) Al Si -xH 4 4 a 20 2 a(0H)+ 40 Fig. 1.—Schematic presentation of crystal structure of montmorillonite (modified after Hofmann, et at., Z. Krist., AI4 Fe4' Mg4- Mgfe 86 [5-6] 340-48(1933)). C-AXIS III. Properties of Clay Minerals Only those properties of clay minerals that affect the physical properties of clays will be considered in the following discussion. (1 ) Lattice Structure The general structural features of the clay minerals ILLITE COH^UKy fAI 4 F«4- Mg4- Mg«XSi _ • Al^, ) Oj. have been well established by work in several labora- 6 y tories following generalizations made by Pauling 1 in Fig. 2.—Schematic presentation of crystal structure of 1930. Two units are involved in these structures. illite. One is the alumina or aluminum hydroxide unit which consists of sheets of closely packed oxygens or hy- two The structure of illite (Fig. 2) is similar to that of droxyls between which aluminum atoms are embedded montmorillonite. It also consists of units of one gibb- position equidistant in such a that they are from six site sheet between two sheets of tetrahedral silica or hydroxyls. two-thirds of the oxygens Actually only groups. In the illite structure, however, considerable possible aluminum positions are occupied in this unit, +++ ++++ Al replaces Si , and the excess charge is bal- which is the gibbsite structure. The mineral, brucite, + anced chiefly by K . The same situation prevails possesses similar structure all possible a except that in muscovite in which one fourth of the Si ++++ is re- aluminum positions are occupied by magnesium. +++ + placed by Al , with K balancing the excess charge. unit consists sheet of tetrahedral silica The second of a In illite, there is less than one fourth of the Si ++++ (Si0.j) groups linked to form a hexagonal network of the +++ replaced by Al . The c dimension of the illite composition S14O10 when repeated indefinitely. This unit cell does not change with varying water content. unit may be viewed as a sheet of loosely packed oxygen Illites, which have been studied to date, indicate that atoms with each oxygen linked to two silicon atoms some magnesium and iron may replace aluminum in the directly beneath. silicon are in tetrahedral The atoms gibbsite layer. Both illite and montmorillonite may positions, three valencies being satisfied by linkage to carry some Mg ++ in the possible Al +++ positions not three oxygens in the overlying sheet. The fourth sili- ++ occupied in gibbsite, i.e., in brucite positions. Ca is satisfied that con valency below by an atom such probably does not replace Al +++ because the atom is silicon valency is analogous to the group common (OH) too large to fit. gibbsite. of The kaolinite structure (Fig. 3) is composed of units of one gibbsite sheet with a single sheet of tetrahedral 1 See reference 103 in Bibliography, p. 151. silica groups. The lattice structure does not expand Relation of Composition to Properties of Clays 143 with varying water content, and no replacements by proper conditions are exchangeable for other bases.
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