Ch. 25] CONTROLLING FACTORS 465 



TABLE 1 

 Common Clay Minerals 



1. Kaolinite group 



(a) Equidimensional flake-shaped units 



Kaolinite: (OH) 8 Al4Si 4 Oio 



(b) Lath-shaped units 



Halloysite minerals: (OITjsAh^Oio 



(OH)8AUSi40io-4H 2 



2. Montmorillonite group 



(a) Equidimensional flake-shaped units 



Montmorillonite : (OH) 4 (Al4 • Fe 4 • Mg4)Si 8 2 o ■ wH 2 



(b) Lath- or needle-shaped units 



Nontronite : (OH) 4 (Fe 4 • Mg 4 • Ah.) Si 8 O 20 • wH 2 

 Hectorite : (OH) 4 (Mg • Li) 6 Si 8 2 o • nH 2 



3. Ilhte group 



Insufficient data to subdivide: (OH)4Kj / (Al4-Mg4-Fe4)(Si8_2 / 'Al 4 )02o 



4. Miscellaneous fiber-shaped units 



Attapulgite : (OH 2 ) 4 (OH) 2 Mg 5 Si 8 2 o • 4H 2 

 Sepiolite-Uke: (OH) 4 Mg 6 Si 8 O 20 - ?H 2 



5. Clay minerals resembling chlorite and vermiculite are known to occur, but in- 

 sufficient data are available for their classification. 



FACTORS THAT CONTROL THE PROPERTIES OF CLAYS 



The factors that control the properties of clays may be listed as 

 follows: 



(1) Clay mineral composition: the kind of clay minerals, their 

 relative abundance, and their particle-size distribution. 



(2) Non-clay mineral composition: the kind of non-clay minerals, 

 their relative abundance, and the size distribution of the particles of 

 each mineral. 



(3) Electrolyte content: the amount and kind of exchangeable bases 

 and water-soluble salts. 



(4) Organic content: the amount and kind. 



(5) Miscellaneous textural characteristics such as shape of quartz 

 grains, degree of parallel orientation of the clay mineral particles, 

 silicification. 



The clay mineral composition is usually the dominant factor con- 

 trolling the properties of clays, but in some clays the other factors play 

 a dominant role. It is particularly significant that relatively small 

 amounts of certain components may exert a very great influence on 

 some of the properties of a clay (Grim, 1948). As will be shown, this 



