between the plasticity index, instead of the liquid limit, and the clay fraction, where 



° c = i , f . — - — ; — TV (21) 



clay fraction percentage < 2\x. 



The same activity classification was retained. Skempton (1953b, p 57) showed that 

 the activities of a number of samples plot about a straight line that extrapolates back 

 to the origin of a plasticity index -clay fraction graph, or activity chart. 



The relationship between surface activity of colloidal particles, clay minerals, 

 and quartz and calcite found by Skempton is shown in Figure 16 together with data 

 from the samples investigated. Agreement of data is reasonably good. The higher 

 montmorillonite content of the two samples from Area C is reflected in higher activ- 

 ities. Although the clay mineralogy of Area A and B samples was found nearly 

 identical, the latter are less plastic and, as a result, have lower activities. Greater 

 scatter about an average line (not drawn in Fig. 17) for samples from each area re- 

 sults when all available samples are plotted on an activity chart (Fig. 17). In this 

 Figure, Hathaway (1961, written communication) is of the opinion that the term 

 "illite" should be replaced with "mixed layered mica-montmorillonite, " having a 

 probable range of activity from about kaolinite to between Ca- and Na-montmoril- 

 lonite. Yoder and Eugster (1955, p. 252-254)' ' discuss problems with the term 

 "illite." 



A further subdivision of Skempton's classification is indicated in Figure 17 with 

 additional lines at activities of 0.25 and 1 .75. A tentative activity classification 

 of: inactive < 0.25, slightly active, 0.25 to 0.75; normal, 0.75 to 1 .25; active, 

 1 .25 to 1 .75; and very active > 1 .75 is suggested for sea-floor sediments. More data 

 are needed, however, before it will be known whether or not these additional cate- 

 gories are significant elsewhere. Using this nomenclature, the very highly plastic 

 Area C core samples are active. Most Area B, E, and all of the F 6 samples are 

 slightly active. Area D samples are mostly inactive, except piston core D 1 . This 

 core is relatively homogeneous in composition and has a plasticity index of 81 and 

 a 20 percent clay-size fraction, that results in the exceptionally high activity of 

 4.0, which is not plotted in Figure 17. All other samples, Area A, C, and F, have 

 normal activity. 



Elsewhere, Fisk and McClelland (1959, p. 1383) found that late Quaternary 

 continental shelf clays off Louisiana have an activity of about 0.9 and hence are 

 normally active. 



This reference kindly was called to my attention by Dr . J . C . Hathaway. 



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