1 . MINERALOGY AND ACTIVITY 



Procedure — A few samples were Investigated for clay mineralogy to examine 

 the relationship between clay mineralogy, clay-size fraction, and plasticity index. 

 X-ray diffraction, differential thermal analyses, and surface area studies were made 

 by Messrs. E. B. Kinter and S. Diamond of the Physical Research Division, Bureau 

 of Public Roads. Their X-ray analytical procedure is described by Kinter and Diamond 

 (1956). In these studies, a General Electric XRD-3D direct-recording X-ray diffrac- 

 tion machine was operated as follows: 40 KV at 20 ma, nickel -filtered CuKoc radia- 

 tion, scans at 2° per minute, and 1° beam and 0.2° detector slit widths. Clay mineral 

 surface areas were determined using the method of Diamond and Kinter (1958) from 

 glycerol retention measurements (Kinter and Diamond, 1958). 



In the Hydrographic Office, the grain-size fraction coarser than 4^ was cursorily 

 examined under a binocular microscope by laboratory personnel to obtain an estimation 

 of the percentage of skeletal material and mineral grains. 



Mineralogy — Results of the greater than 9 ^ fraction are summarized in Table 6. 

 Montmorillonite percentage is based on surface area measurements (Areas A and B 

 composite sample: external area 62 mVg, internal area 173 nryg) and are believed 

 by Kinter to be accurate within a few percent. Other percentages in this table are 

 based on an "educated guess" by Kinter and Diamond, following similar reasoning to 

 that made by Johns and others (1954). These estimates may be in error -40 percent from 

 the stated value, although Kinter believes ~t5 percent is more reasonable. According to 

 J . C. Hathaway (1961, written communication), who also has studied the mineralogy of 

 certain Area C core samples, the mixed layered montmorillonite-mica contains about 30 

 percent mica layers. Percentage of quartz and calcite (15 percentage points) in the 

 whole sample, plasticity index, and computed activity are also presented in Table 6. 



Results of an estimation of carbonate content by hydrochloric acid treatment and 

 microscope estimations of the coarse fraction are summarized in Table 7 for comparison 

 with the data presented in Table 6. It is evident that calcareous skeletal material and 

 shells account for the majority of material in the coarse fraction from Areas A and B, 

 and mineral grains are more abundant than carbonate in Area C. 



Activity — More than a decade ago Skempton (1948) established a relation 

 between the ratio of the liquid limit and the clay fraction greater than 9 j& (less than 

 2/x), which he called activity. By activity is meant the increased surface activity of 

 the clay fraction, for example, the increased ion exchange capacity and adsorption of 

 water with decreasing grain size. Clays were classified as: inactive clay, a c <0.75, 

 normal clay, a c = 0.75 to 1 .25, and active clay, a c > 1 .25. This ratio was redefined 

 later (Skempton, 1953a, p. 42-43; 1953b, p. 58) in terms of the direct linear relationship 



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