X-RAY ANALYSIS 



97 



Table 31. Constituents of the fine fractions of samples of high carbonate content as revealed by 



X-ray powder diagrams 



Symbols refer to intensity of characteristic spacings for substances indicated: dom = charac- 

 teristic spacings are dominant in pattern; vs= very strong; s = strong; m = moderate; mw = moder- 

 ately weak; w = weak; vw=veryweak; ww = very, very weak; tr = trace; ? = spacings are possibly 

 present or some spacings are missing; nf = spacings not observed. 



contain montmorillonite beidellite clay minerals and 

 quartz, and for two California soils, previously men- 

 tioned, containing a beidellite-like mineral and quartz. 



The prominent lines exhibited by all clay minerals 

 are those at 4.4 to 4.5, about 2.55, 1.7, 1.5, 1.29, and 1.24 

 angstroms. The distinctive lines of muscovite are those 

 at 3.7, 3.5, 3.333 (the most prominent line of quartz), 

 3.20, 3.00, 2.87, and 2.13 angstroms. The kaolinite min- 

 erals, including halloysite, exhibit characteristic lines 

 at 7.4 to 7.1 and 3.6 angstroms, whereas kaolinite itself 

 may be distinguished from halloysite by the occurrence 

 of a strong line at approximately 4.2 angstroms and a 

 moderate line at 3.87 to 3.83 angstroms (there is a 

 weak muscovite line at 3.85 to 3.90 angstroms). The 

 weak kaolinite lines at 3.07 to 3.11 and 2.76 to 2.79 are 

 also absent from halloysite. Halloysite differs from ka- 

 olinite principally in the presence of wider and more 

 diffuse lines. This difference is said by some workers 

 to be owing to the finely divided nature of halloysite, in 

 which the individual grains are usually of colloidal di- 

 mensions. Nagelschmidt states that the X-ray diagrams 

 of halloysite are like those which would be expected from 

 kaolinite if the particles of the latter were smaller and 

 only the strong lines remained. That kaolinite is usually 

 relatively coarse-grained is shown by the work of Vin- 

 ther and Lasson (1933), who found by sedimentation anal- 

 yses that only 1 to 9 per cent of kaolin particles from 

 various sources have particle diameters less than 0.1 

 micron, whereas other clays had as much as 55 per cent 

 of the particles smaller than 0.1 micron. On the other 

 hand, Ross and Kerr, basing their opinion on optical and 

 • dehydration data as well as X-ray analyses, believe that 

 halloysite is a mineral distinct from kaolinite.^ 



The minerals of the montmorillonite, beidellite, 

 nontronite group may be distinguished from those of the 

 kaolinite group by the presence of lines at 4.95 to 5.05, 

 and 3.03 to 3.06 angstroms. Both these lines, however. 



are close to muscovite spacings. The essential charac- 

 teristic of the minerals of this group is the presence of 

 only a few lines, namely the basic spacings, previously 

 mentioned, of all clay minerals. In this respect, they 

 are similar to halloysite but differ from it in the two 

 lines mentioned and in the absence of the broad line at 

 2.5 angstroms which characterizes halloysite. 



Recent investigations by Ross and Kerr and others 

 have shown that the clay minerals of higher silica ses- 

 quioxide ratio of the montmorillonite, beidellite, nontro- 

 nite group are far more common in soils and fine-grained 

 sediments than is kaolinite. Hendricks and Fry, in their 

 examination of soil colloids, found that the minerals of 

 these colloids belong in one of three groups (1) mont- 

 morillonite beidellite, (2) montmorillonite and quartz, 

 and (3) halloysite. Bentonite, which is formed from 

 the alteration of volcanic ash, has been found to consist 

 of montmorillonite together with some quartz in certain 

 cases (see Kelley, Dore, and Brown and Ross and Kerr). 



Samples of High CaCOs Content 



The patterns obtained from the Carnegie samples, 

 compared with those of known substances, are illustrated 

 in figures 36 and 37. Figure 36 shows the interplanar 

 spacings of certain Globigerina oozes, together with 

 those of calcite, aragonite, phillipsite, and bog iron ore; 

 the minerals identified in the various samples of Globig- 

 erina ooze are listed in table 31. The presence of sodium 



^Mehmel (1935) has made an X-ray study of the 

 crystal structure of halloysite which he claims is distinct 

 from that of kaolinite. 



