X-RAY ANALYSIS 



81 



X-RAY ANALYSIS 



Discussion of Method 



In order to obtain information on the mineralogical 

 composition of the fine fractions of the sediments, X- 

 ray diffraction patterns of the silt, clay, and colloidal 

 grades of a number of samples were made by the method 

 of Debye and Scherrer (1916) and Hull (1917). 

 Mr. W. H. Dore kindly co-operated in this part of the 

 study. The materials were ground in an agate mortar, 

 packed into thin-walled glass tubes having inside diam- 

 eters of the order of 0.5 mm and exposed for 24 hours 

 in an X-ray diffraction apparatus supplied by the Gener- 

 al Electric Company. A Coolidge tube with a molybde- 

 num target was used as the source of X-rays, and the 

 rays were passed through a zirconium oxide screen, so 

 that their effective wave length was 0.712 angstrom unit. 

 The photographic films on which the diffraction patterns 

 were recorded were placed in cassettes of 8-inch radius 

 at the center of which were located the tubes containing 

 the specimens. Incident X-ray beams were directed 

 through a narrow slit (0.02 by 0.5 in.) on these tubes, 

 and images of the slit were produced on the film by the 

 undeflected beam and by the various beams deflected by 

 the powdered particles. 



The Debye-Scherrer method^ of X-ray analysis of 

 finely powdered crystalline substances depends on the 

 fact that the constituent grains of such fine powders are, 

 in reality, minute crystals which are in random arrange- 

 ment. Even in a small amount of finely powdered crystal- 

 line material there will be some crystalline fragments 

 so arranged with respect to the X-ray beam that they will 

 produce reflections in phase on a suitably placed photo- 

 graphic film corresponding to each interplanar spacing. 



Since no two minerals have exactly the same atomic 

 structure, the diffraction pattern of an unknown mineral 

 may be employed for its identification by comparison of 

 the pattern with those of known substances. 



In a mixture of two or more crystalline minerals 

 the patterns of each mineral remain the same, but the 

 patterns are superimposed on each other. Some of the 

 lines of one pattern often coincide with some of the lines 

 of the other patterns and the greater the number of com- 

 pounds, the greater the probability of overlapping or 

 coincidence of lines. If the number of compounds is not 

 too great, however, there are usually some lines which 

 are characteristic of each compound in the mixture and 

 are not shown by the remainder of the substances. In a 

 mixture the general intensity of each pattern is dimin- 

 ished because of dilution, and coincidences of lines 

 cause some difference m relative intensity. 



As the quantity of any crystalline substance in a 

 mixture decreases, so does the intensity of its pattern. 

 First the less intense lines disappear, and then the 

 stronger ones, until a point is reached when the strong- 

 est lines can no longer be seen. According to Brown- 

 miUer and Bogue (1930) quantities of material less than 

 2 or 3 per cent can rarely be identified even under the 

 most favorable conditions; Hendricks and Fry (1930) 

 state that in the case of clay minerals, amounts less 



^For other discussions of the application of X-ray 

 methods to the study of colloidal fractions of sediments 

 and soils, see Kelley, Dore, and Brown (1931), and 

 Nagelschmidt (1934). 



than 10 per cent are not indicated in the diffraction pat- 

 terns. The minimum amount of material necessary for 

 identification is a function of the simplicity of structure 

 and degree of symmetry of the crystals, also of their 

 size and shape. Sodium chloride, for example, can be 

 readily detected, owing to its high symmetry and sim- 

 ple structure, even when present in very small amounts. 



On the other hand, noncrystalline or amorphous 

 substances, the atoms of which occupy more or less 

 random positions, do not give definite patterns, hence 

 cannot be recognized except by a more or less general 

 fogging of the film, even when present in large amounts. 



Nagelschmidt (1934) has pointed out the special dif- 

 ficulties attending the application of the X-ray method, 

 when applied to fine-grained sediments and soils. These 

 arise from the following causes: 



1. Certain of the minerals occurring in clays have 

 low symmetry, and their X-ray diagrams therefore have 

 many lines. 



2. The crystal structure of many of the minerals of 

 fine-grained sediments is very similar and their powder 

 diagrams are correspondingly difficult to distinguish. 



3. A broadening of the diffraction lines and increase 

 in diffused radiation takes place when crystal particles 

 are smaller than 0.05 micron, as they are in many clays. 



4. The crystal structures of certain minerals oc- 

 curring in fine-grained sediments are not accurately 

 known, hence a theoretical prediction of the applicability 

 and limits of the method for purposes of quantitative 

 identification is not possible at present. 



The precision of measurement of the interplanar 

 spacings increases as the magnitude of the spacing de- 

 creases. In their investigation of the constituents of 

 Portland cement, Brownmiller and Bogue, using a ma- 

 chine similar to that used in this investigation, found 

 an extreme error of less than 1 per cent for interplanar 

 spacings of 2.990 angstroms and 0.4 per cent for inter- 

 planar spacings of 1.202 angstroms. Nagelschmidt, 

 using a more modern type of machine and copper-K- 

 radiation, obtained a maximum deviation of 1 per cent 

 in measurements of interplanar spacings of 4 angstroms, 

 and 0.1 per cent for Interplanar spacings of 1 angstrom. 

 Measurements of weak lines and of diffuse lines are of 

 greater inaccuracy than these figures would indicate. 



Measurements were made on the centers of narrow 

 lines; for broad lines the measurements in brackets 

 given in the tables are for both edges of the lines. 

 These broad lines probably often consist of two or more 

 narrow lines, but molybdenum radiation is said by Nagel- 

 schmidt to be of too short a wave length to give satisfac- 

 tory resolution for lines that are close together. The 

 intensity of the lines was determined visually. 



The results of the X-ray analyses are given in table 

 29. Part A of this table gives the measured interplanar 

 spacings found in eighteen fine fractions of fifteen cal- 

 careous sediments. The silt and clay grades obtained 

 in mechanical analyses by the Bureau of Soils method 

 and the parts of definite particle size, ranging from less 

 than 3 to less than 1 micron obtained in the pipette 

 method of mechanical analyses were investigated. Part 

 B of table 29 gives the meastired interplanar spacings 

 of seventeen samples of low calcium carbonate content, 

 including 11 red clays, 1 green (coprolitic) mud, 1 green 

 clayey mud, 2 volcanic muds, 1 diatom ooze, and 1 



