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A Laboratory Manual for X-Ray Powder Diffraction
Semi-quantitative assessments make the identification of individual components in polymineralic samples much more valuable. Unfortunately, the intensity of a mineral's diffraction peaks can not be directly used as an accurate measure of abundance because sample mounts and X-ray machine conditions vary, and because different minerals, different atomic planes within a mineral, and different samples of the same mineral do not have the same ability to diffract X-rays (Biscaye, 1965). However, Biscaye (1965) also found that useful semi-quantitative comparisons can be made between samples by means of various ratios of peak areas. These ratios vary in part due to mineralogy and in part due to scattering factors inherent to X-ray diffraction. For example, a 17-angstrom peak will have four times the intensity of a ten-angstrom peak if a two theta compensating device is not used (Borchart, 1989).
The method of weighting basal-peak areas described here, which has been modified from his work, is based on the assumptions that only the less than 0.002 mm fraction is recorded on the X-ray diffractograms and that montmorillonite, illite, kaolinite, and chlorite comprise 100 percent of that fraction. The accuracy of this
method increases when replicate x-ray diffraction analyses are performed and the results are averaged, and when the peak areas are not too small.
| PROCEDURE FOR ANALYSIS OF CLAY MINERALS | |
Materials Required:
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| Draw a baseline beneath each of the 17, 10, and 7 angstrom peaks, and the 3.58/3.54 angstrom kaolinite-chlorite doublet. |  |
| Photocopy the 2 to 30 degrees two-theta portion of the glycolated pattern. |  |
| Using the scissors, cut out each peak and divide the 3.58/3.54 angstrom doublet proportionately. |  |
| Weigh the paper copy of each peak and record the weights. Apply the appropriate weighing factor to the weight of each peak, and calculate the ratio of the doublet's parts. The peaks and respective weighing factors are: the weight of the area of the 17 angstrom glycolated peak for montmorillonite; four times (4x) the weight of the 10 angstrom peak on the glycolated pattern for illite; and twice (2x) the weight of the area of the 7 angstrom peak for chlorite and kaolinite. The weighted 7 angstrom peak, common to both chlorite and kaolinite, is divided between the two in proportion to the fraction of each mineral in the total area under the 3.58/3.54 angstrom kaolinite-chlorite doublet. |  |
| Normalize the clay mineral distribution by summing the weighted peak-area weights, and dividing the weighted peak-area weight of each mineral times 100 by the sum of the weighted peak-area weights. | |
