Quantitative determination of mineral content of geological samples by X-ray diffraction: Discussion

The reference-intensity-ratio method of quantitative X-ray diffraction is rapidly coming to the forefront of quantitative X-ray powder difraction analysis. Pawloski (1985) has completed quantitative analysis of prepared mixtures of fine-grained geologic materials by this procedure. Pawloski is to be commended for using this approach and for her method of obtaining reference-intensity constants from calibration curves by the intensity ratio/weight ratio slope method. Nevertheless, I believe that several experimental and interpretive difficulties arise in this paper and bring into question the application to Pawloski's samples, which I feel warrants some discussion here. As a point of clarification, it should be noted that Pawloski's constant K is the reciprocal ofthe well-established reference intensity constant, k, (Chung, I 974), also known as the reference intensity ratio RIR (Hubbard et al., 1976; Cline and Snyder, 1983), but with quartz as the reference material. The theoretical basis for Pawloski's approach is sound, in my opinion, but I have some concern over several other aspects of the presentation, briefly listed as follows: (l) the large size (between 35and 45-pm diameter) of the sample particles, (2) the "backpacking" procedure of sample mounting, (3) the manner in which the errors are presented, and (4) the use of diffused diffraction intensities for glass quantification. The efects of large particle size on X-ray diffraction are well known. The extreme variability of intensities from a layer ofparticles this coarse grained has been well documented by Wolffet al. (1965) and Klug and Alexander (1974). Since the number of particles (spheres) of a material found within a given volume varies as the diameter cubed, it is easy to show, for example, that there will be 125 times more 5-pm particles than 25-pm particles in a given component volume. Thus, the probability that a particle will have the proper orientation for constructive beam interference is increased by a like factor. Compared to a scan taken from the large particles, a set ofintensities from a sample of the smaller particles would not show the large variation in intensities on sample rotation as was observed for 20-30-pm particles by Wolffet al. (1965). Other effects were demonstrated by Cline and Snyder (1983), whose data showed a change in the referenceintensity-ratio values determined with AlrO3 as a standard as the particle size of the material being analyzed increased. This is also a common observation in our laboratory; that is, the reference-intensity ratio will increase markedly with decreasing particle size of the material being analyzed as a result of the reduction in microabsorption and primary extinction with decreasing particle slze. With regard to the second point, there are at least two techniques now available for the preparation of samples without introducing significant preferred orientation (Davis and Cho,1977: Davis and Johnson, 1982a,1982b:. Calvert et al., I 983; Cline and Snyder, I 983; Davis, I 984). One method used in our laboratory (Davis, 1984) consists ofaerosol suspension in a 4or 8-L aspiratorjar and collection on glass fiber filters with corresponding application of corrections for transparency and matrix effects. The sample particles are reduced to an average of under l0-pm diameter by mixer-mill pulverization for such samples. Actual sample loading takes from l-5 min. Since a large proportion of Pawloski's samples contains phyllosilicates and carbonates, it would appear that any sample preparation without due concern for preferred orientation would yield highly suspect results. I believe that the particle size and preferred orientation aspects just discussed will create reference-constant values such as presented in Pawloski's Table I that cannot be used for accurate quantitative analytical work. I have converted most of these K constant values into AlrO3based reference-intensity ratios as formally defined using the calculated qtartz k, as the common basis of conversion. Table I lists values given by Pawloski, values used in our laboratory's aerosol suspension inventory, and cal-