Quantitative X-ray diffraction analysis of mineral and organic phases in organic-rich rocks

• Published in: International journal of coal geology 1995-10, Vol.28 (1), p.51-69
• Main Authors: Mandile, Andrew J., Hutton, Adrian C.
• Format: Article
• Language: English
• Subjects: Coal; Earth sciences; Earth, ocean, space; Exact sciences and technology; Sedimentary rocks
• ISSN: 0166-5162; 1872-7840
• DOI: 10.1016/0166-5162(95)00004-W

Using rapid routine quantitative analyses of X-ray powder diffraction scans both the mineral and organic content of organic-rich rocks, such as coal, carbonaceous shale and oil shale, can be quantified. The minerals are quantified by the Rietveld method and the organic matter is quantified by an organic calibration curve from the same X-ray diffraction (XRD) data. For this single analysis technique, the organic content, mass absorption coefficient and density of the organic matter have to be included in the analysis to account for microabsorption effects caused by the large differences between the mass absorption coefficients of the organic matter and the inorganic fraction of the rock. The methodology was tested on rocks with up to 90wt% organic matter. Empirical calibration curves are used to relate the area of an ‘organic hump’ on an XRD trace to the organic content. These curves are calibrated by either of two calibration methods: the addition method, using mineral mixtures to which various proportions of organic matter are added, or the direct method, using XRD patterns of samples that are spiked with an internal standard. Experimental results for both calibration methods are in good agreement. The calibration curves are dependant upon organic type but, once established, each curve can be used universally for the given type of organic matter, providing the same set of diffractometer conditions are used. Errors may be introduced by: XRD background interference noise, caused by clay minerals; interference from other amorphous phases; and the presence of iron-bearing minerals. The latter is partly eliminated using either a calibration formula and intensity correction curve or by spiking the organic matter with a fixed proportion of an iron-bearing phase.