Vitrinite reflectance measurement — Some technique enhancements and relationships

• Published in: International journal of coal geology 1991-12, Vol.19 (1), p.201-218
• Main Author: Kilby, Ward E.
• 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(91)90021-A

Coals with non-uniaxial negative reflectance indicating surfaces (RIS) are the norm rather than the exception for coals from British Columbia coalfields and have also been reported from elsewhere around the world. Unique description of the reflectance characteristics of these coals requires determination of the three principle reflectance axes. Such axes values are determined from a standard coal particle sample using a crossplot of reflectance versus bireflectance for the apparent maximum and minimum values obtained from each particle. Computer-controlled stage rotation and reflectance measurement collection allow quality control and reflectance ellipse orientations to be assessed for each coal particle measured. The eigenvectors associated with the largest eigenvalue of a symmetrical 2×2 matrix of direction cosines of stage rotation angles, weighted by the associated reflectance value, provide the orientation of R′ max (apparent maximum reflectance) on the pellet surface. The largest and smallest eigenvalues of this matrix divided by the sum of reflectance values measured during stage rotation are the R′ max and R′ min values, respectively. The quality of measurement for each particle can be quantified by the scatter of raw reflectance values about the best fit ellipse defined by R′ max and R′ min . The calculated random reflectance value, R Mc, determined from R max and R′ min with stage rotation is not affected by the problem of light pathway induced polarization of the “non-polarized” light commonly used in the R m (mean random reflectance) measurement procedure. Reflectance variations of 15% for a coal particle with a theoretical R m of 3.06 can result from partially polarized “non-polarized” light. The relationship between R m and R max (mean maximum reflectance) is non-linear and controlled by the shape of the RIS. Errors as large as 13% for coals in this study and greater for RIS reported from other tectonic settings would result from using a single linear conversion factor.