Abstract: Observations on the Use of Vitrinite Maximum Reflectance Versus Vitrinite Random Reflectance for High Volatile Bituminous Coals

James C. Hower, Robert F. Rathbone, Garry D. Wild

The reflectance level at which vitrinite maximum reflectance is significantly different from vitrinite mean or random reflectance has been the subject

of debate. While some researchers draw the limit of effective measurement of both parameters at the sub-bituminous/high volatile C bituminous transition, about 0.4% R_max, others question the reality of the divergence below 1% R_max. Both parameters can be measured at the same point on a coal particle by means of determining the mean of two orthogonal reflectances for R_random and determining R_max as the peak reflectance determined through 360° rotation of the microscope stage.

High-rank coals, such as semianthracites through meta-anthracites, have a large divergence of R_max and R_random and, indeed, are biaxial. Both biaxial negative and biaxial positive reflectance ellipsoids are found in varying tectonic settings. Although bituminous coals are conveniently assumed to have a uniaxial reflectance, examples of biaxial reflectances from folded regions are documented. For example, high-volatile A bituminous coal from the west flank of the Chestnut Ridge anticline, Jefferson County, Pennsylvania, has three primary reflectances (0.94, 0.88, 0.75).

Biaxial reflectances complicate the measure of R_max on particulate pellets, as the measured R_max is actually a function of a true R_max and the intermediate reflectance value, both measurable directly with oriented samples. For most bituminous coals, particularly within the high-volatile bituminous range, the divergence of R_max and R_int would not be large and therefore should not inhibit the determination of R_max on particulate pellets. Ideally, both R_max and R_random should be determined to maximize the information measured on the coal and to optimize the comparison of the sample to national and international standards.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994