On the optically biaxial character and heterogeneity of anthracites

The results of the study of optical properties of 13 anthracites from different parts of the world are presented in this paper, Measurements of reflectance values were made on non-oriented vitrinite grains Tor a minimum of 300 points per sample. The reconstruction of Reflectance Indicating Sur faces (RIS) were made by Kilby's method [Kilby, W.E., 1988, Recognition of vitrinite with non-uniaxial negative reflectance characteristics. Int. J, Coal Geol. 9, 267-285; Kilby, W.E., 1991. Vitrinite reflectance measurement - some technique enhancements and relationships, int. J, Coal Geol. 19, 201-218]. It was found that the use of Kilby's method for strongly anisotropic materials like anthracites did not give unambiguous results. Some improve ment in Kilby's method, consisting of the division of the cumulative cross-plot into several elemental components, is suggested. Each elemental cross-plot corresponds to a textural class of anthracite, which is characterized by the values of RIS main axes R-MAX((k)) R-INT((k)) and R-MIN((k)) (k = 1,2,... n; n - number of classes). The global texture of anthracite is characterized as a RIS with main axes calculated as the weighted means of <(R-MAX)over bar>, <(R-INT)over bar> and <(R-MIN)over bar> for each class of this anthracite. The division of cumulative Kilby's cross-plot on elemental components makes possible the calculation of new coefficients H-t and H-10 characterizing the heterogeneity of the structure and texture of anthracites. The results of our study show that all anthracites have biaxial negative textures, but their heterogeneity varies in a wide range of H-t and H-10 coefficients depending upon the individual coal basin. (C) 2000 Elsevier Science B.V. All rights reserved.