Textural transformation of thermally treated anthracites

Abstract

The organic matter of anthracites consists of turbostratic (two-dimensional crystalline order) stacks of three to five aromatic layers called the Basic Structural Units (BSU). The arrangement of BSU forms the texture of anthracites, which corresponds to their chemical and physical properties including optical properties (reflectance values). Thermal treatment leads to the changes in the arrangement of BSU. The character of structural and textural transformation of anthracites during heating can be determined based on the changes of reflectance values of anthracites. Several anthracites of various ranks were thermally treated over the temperature range of 400 (673 K)–1000 °C (1273 K) in an inert atmosphere (N 2), at atmospheric pressure. The reflectance indicating surfaces (RIS) were reconstructed and reflectance indicatrix parameters were calculated according to the Kilby method specially modified for the study of anthracites. It was found that the relative increase of reflectance values ( R max, R min) was greater for the lower metamorphosed samples than for the higher ones. However, the higher rank of initial samples, the greater their final reflectance values and the anisotropy of the texture attained at 1000 °C. It indicates that the texture of the higher metamorphosed anthracites (meta-anthracites) is more flexible for rearrangement during heating, than the texture of the lower metamorphosed anthracites. Moreover, the difference in the changes of reflectance values versus temperature for the higher and for the lower rank anthracites suggests that meta-anthracites have undergone stronger natural metamorphism equivalent to about 200 °C in comparison with the other anthracites. It could be said that the character of textural transformations of individual anthracites results from the degree of their metamorphism and, in a consequence, from the texture of parent sediments.