A study of chemical structure in “barkinite” using time-of-flight secondary ion mass spectrometry

Abstract

The Late Permian coals from south China are unique in their high content of “barkinite” and elemental hydrogen, and typically produce high yields of tar. “Barkinite” has often been identified as suberinite for a long time, but now many Chinese workers have concluded that “barkinite” is not suberinite, rather it is a distinct maceral. The term “barkinite” was formally certified and named by the State Bureau of Technical Supervision of the People's Republic of China in 1991, 1995 and 1999; however, it has not been recognized as a scientific term by international coal geologists and ICCP, and TSOP have not recognized the term, either. The petrographic characteristics of “barkinite” and suberinite show that the biggest difference between them is in the cell fillings. Therefore, in the paper, a new in situ microprobe mass chemical analytical method, time-of-flight secondary ion mass spectrometry (TOF-SIMS) was first used to study the chemical structure of cell walls and cell fillings of “barkinite”. The results indicate that the chemical structures of cell fillings and cell walls are similar to each other. They are all mostly composed of aliphatic compounds (such as alkane and alkene), and followed by aromatic compounds and alkyl aromatics, in addition to minor amounts of heteroatomic compounds (such as alkoxyl and alkylamine). Based on these, we can deduce that the cell cavities of “barkinite” were mainly filled by original lipid, which were derived from cell secretion of “barkinite” itself. But suberinite is defined only as suberinized cell walls in cork tissue, and cell cavities were usually filled by secondary gelification materials (such as corpocollinite), these characteristics are quite different from those of “barkinite”.The chemical characteristics of “barkinite” shown in SIMS spectra coincide with those inferred from its petrographic characteristics. The high content of hydrogen, the high tar yields and the high hydrocarbon generation potential of Late Permian coals from south China are attributed to the high aliphatic content of “barkinite”.