ORIGIN OF IRON SULFIDE IN COAL

The Late Permain Longtang coal of Geizhou, China is chosen for the study because it was affected by the Late Mesozoic Hg and As mineralization. The over 10,000 times magnification investigation of the environmental scanning electron microscope (ESEM) of this coal revealed that the occurrence of framboidal and polyframboidal texture is not just limited to disseminated but also pyrites with massive appearance at lower magnification. The sizes of these framboids are generally varying within 10-15µ and individual crystal can be as small as 0.5µ.

The origin of these pyrites could be the result of rapid nucleation in environments where pyrite is strongly supersaturated or through the action of bacterial activities. The former process produced relatively uniform cubic crystals with sharp terminations. The latter process produced crystals of variable sizes and forms without sharp terminations. The ESEM analyses indicated that these pyrites are relatively pure iron sulfide. This suggests that the epigenetic mineralization processes did not change the chemical composition of these pyrites significantly. The high Hg and As contents in these coals appear to reflect their concentration in organic portion rather than in sulfide minerals. Marcasite is rare and occurs either as orthorhomibic or cockscomb in crystallographic habit. The presence of marcasite is an indication that this coal has never been metamorphosed above 157°C.