Surface analysis of pyrite in the No. 9 coal seam, Wuda Coalfield, Inner Mongolia, China, using high-resolution time-of-flight secondary ion mass-spectrometry

Abstract

The chemical composition of pyrite in coal can be used to investigate its geological and mineralogical origin. In this paper, high-resolution time-of-flight secondary ion mass spectrometry (TOF-SIMS) was used to study the chemical composition of various pyrite forms in the No. 9 coal seam ( S t,d=3.46%) from the Wuda Coalfield, Inner Mongolia, northern China. These include bacteriogenic, framboidal, massive, cell-filling, fracture-filling, and nodular pyrites. In addition to Fe + ( 54Fe +, 56Fe +, 57Fe +), other fragment ions were detected in bacteriogenic pyrites, such as 27Al +, Si + ( 28Si +, 29Si +, 30Si +), 40Ca +, Cu + ( 63Cu +, 65Cu +), Zn + ( 64Zn +, 66Zn +, 67Zn +, 68Zn +), Ni + ( 58Ni +, 60Ni +, 62Ni +), and C 3H 7 +. TOF-SIMS images show bacteriogenic pyrites are relatively rich in Cu, Zn, and Ni, suggesting that bacteria probably play an important role in the enrichment of Cu, Zn, and Ni during their formation. Intense positive secondary ion fragments from framboidal aggregates, such as 27Al +, 28Si +, 29Si +, AlO +, CH 2 +, C 3H 3 +, C 3H 5 +, and C 4H 7 +, indicate that formation of the framboidal aggregates may have occurred together with clay mineral and organic matter, which probably serve as the binding substance. The intense ions of 28Si + and 27Al + from massive pyrites also suggest that their pores incorporated clay minerals during crystallization. Together with the lowest 28Si +/ 23Na + value, the intense organic positive secondary ion peaks from cell-filling pyrites, such as C 3H 3 +, C 3H 5 +, C 3H 7 +, and C 4H 7 +, indicate that pyrite formation may have accompanied dissolution or disintegration of the cell. The intense P + peak was observed only in the fracture-filling pyrite and the highest 28Si +/ 23Na + value of fracture-filling pyrite reflects its epigenetic origin. Together with XRD and REEs data, the stronger 40Ca + in nodular pyrite than other pyrite forms shows seawater influence during its formation.