Arsenic-Bearing Cell-Wall Replacement Pyrite in Upper Freeport Coal Bed, West-Central Pennsylvania

Leslie F. Ruppert, Jean A. Minkin, James J. McGee, C. Blaine Cecil

Arsenic-bearing cell-wall replacement pyrite was observed in coal samples from the Upper Freeport coal in west-central Pennsylvania. Scanning electron microscopy (SEM) and electron microprobe analysis were used to characterize pyrite morphologies and quantify arsenic in different forms of pyrite. The framboidal, massive, and cleat pyrite forms contained no detectable arsenic (minimum detection level = 0.04 wt. %). In contrast, arsenic values ranged from not detectable to 1.87 wt. % in cell-wall replacement pyrite. No obvious correlation was found between sulfur and arsenic totals in the pyrite. Cell-filling pyrite contained arsenic rarely and only where it was in direct contact with cell-wall replacement pyrite. Arsenic in cell-filing pyrite was highest at the edges; valu s ranged from not detectable to 1.64 wt. %.

Lack of contrast differences in the backscattered mode of the SEM and relatively homogeneous arsenic values for individual cell walls suggest that arsenic is in solid solution with the pyrite. Cell-wall pyritization occurred after peat formation but before compaction; arsenic was probably not available at other times. Framboids form after compaction and massive and cleat pyrite form after compaction; our analyses indicate no detectable arsenic in these phases. The zoned character of cell-filling pyrite suggests that remobilization of arsenic from cell-wall pyrite to adjoining cell-filling pyrite may have occurred during later stages of coalification.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.