Five genetically different forms of microscopic pyrite occur in samples of the upper Freeport coal obtained near Homer City, Pennsylvania. At least two facies of pyrite forms and associations can be observed throughout the study area.

The five forms of pyrite are: (1) maceral encapsulated crystals which formed in cell cavities of plants shortly after death of the plants; (2) framboids, which form early in peat accumulation; (3) maceral cell fillings, which form before compaction of the plant material and in association with calcite and kaolinite; (4) replacement of pre-macerals before compaction; and (5) cleat and fracture filling during or after compaction and the formation of cleat.

Facies of pyrite forms and associations may include the five pyrite forms in differing concentrations. Facies 1 is dominantly composed of framboids associated with vitrinite and clay; facies 2 is dominantly composed of massive pyrite that replaced vitrinite pre-macerals. Facies 1 is in the lowermost coal type, which was believed to have been deposited on previously (subaerially) exposed freshwater lime muds and argillaceous sediments.

Microscopic analyses of the pyrite forms, associations, and grain size of the iron disulfide minerals may be used as tools for estimating the washability of a coal bed before mining.

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