Occurrences of iron sulfides in Ohio coals

The iron sulfides in seven different Ohio coals have been studied by polished-section ore microscopy, scanning electron microscopy augmented with EDS analyses, and secondary ion mass spectrometry. The iron sulfides in the coals exhibit a large array of textures and interrelationships which reflect site-specific environmental changes that occurred during the deposition of the sulfides. Sulfide deposition occurred principally during the depositional and diagenetic phases of the formation of the precursor peats. Pyrite and marcasite are present in most of the samples examined. Pyrite occurs as isolated and clustered euhedra, isolated and clustered framboids and spheres, cell-fillings, cleat- and fracture-fillings, replacements of plant debris, and as a porous or spongy-textured variety deposited within and around sulfide masses and grains. Marcasite occurs as polycrystalline spheres, polycrystalline rims and bands within and around clusters of pyrite spheres and framboids, as cell-fillings, and as replacements of plant debris. A typical sequence of iron sulfide deposition in texturally complex sulfide grains and masses is: (1) pyrite framboids or spheres; (2) deposition of marcasite around the relict framboids and clusters of framboids; and (3) spongy pyrite deposited as an outer fringe around sulfide masses and as infillings within the masses. The sulfides exhibit a persistent, although not universal, association with clays, and it is likely that much of the iron now present as sulfides was delivered to the depositional environment adsorbed on clay minerals. The iron sulfides tend to be localized in zones parallel to the banding in the coals. Such localization is most pronounced with respect to specific varieties of iron sulfides such as marcasite spheres, pyrite framboids, zones of pyrite euhedra, and occurrences of texturally complex grains and masses. Such zones are believed to represent depositional environments favorable for the precipitation of specific types of iron sulfides. These stratigraphic microenvironments changed during the times of deposition and diagenesis of the precursor peats and resulted in sequential deposition of the different forms of iron sulfides particularly evident in texturally complex sulfide grains. Chemically significant variables most likely were pH and availability of certain trace elements. The factors favoring precipitation of marcasite rather than pyrite are not clearly understood.The textures of the iron sulfides will prove to be important in understanding the reactivity of pyrite and marcasite in causing acid mine drainage and possibly spontaneous combustion of coal and mine waste, and will be important in the continuing development of effective methods of coal cleaning.