Spontaneous formation of hydrated iron sulfates on laboratory samples of pyrite- and marcasite-bearing coals

Abstract

Hydrated iron sulfates identified as melanterite, rozenite, szomolnokite and halotrichite have been observed growing on iron sulfides in polished and rough-cut surfaces of samples of Ohio and Utah coals stored under normal atmospheric conditions. Melanterite and rozenite are likely the earliest hydrated sulfates to form. Szomolnokite and halotrichite appear to be the most stable sulfates in the samples studied. The formation of halotrichite involves the interaction between the oxidation of sulfides and the breakdown of clays. In many cases the oxidation process occurs below the polished surfaces of the sulfides, and the sulfates are extruded from pores and grain boundaries within and marginal to the sulfide grains. The first stages of sulfate formation may involve development of amorphous sulfate phases that subsequently become crystalline. There appear to be no significant differences between reactivities of pyrite and marcasite with respect to sulfate formation. Porous, or spongy-textured, sulfides are more prone to oxidation and sulfate formation than are well-crystallized sulfides. In addition to conditions of high humidity, the presence of clays associated with the sulfides is an important contributory factor in the oxidation of the sulfides. The oxidation of iron sulfides contained in coals and associated strata is the principal cause of acid waters generated by mining activities and of the spontaneous combustion of coal storage piles and mine waste dumps. The environmental impact of these phenomena depends upon reaction kinetics.