OCCURRENCE AND MINERALOGY OF EFFLORESCENCE IN LATE DEVONIAN BLACK SHALES, NORTH-CENTRAL OHIO

Prominent and mineralogically diverse efflorescences are found on sheltered exposures of pyrite-bearing Late Devonian black shales in north-central Ohio. Efflorescences were studied along the West Branch of the Huron River (Huron Shale), Huron County, and Vermilion River (Cleveland Shale), Lorain County, to determine factors contributing to their occurrence and composition. The mineralogy of the efflorescence of the two rock units are similar, being comprised of the following Mg, Fe, Al and Ca sulfates: alunogen, botryogen, copiapite, epsomite, gypsum, hexahydrite, melanterite, ferroan pickeringite, and rozenite. The dominant phase is ferroan pickeringite, which occurs in porous silky white masses up to 8 cm across, although significant amounts of epsomite, copiapite, and gypsum are found also. Associations of ferroan pickeringite and epsomite are common while those of ferroan pickeringite and copiapite are less common. Hexahydrite is present as loose accumulations growing on the surface of masses of epsomite. Gypsum typically occurs alone as coatings of crystals 1 mm long on bedding and joint surfaces. Drier south facing exposures are distinguished by Mg-Fe-Al sulfates like ferroan pickeringite, whereas wetter north facing exposures are dominated by chemically simpler sulfates such as epsomite or copiapite.

Weathering of pyrite provides a ready source of sulfate-rich acidic pore water, which is drawn to the surface of the shale by evaporation, forming the efflorescence. Published analyses of Late Devonian shales indicate Fe exceeds Mg whereas in ferroan pickeringite Mg exceeds Fe. The higher solubility and broader Eh-pH field of magnesium sulfate relative to iron sulfate best explain this relationship. Alunogen, which is somewhat soluble in acidic pore water, apparently is unstable with respect to epsomite and the two react to form pickeringite.