Reactivity of alum and black shale in the Oslo region, Norway

Abstract

Abstract

Mineralogical and chemical analysis of alum shale and black shale samples were done using the XRD, XRF, sulfur chemical analysis, petrographic and scanning electron microscopy. Samples from two study areas in the Oslo region, Konows gate and Slemmestad have abundant of quartz and pyrite. Calcite and gypsum minerals were also found in the Konows gate and Slemmestad areas samples. Other minerals in small concentrations such as barite, chalcopyrite, pyrrhotite, sphalerite and dolomite were detected by the petrographic and scanning electron microscopy.

The microscopic analysis (SEM and reflected light ore microscopy) showed recrystallization of the pyrite and graphite minerals in the alum shale. The alum shale from the Konows gate area had high graphite content and recrystallized cubic pyrite, while the Slemmestad samples had small amount of pyrite, most in framboidal form. The vein formations of calcite, pyrite and graphite depicted active site of transformation or reactions, as here there is access for the oxygen and water to come in contact with the minerals of the alum shale.

Geochemical simulations showed that the most reactive and at the same time most abundant sulfide mineral in the alum shale when coexisting with calcite a high reaction was demonstrated. The pyrite undergoes both aqueous and electrochemical oxidation. The oxidation of pyrite changed the composition and also property of the rock through time. The change in composition may cause swelling problem due to loss of density, because of formation of gypsum due to oxidation of pyrite, and dissolution of calcite. The minerals calcite and pyrrhotite have an effect on the rate of pyrite oxidation. The calcite caused inhibition on the rate of pyrite oxidation but it was important for the growth of gypsum, and while the mineral pyrrhotite had a catalytic effect on the process of oxidation. Free swelling test showed that there was no expanding clay or smectite in the alum shale sample from the Konows gate and the black shale sample from the Slemmestad.