Saturation state sulfate minerals

Fig. 30.2. Saturation states (Q/K) of supersaturated sulfate minerals over the courses of simulations in which seawater mixes at reservoir temperature with formation fluids from three North Sea oil fields. Reaction paths are the same as shown in Figure 30.1, except that minerals are not allowed to precipitate.

Many studies of the impact of chemical diagenesis on the carbonate chemistry of anoxic sediments have focused primarily on the fact that sulfate reduction results in the production of alkalinity, which can cause precipitation of carbonate minerals (see previous discussion). However, during the early stages of sulfate reduction (—2-35%), this reaction may not cause precipitation, but dissolution of carbonate minerals, because the impact of a lower pH is greater than that of increased alkalinity (Figure 4). Carbonate ion activity decreases rapidly as it is titrated by CO2 from organic matter decomposition leading to a decrease in pore-water saturation state. This process is evident in data for the Fe-poor, shallow-water carbonate sediments of Morse et al. (1985) from the Bahamas and has been confirmed in studies by Walter and Burton (1990), Walter et al. (1993), and Ku et al. (1999) for Florida Bay, Tribble (1990) in Checker Reef, Oahu, and Wollast and Mackenzie (unpublished data) for Bermuda sediments. 

Nesbitt et al. (1995) conducted a detailed study of the oxidation of arsenopyrite in oxygenated solutions. Arsenic and sulfur were observed to exist in multiple oxidation states near the pristine surface. After reaction with air-saturated distilled water, Fe(III) oxyhydroxides formed the dominant iron surface species, and As(V), As(III), and As(I) were as abundant as As(—I) surface species. An appreciable amount of sulfate was observed on the mineral surface. Arsenic was more readily oxidized than sulfur, and similar rates of the oxidation of As(—I) and Fe(II)" surface species were observed. Nesbitt et al. (1995) concluded that continued dilfusion of arsenic to the surface under these conditions can produce large amounts of As " " and As, promoting rapid selective leaching of arsenites and arsenates. 

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