Mineralogcial Biosignature of High-Magnesian Calcite

Some dolomite formations are associated with sediments where sulfate reduction is active. In general, sulfate has been considered to inhibit dolomite formation. However, it was also suggested that the reaction products of sulfate reduction, such as increased alkalinity, may play a more important role in increasing the formation rate of dolomite than sulfate does in inhibiting it. We have investigated the effect of sulfide on Mg-Ca carbonate precipitation. Our results show that high-magnesian calcite which contains as much as 34 mol% magnesite can precipitate from solutions with the presence of certain amount of dissolved sulfide and pyrite crystals. These high-magnesian calcites form unusual micro-scale spheres with cracks on them. High- resolution SEM and TEM results show that these spheres are actually composed of a great number of nano- crystals (~50 nm) stacking together with low-angle grain boundaries. Both synthesized and natural high-magnesian calcites indicate that dissolved and solid sulfides can catalyze the Mg incorporation into calcite lattice. We hypothesize that S2- (or HS-) ions and >S2 on pyrite surface will decrease the hydration / slovation energy of magnesium ions and thus promote nucleation and crystallization of the high-magnesian calcite. In nature environment, the sulfide could be related to microbe-induced sulfate reduction at low temperature using organic carbon and methane as electron donors. Such kind of high-magnesium calcite that is rich in light carbon also indicates involvement of sulfate reducing bacteria. This work is supported by NASA Astrobiology Institute (N07-5489).