AQUEOUS SULFIDE-CATALYZED NUCLEATION AND GROWTH OF DISORDERED DOLOMITE, A POTENTIAL PRECURSOR OF SEDIMENTARY DOLOMITE

Dolomite (CaMg(CO₃)₂) is a common mineral in ancient rock records. However, the rare occurrence of modern dolomite and the notorious difficulty in synthesizing dolomite abiotically under ambient conditions result in the longstanding “dolomite problem” in sedimentary geology. Some modern dolomite formations are found to be associated with sediments where microbial sulfate reduction (SR) is active. However, the role of SR in dolomite formation is still poorly constrained. Our synthesis experiments demonstrated that disordered dolomite can precipitate from aqueous sulfide-bearing solutions at 25 °C in the presence of calcite seed crystals. Also, in situ atomic force microscopy studies showed that the presence of aqueous sulfide can diminish the inhibitory effect of Mg²⁺ ions on Mg-calcite growth and increase its growth rate. We propose that aqueous sulfide can lower the energy barrier to the dehydration of Mg²⁺-water complexes at the surface of a growing dolomite nucleus, and thereby enhance Mg²⁺ incorporation into the precipitating carbonate. In natural environments, aqueous sulfide produced by SR can trigger the formation of disordered dolomite, which can be a precursor to sedimentary stoichiometric dolomite. Calcite seed crystals were also found to promote dolomite formation, by providing nucleation sites for the heterogeneous nucleation of dolomite, which offers a possible explanation of the concurrence of dolomite abundance and “calcite sea” during the Phanerozoic Eon.