New Insights into the “Dolomite Problem”

Calcium carbonate buffers the pH of natural waters by dissolution and precipitation. Dolomite, CaMg(CO3)2, is often supersaturated in natural waters and does not precipitate in the laboratory experiments. The “dolomite problem” is the discrepancy between these laboratory and field observations of dolomite deposits in the geological record.

X-ray reflectivity and lateral force microscopy have provided new fundamental insights into the “dolomite problem”:

• Dolomite surfaces readily react in contact with supersaturated solutions. The reaction, however, was both self-limiting and partially irreversible, explaining the apparent lack of reactivity seen by macroscopic approaches.
   
• X-ray reflectivity demonstrated that the reacted surface is coated by a two-layer thick, Ca-rich CaxMg1-x(CO₃)₂ film with x~3/4. The self-limiting nature of the reaction is due to lattice strain due to the dependence of the lattice parameter with composition.

(A) Schematic side view of the dolomite (104) surface. (B) Lateral-force microscopy image of a dolomite surface with pristine and reacted regions, imaged during reaction. The reaction effectively stops when the surface is coated by the reaction product. (C) Real-time X-ray reflectivity data of dolomite in saturated and supersaturated solutions, showing the reaction kinetics, the self-limiting reaction and the irreversibility with respect to the pristine surface, R(t)/R(0) = 1.

References

P. Fenter, Z. Zhang, C. Park, N. C. Sturchio, X. M. Hu and S. R. Higgins, “Structure and Reactivity of the Dolomite (104)-Water Interface: New Insights into the Dolomite Problem,” Geochimica et Cosmochimca Acta, 71(3) 566-579 (2007).

S. R. Higgins, X. Hu, P. Fenter, “A quantitative lateral force microscopy study of the dolomite (104)-water interface,” Langmuir, 23 8909-8913 (2007).