[M-IS17] Interface- and nano-phenomena on crystal growth and dissolution

[MIS17-09] Hidden stability of norsethite revealed by phase diagram

*Harutoshi Asakawa¹, Kaoru Isobe¹, Hiroshi Uneda¹, Itaru Echigo¹, Ryuichi Komatsu¹ (1.Graduate School of Sciences and Technology for Innovation Yamaguchi University)

Keywords:Norsethite, phase diagram, solution-mediated transformation

Since the norsethite BaMg(CO₃)₂ can be relatively easily synthesized at atmospheric pressure, the utilization of the norsethite is promising as a model crystal for solving the dolomite problem, which is the longstanding mineralogical problems. It is known that the norsethite is formed by the solution-mediated transformation from the BaCO₃. Hence, the driving force of the norsethite crystallization corresponds to the difference of solubility between the norsethite and BaCO₃. In contrast, our preliminary study has shown a two-step pH change during the dissolution process. When the dissolution of the norsethite started, pH steeply increased owing to the appearance of HCO₃^- at first. Then, pH remained constant. In 20 minutes, pH gradually decreased owing to the appearance of BaCO₃, pH, finally, remained constant in 20000 min again. When comparing between the solution-mediated transformation and the two-step dissolution, the inferences based on these results are thermodynamically difficult. This incoherence is resulting in the difficulty of understanding on the formation of the norsethite. Then, we expect that the ratio of Ba²⁺ and Mg²⁺ in an aqueous solution plays an important role in the norsethite crystallization from the difference of cations between the noresethite and BaCO₃ from the viewpoint of thermodynamics. In this study, we examined the relationship between the stability of the norsethite and the ratio of Ba²⁺ and Mg²⁺.

The 100mg norsethite powder was added into 50 ml pure water heated at growth temperature T=40~90^OC, and the solution was kept stirring with measuring pH for one month. After the elution, the concentration of Ba²⁺, Mg²⁺, and CO₃^2- in the solution was evaluated by inductivity coupled plasma optical emission spectrometer and total organic carbon measurement. To confirm the stability of the norsethite, the sample was measured by XRD measurement.
Consequently, a small amount of the BaCO₃ appeared from the norsethite powder after the dissolution of the norsethite at more than 60 ^OC. In addition, the ratio of Ba²⁺ and Mg²⁺ in the solution became Mg²⁺ rich at less than 70 ^OC and Ba²⁺ rich at more than 70 ^OC. In contrast, the BaCO₃ became undersaturated in the saturated solution of the norsethite at more than 70 ^OC. These results demonstrate that the aqueous solution with the norsethite exhibits the saddle point composition, which is more stable than the stoichiometric composition.