Siderite nucleation pathways as a function of aqueous solution saturation state at 25 °C

The mechanisms of siderite (FeCO3) nucleation from aqueous solution depend on the fluid saturation state. In this study we investigate the effect of aqueous fluid saturation on the nucleation pathway of siderite. Thereof, we performed a series of turbidity and batch experiments. Turbidity measurements indicate that at low saturation indices (SI < 3.69), siderite nucleation proceeds via the direct nucleation of crystalline siderite from the aqueous fluid. The direct precipitation of crystalline siderite is confirmed by X-Ray diffraction (XRD) analysis. This is the first study to report the direct nucleation of siderite at near-equilibrium conditions. At higher saturation indices (SI >= 3.69), nucleation proceeds via the initial precipitation of amorphous ferrous carbonate (AFC) followed by the nucleation and growth of crystalline siderite. XRD spectra confirm the initial precipitation of AFC and its transformation to crystalline siderite. Such differences in the nucleation mechanism likely affect both the reactivity of the resulting siderite and its isotopic composition. Insight into the aqueous fluid saturation state during siderite nucleation could thus be crucial for the accurate interpretation of the isotopic signature in natural systems.

Automated summary

The study investigates the effect of aqueous fluid saturation on the nucleation pathway of siderite (FeCO3) and demonstrates that the nucleation mechanism depends on the fluid saturation state. Under low saturation indices, siderite nucleation proceeds via direct crystallization from the aqueous fluid, while at higher saturation indices, nucleation occurs through the initial precipitation of amorphous ferrous carbonate followed by the formation of crystalline siderite. Understanding the aqueous fluid saturation state during siderite nucleation is crucial for interpreting the isotopic signature in natural systems.