Growth kinetics of siderite at 298.15 K and 1 bar

Abstract

The widespread occurrence of siderite at the Earth’s surface has motivated investigations into the thermodynamic factors controlling its stability for most of the last century. However, despite a new appreciation for multiple Fe(II)-carbonate mineralisation pathways, the rates of siderite growth have not been accurately predicted as a function of solution chemistry. This has impeded a quantitative understanding of myriad geochemical systems, both modern and ancient. To address this issue, we investigated the growth kinetics of synthetic siderite seeds in anoxic closed-system conditions at 298.15 K and 1 bar. On the basis of monitoring the chemical evolution of the bulk solutions over the course of 46 days, our results demonstrate two distinct relationships between kinetic behaviour and solution saturation (Ω) with respect to siderite. These relationships are best explained by chemical affinity-based rate laws that have been extensively applied to the growth kinetics of calcite (a mineral isostructural with siderite). More specifically, the surface area-normalised siderite growth rates (mol·m−2·s−1) display a linear correlation with supersaturation (Ω-1) when Ω≳5, suggesting a growth rate controlled by the transport of ions to the mineral surface (rtr):rtr=10-12.60±0.16(Ω-1)1.057±0.112.At low solution saturation, growth rates are consistent with spiral mechanism-dominated surface reactions (rsr), exhibiting a parabolic correlation with (Ω-1):rsr=10-13.42±0.14(Ω-1)1.868±0.307.These data show that at comparable solution saturation at 25 °C, siderite growth is nearly 7-orders of magnitude slower than that of calcite. These kinetic behaviours imply that most examples of natural Fe(II) supply fluxes (e.g., dissimilatory iron reduction, hydrothermal activity, or dissolution of other Fe(II)-minerals) would only be balanced by siderite precipitation rates at very high supersaturation, or, if the barrier to Fe(II)-carbonate nucleation is surpassed, balanced by episodic fluctuations in precipitation rate in response to Fe(II) accumulation. In combination with kinetic barriers influencing nucleation, this slow growth rate provides a straightforward explanation for the common observation that many anoxic water bodies are persistently supersaturated with respect to siderite, and implies that the precipitation of siderite may be more dynamic than previously appreciated.