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 ( r tr ): r tr = 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 ( r sr ), exhibiting a parabolic correlation with ( Ω - 1 ): r sr = 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.

中文翻译：

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菱铁矿在 298.15 K 和 1 bar 下的生长动力学

摘要 菱铁矿在地球表面的广泛存在激发了对控制其稳定性的热力学因素在上个世纪的大部分时间的研究。然而，尽管对多种 Fe(II)-碳酸盐矿化途径有了新的认识，但菱铁矿的生长速率还没有被准确预测为溶液化学的函数。这阻碍了对现代和古代无数地球化学系统的定量理解。为了解决这个问题，我们研究了合成菱铁矿种子在 298.15 K 和 1 bar 的缺氧封闭系统条件下的生长动力学。在监测 46 天过程中本体溶液的化学演化的基础上，我们的结果表明动力学行为与关于菱铁矿的溶液饱和度 (Ω) 之间存在两种不同的关系。这些关系最好通过基于化学亲和力的速率定律来解释，该定律已广泛应用于方解石（一种与菱铁矿同构的矿物）的生长动力学。更具体地说，当 Ω ≳ 5 时，表面积归一化菱铁矿生长速率 (mol · m−2 · s−1) 显示出与过饱和度 (Ω - 1 ) 的线性相关性，表明生长速率受离子向矿物表面 (r tr ): r tr = 10 - 12.60 ± 0.16 (Ω - 1 ) 1.057 ± 0.112 。在低溶液饱和度下，增长率与螺旋机制主导的表面反应 (r sr ) 一致，与 (Ω - 1 ) 呈抛物线相关性：r sr = 10 - 13.42 ± 0.14 (Ω - 1 ) 1.868 ± 0.307。这些数据表明，在 25 °C 的类似溶液饱和度下，菱铁矿的生长比方解石慢近 7 个数量级。这些动力学行为意味着大多数天然 Fe(II) 供给通量的例子（例如异化铁还原、水热活动或其他 Fe(II) 矿物的溶解）只能通过非常高过饱和度下的菱铁矿沉淀速率来平衡，或者，如果超过了对 Fe(II)-碳酸盐成核的障碍，则通过响应 Fe(II) 积累的沉淀速率的间歇性波动来平衡。与影响成核的动力学障碍相结合，这种缓慢的增长速度为许多缺氧水体相对于菱铁矿持续过饱和的普遍观察提供了一个直接的解释，并暗示菱铁矿的沉淀可能比以前认为的更具动态性。异化铁还原、水热活动或其他 Fe(II)-矿物的溶解）只能通过非常高的过饱和度下的菱铁矿沉淀速率来平衡，或者，如果超过了 Fe(II)-碳酸盐成核的障碍，则通过偶发性响应 Fe(II) 积累的沉淀率波动。与影响成核的动力学障碍相结合，这种缓慢的增长速度为许多缺氧水体相对于菱铁矿持续过饱和的普遍观察提供了一个直接的解释，并暗示菱铁矿的沉淀可能比以前认为的更具动态性。异化铁还原、水热活动或其他 Fe(II)-矿物的溶解）只能通过非常高的过饱和度下的菱铁矿沉淀速率来平衡，或者，如果超过了 Fe(II)-碳酸盐成核的障碍，则通过偶发性响应 Fe(II) 积累的沉淀率波动。与影响成核的动力学障碍相结合，这种缓慢的增长速度为许多缺氧水体相对于菱铁矿持续过饱和的普遍观察提供了一个直接的解释，并暗示菱铁矿的沉淀可能比以前认为的更具动态性。如果超过了 Fe(II)-碳酸盐成核的障碍，则通过响应 Fe(II) 积累的沉淀速率的间歇性波动来平衡。与影响成核的动力学障碍相结合，这种缓慢的增长速度为许多缺氧水体相对于菱铁矿持续过饱和的普遍观察提供了一个直接的解释，并暗示菱铁矿的沉淀可能比以前认为的更具动态性。如果超过了 Fe(II)-碳酸盐成核的障碍，则通过响应 Fe(II) 积累的沉淀速率的间歇性波动来平衡。与影响成核的动力学障碍相结合，这种缓慢的增长速度为许多缺氧水体相对于菱铁矿持续过饱和的普遍观察提供了一个直接的解释，并暗示菱铁矿的沉淀可能比以前认为的更有活力。