Abstract Crystallization from an amorphous precursor is an important pathway of carbonate precipitation in nature. However, the mechanistic details of the transformation from an amorphous phase to a crystalline phase of carbonates remain a topic of intense debate. Two competing mechanisms, including solid-state transition and coupled dissolution-reprecipitation, have been proposed to explain this transformation process. Magnesium is a common element in carbonate crystal lattices and its isotopes may provide unique insights into this problem. In this study, we investigated the transformation of the amorphous carbonate (AC) precursor for norsethite [BaMg(CO3)2], a dolomite analogue mineral, by in situ XRD analysis and isotope exchange experiments using a 25Mg enriched tracer coupled with high precision isotope analyses of δ26Mg and δ25Mg values for aqueous and solid phases. In situ XRD experiments revealed that the AC can transformed to crystalline norsethite at various temperatures (25 °C, 50 °C and 70 °C) and no intermediate mineral formed during the AC transformation process. 25Mg tracers indicated that near-complete Mg isotope exchange occurred in all exchange experiments during AC transformation. More importantly, after the AC transformation, the system showed surprising apparent non-mass dependent fractionation relationship, that the δ25Mg value of solid phase became greater than that of aqueous solution from a lower value, producing positive Δ25Mgsolid-aq fractionation, whereas the Δ26Mgsolid-aq fractionation remained negative. We numerically modeled the behavior of Mg isotopes (in both δ26Mg and δ25Mg) for the experimental system according to the two competing mechanisms of AC transformation. The modeling results suggest that the apparent non-mass dependent isotope behavior can only be explained by the coupled dissolution-reprecipitation process. Therefore, this study does not support the solid-state transition mechanism for AC transformation. Further, this study rigorously proves that norsethite can form by precipitation from aqueous solution without replacement, and implies that Mg2+ in aqueous solutions can be efficiently dehydrated and incorporated into a well ordered dolomite-group mineral (norsethite) under abiotic, low temperature conditions, thus providing new insights for understanding dolomite precipitation in nature.

中文翻译：

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无定形前体向结晶碳酸盐的转化：从白云石类似矿物 Norsethite [BaMg(CO3)2] 中 Mg 同位素的见解

摘要 从无定形前体中结晶是自然界中碳酸盐沉淀的重要途径。然而，碳酸盐从无定形相转变为结晶相的机理细节仍然是一个激烈争论的话题。两种相互竞争的机制，包括固态转变和耦合溶解-再沉淀，已被提出来解释这种转变过程。镁是碳酸盐晶格中的常见元素，它的同位素可以为解决这个问题提供独特的见解。在这项研究中，我们研究了无定形碳酸盐 (AC) 前驱体 [BaMg(CO3)2] 白云石类似矿物的转化过程，通过原位 XRD 分析和同位素交换实验，使用富含 25Mg 的示踪剂，结合对水相和固相的 δ26Mg 和 δ25Mg 值进行高精度同位素分析。原位 XRD 实验表明，AC 可以在各种温度（25°C、50°C 和 70°C）下转变为结晶 Norsethite，并且在 AC 转变过程中没有形成中间矿物。25Mg 示踪剂表明在 AC 转化过程中的所有交换实验中都发生了近乎完全的 Mg 同位素交换。更重要的是，经过 AC 转化后，系统表现出惊人的明显的非质量依赖分馏关系，固相的 δ25Mg 值从较低的值变得大于水溶液的值，产生正的 Δ25Mgsolid-aq 分馏，而 Δ26Mgsolid-aq 分馏仍为负值。我们根据 AC 转变的两种竞争机制对实验系统的 Mg 同位素行为（在 δ26Mg 和 δ25Mg 中）进行了数值模拟。建模结果表明，表观的非质量依赖性同位素行为只能通过耦合溶解-再沉淀过程来解释。因此，本研究不支持交流转化的固态转变机制。此外，该研究严格证明，通过从水溶液中沉淀而无需置换，可以形成 Norsethite，并暗示水溶液中的 Mg2+ 可以在非生物低温条件下有效脱水并结合到有序的白云石族矿物（norsethite）中，