无定形MgCO3作为前驱体介导无水碳酸镁矿物矿化的实验研究

碳酸盐矿物在地球系统中广泛存在，一直是地球科学的重要研究课题之一。目前，人们对CaCO3系统的研究比较深入，但是对MgCO3系统的研究不足。尽管Mg2+和Ca2+在化学和热力学性质上相似且菱镁矿和方解石在结构上相似，但是常温常压下菱镁矿却不能像方解石一样在水溶液中结晶。大多数研究认为这是Mg2+的强水合性导致的。然而，无水环境中的合成实验也只形成无定形MgCO3(AMC)，说明除水合外还存在其他制约因素，进而推测直接成核结晶的经典模式不能有效形成菱镁矿，非晶相-晶相转变的非经典结晶模式也许是常温常压下菱镁矿矿化的可行途径。本项目将通过改变溶剂水活度合成不同种类的AMC，利用同步辐射X射线/中子全散射、X射线吸收边精细结构谱（XAFS）和分子动力学模拟等多种研究手段解析其局域近邻结构，探明控制其形成的关键因素；进一步进行晶相转变实验，验证AMC介导无水碳酸镁矿物形成的假设。

Owing to their wide occurrence in geological formations, high chemical reactivity, and potential for CO2 sequestration, carbonate minerals have been an important research subject in Earth sciences for the past one and a half centuries. So far, the CaCO3 system have been studied in depth, but the research on the MgCO3 system is insufficient. Despite the similarities of Mg2+ to Ca2+ in chemical/thermodynamic properties and magnesite to calcite in crystal structures, magnesite and dolomite differ fundamentally from calcite in their inability to crystallize in aqueous solutions at ambient conditions. The inability of dolomite and magnesite to mineralize at ambient conditions has been attributed largely to the stronger hydration of Mg2+ relative to that of Ca2+. However, our preliminary experimental results in water-free environment only revealed the precipitation of amorphous MgCO3 anhydrate, disagreeing with the existing supposition and indicating the presence of additional fundamental retardation forces. Thus, we hypothesize that the direct crystallization under the framework of classical nucleation theory cannot effectively form magnesite, instead, non-classical crystallization mode of amorphous phase-crystal phase transformation may be a feasible way for magnesite mineralization under ambient conditions. In this project, we will synthesize different kinds of AMCs in both organic and organic-water binary solvent and analyze their local structures with various methods such as synchrotron radiation X-ray/neutron total scattering pattern, X-ray absorption edge fine structure spectrum (XAFS) and molecular dynamics simulation. The local structures of different kinds of AMCs will be used to identify the key factors controlling AMC formation. Furthermore, phase transformation experiments will be carried out to verify the assumption that AMC mediates the formation of anhydrous magnesium carbonates.