Time-Resolved in Situ Raman Spectroscopy of the Nucleation and Growth of Siderite, Magnesite, and Calcite and Their Precursors
详细信息 查看全文
- 作者：German Montes-Hernandez ; François Renard
- 刊名：Crystal Growth & Design
- 出版年：2016
- 出版时间：December 7, 2016
- 年：2016
- 卷：16
- 期：12
- 页码：7218-7230
- 全文大小：913K
- ISSN：1528-7505
文摘
The nucleation and growth processes of particles and crystals from aqueous systems are actively investigated and occasionally open to debate because multistep nucleation and prenucleation events often exist. This study demonstrates that time-resolved Raman spectroscopic measurements can provide complementary and useful information on the nucleation and growth of particles and crystals from homogeneous and heterogeneous systems at different pressure–temperature conditions. Three minerals were chosen for this study: siderite, magnesite, and calcite, three carbonate minerals that are widespread in geological environments and have numerous industrial and medical applications. As expected, siderite and calcite can rapidly form via an amorphous precursor when using concentrated solutions of reactants directly mixed at ambient temperature (∼25 °C) and pressure (∼1 bar) (i.e., via spinodal decomposition or when energetic barrier is close to zero). In addition, calcium carbonate clusters and/or probably also amorphous calcium carbonate (ACC) were detected in the first 5 min prior to calcite nucleation in a Ca(OH)2–H2O–CO2 concentrated slurry at 25 °C and 50 bar. The calcite and siderite crystals, nucleated from their respective amorphous phases, grow by oriented aggregation of crystalline nanoparticles leading to porous spherical siderite-mesocrystals or nonporous rhombohedral calcite crystals after 24 h of reaction, as deduced from field emission scanning electron microscopy images and X-ray diffraction analyses. Moreover, the possibility of an Ostwald ripening mechanism contributing to crystal growth is not excluded. Conversely, magnesite formation was not detected by Raman spectroscopy at ambient room temperature; only nesquehonite (MgCO3·3H2O) and dypingite (Mg5(CO3)4–(OH)2·5H2O) were formed, depending on the reacting solution chemistry. These two minerals transform rapidly into hydromagnesite (Mg5(CO3)4–(OH)2·4H2O) by a heat-aging step at 50 °C. In fact, magnesite formation (MgCO3: anhydrous Mg carbonate) was only measured under hydrothermal conditions (e.g., 90 °C and 50 bar of initial CO2 pressure), and systematically a transient crystalline phase such as hydromagnesite was observed prior to magnesite nucleation and growth at the investigated conditions. This confirms that crystalline phases (not necessarily polymorphs) can also act as precursors during nucleation and growth of more stable phases. The fact that direct nucleation of magnesite from ionic solutions and slurries at ambient temperature is not observed is probably due to the high level of hydration of Mg ions in aqueous systems.