菱铁矿，菱镁矿和方解石及其前体成核和生长的时间分辨原位拉曼光谱,Crystal Growth & Design

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菱铁矿，菱镁矿和方解石及其前体成核和生长的时间分辨原位拉曼光谱
Crystal Growth & Design ( IF 3.2 ) Pub Date : 2016-11-23 00:00:00 , DOI: 10.1021/acs.cgd.6b01406
German Montes-Hernandez _{1,

2} , François Renard _{1,

2,

3}

Affiliation

由于经常存在多步成核和预成核事件，因此正在积极研究水性系统中颗粒和晶体的成核和生长过程，并且偶尔会引起争议。这项研究表明，时间分辨拉曼光谱测量可以提供有关在不同压力-温度条件下来自均质和非均质系统的粒子和晶体的成核和生长的补充和有用信息。本研究选择了三种矿物：菱铁矿，菱镁矿和方解石，这三种碳酸盐矿物广泛分布于地质环境中，并具有大量的工业和医学应用。不出所料当使用在环境温度（〜25°C）和压力（〜1 bar）下直接混合的反应物浓溶液（即通过旋节线分解或高能势垒接近零）时，菱铁矿和方解石可以通过无定形前体快速形成。此外，在方解石在Ca（OH）中成核之前的前5分钟内，检测到碳酸钙簇和/或无定形碳酸钙（ACC）。在25°C和50 bar下的₂ –H ₂ O–CO ₂浓缩浆液。由场发射扫描电子显微镜图像和X-射线推论得出，方解石和菱铁矿晶体从其各自的非晶相成核，通过晶体纳米粒子的定向聚集而生长，从而在反应24小时后形成多孔球形菱铁矿-中晶或无孔菱形方解石晶体。射线衍射分析。此外，不排除奥斯特瓦尔德成熟机制有助于晶体生长的可能性。相反，在室温下用拉曼光谱法未检测到菱镁矿的形成。仅三水菱镁矿（MgCO ₃ ·3H ₂ O）和绿霞石（Mg ₅（CO ₃）取决于反应溶液的化学性质，形成了₄ –（OH）₂ ·5H ₂ O）。通过在50°C的热老化步骤，这两种矿物质迅速转变为菱镁矿（Mg ₅（CO ₃）_4-（OH）₂ ·4H ₂ O）。实际上，菱镁矿的形成（MgCO ₃：无水碳酸镁）仅在水热条件下（例如，90°C和50 bar的初始CO _{2进行测量）}压力），并且在研究条件下，在菱镁矿成核和生长之前，系统地观察到过渡结晶相（例如菱镁矿）。这证实了晶相（不一定是多晶型物）也可以在更稳定的相的成核和生长过程中充当前体。在环境温度下未观察到离子溶液和浆液中菱镁矿直接成核的事实，可能是由于Mg离子在水体系中的高水合程度所致。

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Time-Resolved in Situ Raman Spectroscopy of the Nucleation and Growth of Siderite, Magnesite, and Calcite and Their Precursors

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.

更新日期：2016-11-23

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