颗粒介导的成核途径在硫酸钙单晶的内部结构中留下印记
Particle-mediated nucleation pathways are imprinted in the internal structure of calcium sulfate single crystals
摘要：
硫酸钙矿物以三种晶相的形式存在：石膏（CaSO4.2H2O）、石膏（CaSO4.0.5H2O）和硬石膏（CaSO 4）。由于其在自然和工业过程中的相关性，从水溶液中形成这些硫酸钙相的途径一直是深入研究的主题。有越来越多的文献表明，硫酸钙基本上是通过非经典的纳米颗粒或簇介导的结晶过程形成的。在沉淀的早期阶段，硫酸钙晶体通过聚集体的重组和聚结而不是通过经典的单元加成而生长。在这里，我们表明，通过使用低剂量暗场（DF）透射电子显微镜（TEM）和电子衍射，这些重组过程绝不会继续，直到获得一个近乎完美的均匀的单晶。相反，生长过程产生了最终的不完美的介晶，其整体形态类似于单晶，但由较小的纳米畴组成。因此，我们的数据表明，通过粒子介导途径生长的无有机介晶可能会在最终的晶体结构中保留非经典生长过程的“记忆”或“印记”，这在很大程度上被忽视了。此外，晶体内的结构子单元的纳米级未对准可能通过长度尺度传播，并且可能在宏观上表示为大单晶中的未对准区域/域，例如，如在来自墨西哥奇瓦瓦州奈卡矿的一些巨型晶体中观察到的那些。
英文摘要：
Calcium sulfate minerals are found under the form of three crystalline phases: gypsum (CaSO4.2H2O), bassanite (CaSO4.0.5H2O), and anhydrite (CaSO4). Due to its relevance in natural and industrial processes, the formation pathways of these calcium sulphate phases from aqueous solution have been the subject of intensive research. There is a growing body of literature, that calcium sulfate forms essentially through a non-classical nanoparticle- or cluster-mediated crystallisation process. At the early stages of precipitation calcium sulfate crystals grow through the reorganization and coalescence of aggregates rather than through classical unit addition. Here, we show by using low-dose dark field (DF) transmission electron microscopy (TEM) and electron diffraction, that these re-structuring processes by no means continue until a near-perfectly homogeneous single crystal is obtained. Instead the growth process yields a final imperfect mesocrystal of the overall morphology resembling that of a single crystal, yet composed of smaller nano-domains. Hence, our data indicate that organic-free mesocrystal grown by a particle mediated-pathway may preserve a 'memory or 'imprint' of the non-classical growth process in the final crystal structure, something that has been largely overlooked until now. Furthermore, the nano-scale misalignment of the structural sub-units within crystals might propagate through the length-scales, and potentially is expressed macroscopically as misaligned zones/domains in large single crystals, e.g. as those observed in some of the giant crystals from the Naica Mine, Chihuahua, Mexico.
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