Characterization of mineral surfaces using FIB and TEM: A case study of naturally weathered alkali feldspars
Characterization of mineral surfaces using FIB and TEM: A case study of naturally weathered alkali feldspars
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DOI：
10.2138/am.2007.2453
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发表时间：
2007-08
影响因子：
3.1
通讯作者：
Martin R. Lee;David J. Brown;Caroline L. Smith;M. Hodson;M. Mackenzie;R. Hellmann
中科院分区：
文献类型：
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作者：
Martin R. Lee;David J. Brown;Caroline L. Smith;M. Hodson;M. Mackenzie;R. Hellmann
摘要利用聚焦离子束(FIB)仪器，从英格兰西北部Shap花岗岩上的土壤中回收的过硫质碱性长石的自然风化表面切割出电子透明的样品(称为箔)。通过透射电子显微镜(TEM)对这些薄片的表征，可以确定长石近表面区域的结晶度和化学成分，并评估晶内微结构对颗粒表面微形貌和蚀坑发展的影响。在离子研磨之前，用于成像和沉积保护性铂的30千伏Ga+离子的注入伴随着损伤，在颗粒外表面下形成无定形层，但可以在FIB工作之前在颗粒表面涂上85 nm的金来克服。在离子研磨的后期阶段，围绕原始空隙的箔和长石的侧壁也部分非晶化。没有发现非晶态或晶态风化产物或紧贴在颗粒外表面之下的无定形“淋滤层”的证据。没有淋溶层表明沙土中长石的化学风化是化学计量的，或者如果是非化学计量的，要么是由于淋溶层太薄而不能用透射电子显微镜技术(即≤~2.5 nm)分解，要么是从近地表淋洗的离子比例不足，从而保持了长石的结晶度。没有发现任何证据表明，长石表面存在微生物细丝的风化机制有任何不同。在物理风化过程中，亚颗粒和出溶片层影响了裂缝的扩展，在颗粒表面形成了亚微米级台阶，而由于钠长石片层和包裹斜长石的成分或应变相关的溶解速度的差异，形成了更细小的波纹。随着逐渐的风化，在谷物表面开始的蚀坑通过利用在谷物的火成岩历史中形成的先前存在的纳米孔网络，作为蚀刻管延伸到谷物内部。FIB和TEM技术的结合是一种特别有效的方法来探索外部颗粒表面下的“内部区域”的风化机制，但必须谨慎地解释结果，因为在制备和表征箔的过程中使用的高能离子和电子束很容易产生伪影。
Abstract Using a focused ion beam (FIB) instrument, electron-transparent samples (termed foils) have been cut from the naturally weathered surfaces of perthitic alkali feldspars recovered from soils overlying the Shap granite, northwest England. Characterization of these foils by transmission electron microscopy (TEM) has enabled determination of the crystallinity and chemical composition of near-surface regions of the feldspar and an assessment of the influence of intragranular microtextures on the microtopography of grain surfaces and development of etch pits. Damage accompanying implantation of the 30 kV Ga+ ions used for imaging and deposition of protective platinum prior to ion milling creates amorphous layers beneath outer grain surfaces, but can be overcome by coating grains with >85 nm of gold before FIB work. The sidewalls of the foil and feldspar surrounding original voids are also partially amorphized during later stages of ion milling. No evidence was found for the presence of amorphous or crystalline weathering products or amorphous “leached layers” immediately beneath outer grain surfaces. The absence of a leached layer indicates that chemical weathering of feldspar in the Shap soils is stoichiometric, or if non-stoichiometric, either the layer is too thin to resolve by the TEM techniques used (i.e., ≤~2.5 nm) or an insufficient proportion of ions have been leached from near-surface regions so that feldspar crystallinity is maintained. No evidence was found for any difference in the mechanisms of weathering where a microbial filament rests on the feldspar surface. Sub-micrometer-sized steps on the grain surface have formed where subgrains and exsolution lamellae have influenced the propagation of fractures during physical weathering, whereas finer scale corrugations form due to compositional or strain-related differences in dissolution rates of albite platelets and enclosing tweed orthoclase. With progressive weathering, etch pits that initiated at the grain surface extend into grain interiors as etch tubes by exploiting preexisting networks of nanopores that formed during the igneous history of the grain. The combination of FIB and TEM techniques is an especially powerful way of exploring mechanisms of weathering within the “internal zone” beneath outer grain surfaces, but results must be interpreted with caution owing to the ease with which artifacts can be created by the high-energy ion and electron beams used in the preparation and characterization of the foils