Microchemistry and microstructure of Australian Opal

胶体无定形二氧化硅(蛋白石-A)的沉淀存在于S地表和地下的各种重要地质环境中。普通蛋白石由无序的非均质球体组成。相比之下，珍贵的蛋白石由以高度规则的阵列排列的均匀球体组成，出现在极少数分散的沉积物中。目前，由于地质系统中蛋白石形成流体的复杂性，对其形成机制尚无令人满意的解释。迫切需要对蛋白石及其寄主岩石进行详细的调查，以建立对硅酸盐岩石蚀变和二氧化硅成岩过程中化学再分布规模和机制的矿物学理解。在这项建议中，我们要求支持针对澳大利亚天然蛋白石-A中纳米胶体球体形成机制的分析研究。这项研究将集中在蛋白石形成的关键过程-具有个体尺寸分布的球体的形成及其三维排列。我们将使用一种新的方法，结合蛋白石微结构和微量化学，来限制二氧化硅胶体体系的地球化学，并缩小现有高空间分辨率分析方法与现有知识之间的差距。在沉积或假形变过程中形成的蛋白石的地球化学指纹将深入了解本质上不同条件下的系统元素交换。对寄主岩石和共生矿物的化学和矿物学性质的详细研究将提供有关蛋白石形成溶液的pH和盐度的线索。微观结构分析将揭示杂质的类型和数量如何改变天然无定形二氧化硅的短程有序性。此外，对蛋白石假象的结构和组成的详细研究将揭示二氧化硅假象背后的结构和化学重组机制的线索。蛋白石和共生相的微观形态和化学成分将通过扫描电子显微镜(SEM)和电子探针分析(EMPA)来确定。蛋白石的精确结构表征将通过显微X射线衍射(Micro-X射线衍射)和显微拉曼光谱(Micro-RS)来完成。我们将使用NanoSIMS离子微探针和高空间分辨率的透射电子显微镜技术，如HRTEM、EFTEM-EELS和TEM-EDX来研究假晶石中置换界面的浓度分布。到目前为止，对蛋白石地球化学的研究主要局限于用质谱学方法对蛋白石样品进行整体分析。这些方法忽略了化学成分与硅球大小、粒度分布和包裹体有序性之间的任何系统关系，一方面忽略了矿物包裹体的化学污染，另一方面忽略了从包裹体和共生矿物中获得的有关溶液组成的信息。我们请求支持，以带来研究成果

Precipitation of colloidal amorphous silica (opal-A) occurs in various geologically important settings on earth´s surface and subsurface. Common opal is composed of disordered heterogeneous spheres. In contrast, precious opal composed of uniform spheres packed in highly regular arrays, occurs in very few scattered deposits. At present, there is no satisfactory explanation of the opal formation mechanisms available due to the complex nature of opal-forming fluids in geological systems. Detailed investigations of opals and their host rocks are urgently needed to establish a mineralogic understanding of chemical re-distribution scales and mechanisms during silicate rock alteration and silica diagenesis.In this proposal we ask for support of analytical investigations that aim at the mechanisms of nanocolloidal sphere formation in natural Australian opal-A. This study will focus on the key processes in opal genesis - the formation of spheres with an individual size distribution and their three-dimensional arrangement. We will use a novel approach, combining opal microstructure and microchemistry, to confine the geochemistry of the silica colloid system, and to close the gap between available analytical methods at high spatial resolution and current knowledge.The geochemical fingerprint of opals formed during sedimentation or pseudomorphism will give insights into systematic element exchange at essentially different conditions. The detailed investigation of chemical and mineralogical properties of host rocks and cogenetic minerals will provide clues about pH and salinity of the opal-forming solutions. Microstructural analysis will reveal how the type and quantity of impurities modify the short-range order of natural amorphous silica. Additionally, detailed study of opal pseudomorphs´ texture and composition will reveal clues for structural and chemical reorganization mechanisms behind silica pseudomorphism.The micromorphology and chemistry of opals and cogenetic phases will be determined with scanning electron microscopy (SEM) and electron microprobe analysis (EMPA). The precise structural characterization of opals will be accomplished with micro-X-ray diffraction (micro-XRD) and micro-Raman spectroscopy (micro-RS). Concentration profiles at replacement interfaces in pseudomorphs will be studied with a NanoSIMS ion microprobe and high spatial resolution transmission electron microscopy (TEM) techniques, such as HRTEM, EFTEM-EELS, and TEM-EDX.Research on opal geochemistry is so far largely restricted to bulk analysis of opal samples by mass spectroscopic methods. Any systematic relation between chemical composition and silica sphere size, size distribution, and package ordering is ignored by these approaches, as well as chemical contamination by mineral inclusions, on the one hand, and information on solution composition to be gained from included and cogenetic minerals on the other. We ask for support to bring the resea