Microchemistry and microstructure of Australian Opal

• Applicant: Privatdozent Dr. Ralf Milke
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2014 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 268285275

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

• DFG Programme: Research Grants