LOCAL STRUCTURE OF OPAL-CT: MULTISCALE MODELING OF STACKING DISORDER IN LAYERED MATERIALS (Invited Presentation)

Opal, a hydrous mineral(oid)(SiO₂-nH₂O), is common in low-temperature geochemical environments. It occurs in a number of structural states that typically are designated as: amorphous opal (opal-A); disordered cristobalite with significant tridymitic stacking (opal-CT); and disordered cristobalite with minor (or no) tridymitic stacking (opal-C). During diagenesis, opal matures from opal-A to -CT and to -C, and this reaction series effectively buffers the activity of silica, influencing important low-temperature reactions such as illitization and zeolitization. Given its wide occurrence as a precursor to quartz, details on local disorder in opals are important. The key to describing the structural state of opals lie in their intermediate-range (10-100 Å) ordering. In opal-CT, the inherent disorder in the stacking of layered units has hampered the determination of fine-scale structures by diffraction methods that rely on long-range order. Pair distribution function analysis (PDF) is a promising characterization method for the study of the short- (<10 Å) as well as intermediate-range order in amorphous and poorly ordered solids. However, creation of multiscale structural models that include short-range bonding constraints and higher-level parameters (e.g., stacking fault probabilities), and the use of such parameters as fitting variables are challenging. Here, we present a step-by-step modeling scheme that considers stacking layers in correlation with the abundance of layer types, and optimizes Si-O-Si angles while preserving the regularity of the silica tetrahedra. Simulated PDF and diffraction patterns of the statistical opal-CT models are compared with experimental data, featuring agreements in peak intensities, width and asymmetry never obtained before. This work presents a complete multiscale structural description of the natural precursors to quartz formation and offers novel approaches for modeling of layered materials and their assembly into 3D structures.