GSA Connects 2022 meeting in Denver, Colorado

Paper No. 83-12
Presentation Time: 11:05 AM

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


WANG, Hsiu-Wen, Richmond, TX 77406, BISH, David, Department of Chemistry, Indiana University, Molecular Structure Center, Chemistry Building, 800 E. Kirkwood Ave., Bloomington, IN 47405, PAGE, Katharine, Material Science and Engineering, The University of Tennessee, Knoxville, TN 37996 and NEDER, Reinhard B., Institute for Crystallography and Structural Physics, Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Germany

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.